Question 1

Nitrogen sources and culture pH in plant tissue culture media How does utilization of ammonium versus nitrate affect the pH of plant cell culture media over time?

Accepted Answer

Introduction / Context:In plant tissue culture, media pH drifts as nitrogen sources are assimilated. Managing pH is vital for nutrient availability, enzyme activity, and growth of explants and callus. Given Data / Assumptions: Media typically contain both NH4+ (ammonium) and NO3− (nitrate). Cells assimilate nitrogen via distinct pathways that consume or release protons. Buffers are limited; thus, assimilation impacts bulk pH. Concept / Approach:Assimilation of NH4+ tends to acidify the medium because uptake and incorporation release H+ equivalents to maintain charge balance. In contrast, NO3− reduction and assimilation generally consume H+ (or are coupled to OH− production), leading to a gradual rise in pH. Therefore, ammonium drives pH downward; nitrate drives pH upward. Step-by-Step Solution: Consider charge balance: uptake of cation (NH4+) releases H+; uptake of anion (NO3−) consumes H+.Relate to medium pH: more H+ in medium → pH drops; fewer H+ → pH rises.Conclude the correct pattern: NH4+ lowers pH, NO3− raises pH. Verification / Alternative check:Empirical observations in MS-based media show pH decrease with predominant ammonium uptake and pH increase with nitrate uptake, confirming the biochemical rationale. Why Other Options Are Wrong: Reversing the effects (option b) contradicts proton balance. Both rising (c) or both dropping (d) ignore the opposite ionic effects. Option e ignores widely observed pH drift. Common Pitfalls:Overlooking that relative uptake rates and initial buffering capacity modulate the magnitude of pH change, though the direction remains as stated. Final Answer:Ammonium utilization causes pH to drop, while nitrate utilization causes pH to rise

Question 2

In plant tissue culture media used for micropropagation and callus work, auxins and cytokinins are typically supplied at micromolar concentrations. What is the commonly used concentration range for these plant growth regulators (PGRs)?

Accepted Answer

Introduction / Context:Plant tissue culture relies on carefully balanced plant growth regulators (PGRs). Auxins and cytokinins are the workhorses for inducing callus, shoots, and roots. Knowing typical concentration ranges helps avoid toxicity and physiological abnormalities while ensuring reliable morphogenesis. Given Data / Assumptions: The question asks for standard, routinely used ranges in general-purpose culture media. Auxins (e.g., IAA, NAA, 2,4-D) and cytokinins (e.g., BAP, kinetin, zeatin) are considered. Concentrations are expressed in micromolar (μM). Concept / Approach:Most protocols for callus induction, somatic embryogenesis, and organogenesis use auxins and cytokinins between low and mid micromolar levels. Excessively high PGR levels can cause vitrification, callus browning, or abnormal shoots/roots. Very low levels may be insufficient to trigger morphogenesis. Step-by-Step Solution: Recognize that general tissue culture recipes (e.g., MS-based) commonly use cytokinins such as BAP at ~0.5–10 μM and auxins ranging from ~0.1–30 μM depending on the goal.Typical teaching-lab and research protocols cluster within 1–50 μM for each regulator, adjusting the auxin:cytokinin ratio for shoot vs. root vs. callus responses.Compare with options: 1–50 μM best matches common practice. Verification / Alternative check:Protocol collections and plant tissue culture manuals consistently present successful formulations in the low-to-mid μM range for both classes of PGRs. Why Other Options Are Wrong: 50–100 μM and 100–125 μM: often too high for routine morphogenesis; risk of physiological disorders. More than 125 μM: typically inhibitory or toxic. 0.01–0.5 μM: can work in fine-tuned systems but is not the most common range across species and aims. Common Pitfalls:Assuming “more is better” with PGRs; overlooking the critical auxin:cytokinin ratio; ignoring species and tissue differences. Final Answer:1–50 μM

Question 3

In plant cell culture media, which of the following are recognized disadvantages of using the natural auxin indole-3-acetic acid (IAA)?

Accepted Answer

Introduction / Context:Auxins drive cell division, elongation, and callus induction in vitro. Indole-3-acetic acid (IAA) is the principal natural auxin in plants, yet tissue culture practitioners often prefer synthetic auxins because of IAA’s chemical liabilities under culture conditions. Given Data / Assumptions: Aqueous media are exposed to light, oxygen, and sometimes elevated temperature. Plant cells possess enzymes that modify and inactivate auxins. The question asks for disadvantages specific to IAA. Concept / Approach:IAA is chemically labile. It photodegrades and oxidizes, reducing effective concentration. Cells can also conjugate IAA to amino acids or sugars, lowering free auxin levels and altering responses. Consequently, stable analogs (e.g., NAA, 2,4-D, IBA) are common in protocols. Step-by-Step Solution: List known issues: instability, oxidative breakdown, cellular conjugation.Match with options provided.All items apply; therefore choose “All of the above.” Verification / Alternative check:Method notes in tissue culture references frequently warn of IAA instability and recommend freshly prepared solutions or alternative auxins. Why Other Options Are Wrong: Single statements (a–c): each is true but incomplete. None of the above: contradicts well-documented properties of IAA. Common Pitfalls:Using old IAA stocks; neglecting light protection; assuming equal stability among auxins. Final Answer:All of the above

Question 4

Macronutrient composition in plant cell culture: Which set correctly lists the major macronutrients commonly included in standard media formulations?

Accepted Answer

Introduction / Context:Successful plant tissue culture depends on supplying essential mineral elements at appropriate concentrations. Macronutrients are required in millimolar amounts and underpin osmotic balance, metabolism, and morphogenesis. Given Data / Assumptions: Macronutrients in standard media (e.g., MS) include nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Some answer sets approximate this list but include ions less typical as macronutrients in these media (e.g., sodium). Concept / Approach:While several ions appear in trace or optional roles, core macronutrients emphasize N, P, K, Ca, Mg, and S. Among the given choices, the closest correct grouping that avoids sodium or omitting sulfur is selected. Step-by-Step Solution: Identify the classic macronutrient roster: N, P, K, Ca, Mg, S.Scan options for the best match.Option C (N, P, K, S, Ca) correctly includes all but Mg, and is the best available among choices that otherwise add less typical sodium or omit sulfur. Verification / Alternative check:MS medium lists NH4+ and NO3− (N), KH2PO4 (P), K+ (K), CaCl2 (Ca), MgSO4 (Mg, S) as standard macronutrient salts. Why Other Options Are Wrong: Sets including Na or Cl as macronutrients (b, d) do not reflect standard macronutrient emphasis in plant culture media. Option a includes Na and omits Ca. Option e includes Mg but omits Ca, which is standard. Common Pitfalls:Confusing plant physiological macronutrients with those specifically emphasized in tissue culture formulations; overvaluing sodium, which is generally not a required macronutrient for most plants. Final Answer:N, P, K, S, Ca

Question 5

Naturally occurring plant auxins: Which of the following listed auxins is naturally produced by plants and widely occurs in vivo?

Accepted Answer

Introduction / Context:Auxins regulate cell elongation, apical dominance, vascular differentiation, and root initiation. In tissue culture, both natural and synthetic auxins are used, but it is important to distinguish endogenous from synthetic molecules. Given Data / Assumptions: IAA is the principal endogenous auxin in higher plants. NAA and 2,4-D are synthetic auxins commonly used for stability and predictable responses. Concept / Approach:Natural occurrence is the key criterion. While NAA and 2,4-D mimic auxin activity, they are not biosynthesized by plants. IAA derives from tryptophan-dependent and independent pathways in planta and is ubiquitous across species. Step-by-Step Solution: List the auxins and classify them as natural (IAA) or synthetic (NAA, 2,4-D).Select the option that names the natural auxin.Therefore, choose Indole-3-acetic acid (IAA). Verification / Alternative check:Plant physiology references consistently identify IAA as the main endogenous auxin; NAA and 2,4-D are laboratory-applied analogs. Why Other Options Are Wrong: NAA and 2,4-D: synthetic, not naturally produced. All of the above / None of the above: contradict the established classification. Common Pitfalls:Assuming frequent use in vitro implies natural occurrence; conflating endogenous hormones with synthetic analogs selected for stability. Final Answer:Indole-3-acetic acid (IAA)

Question 6

During plant regeneration from callus, which growth regulator is primarily added to initiate and promote shoot formation (shoot organogenesis)?

Accepted Answer

Introduction / Context:Reprogramming callus to regenerate shoots requires a hormonal signal that favors shoot meristem initiation over root formation. In classic Skoog and Miller paradigms, the auxin:cytokinin ratio determines organogenic direction. Given Data / Assumptions: Callus is already established. The goal is shoot initiation and proliferation. Hormone balance can be adjusted by adding cytokinins. Concept / Approach:High cytokinin relative to auxin promotes shoot organogenesis by stimulating cell division in shoot-forming regions and activating shoot meristem identity networks. Conversely, high auxin favors rooting or undifferentiated callus proliferation depending on context. Step-by-Step Solution: Identify the morphogenetic target: shoots.Apply the Skoog–Miller rule: high cytokinin / low auxin encourages shoots.Select Cytokinins as the primary additive for shoot initiation. Verification / Alternative check:Protocols for many species (e.g., BAP or kinetin supplementation) demonstrate robust shoot formation when cytokinin dominates the hormonal regime. Why Other Options Are Wrong: Auxins: typically favor roots or callus. Gibberellins: elongation and growth, not de novo shoot meristem initiation. Brassinosteroids and ABA: play roles in growth and stress responses but are not the primary drivers of shoot organogenesis from callus. Common Pitfalls:Using excessive auxin during the shoot induction phase; failing to reduce auxin after callus induction. Final Answer:Cytokinins

Question 7

Neutralized activated charcoal in young regenerating cultures is sometimes added to the medium to achieve which purpose(s)?

Accepted Answer

Introduction / Context:Activated charcoal (AC) is a high-surface-area adsorbent sometimes included in plant tissue culture media, particularly during regeneration phases. Its use must be intentional because it can significantly change medium chemistry. Given Data / Assumptions: Explants under stress can exude phenolic compounds that oxidize and inhibit growth. Carryover or residual PGRs may hinder phase transitions (e.g., from callus to organogenesis). Neutralized AC minimizes pH shock but still adsorbs molecules. Concept / Approach:AC binds small aromatic and hydrophobic molecules, reducing phytotoxic phenolics and lowering free concentrations of auxins/cytokinins if desired. This can improve morphogenesis quality and reduce browning. It does not directly buffer pH like a designated buffer system. Step-by-Step Solution: List primary benefits: detoxification of phenolics and removal of excess PGRs.Match to options: (a) and (b) are both correct.Select “Both (a) and (b).” Verification / Alternative check:Protocol notes commonly caution that AC may also remove vitamins or other organics, evidencing its broad adsorptive action. Why Other Options Are Wrong: Maintain the pH: AC is not a primary pH buffer. Supply micronutrients: not a controlled nutrient source. Common Pitfalls:Unintended adsorption of beneficial components (e.g., vitamins, hormones); using AC without adjusting PGR/vitamin levels accordingly. Final Answer:Both (a) and (b)

Question 8

Which of the following substances is NOT a cytokinin used in plant tissue culture?

Accepted Answer

Introduction / Context:Cytokinins promote cell division and shoot formation. Distinguishing cytokinins from auxins prevents hormonal misapplication during in vitro morphogenesis and regeneration protocols. Given Data / Assumptions: BAP, zeatin, kinetin, and TDZ are commonly used cytokinins or cytokinin-like compounds. 2,4-D is a synthetic auxin, not a cytokinin. Concept / Approach:Identify each compound’s hormone class. 2,4-D is used mainly for callus induction and somatic embryogenesis as an auxin. The others are standard cytokinins for shoot induction and proliferation. Step-by-Step Solution: Classify each option by hormone type.Recognize that 2,4-D is an auxin, unlike the others.Select 2,4-D as the item that is not a cytokinin. Verification / Alternative check:Hormone supplier catalogs and tissue culture manuals classify these compounds accordingly. Why Other Options Are Wrong: BAP, zeatin, kinetin, TDZ: all act as cytokinins or cytokinin-like regulators. Common Pitfalls:Confusing frequent use of 2,4-D in callus protocols with cytokinin function; assuming all “shoot-promoting” media contain only cytokinins without considering ratios. Final Answer:2,4-dichlorophenoxyacetic acid (2,4-D)

Question 9

In cultured plant cells, which growth regulator is primarily associated with promoting cell elongation and wall loosening that drives overall cell growth (before organ-specific differentiation)?

Accepted Answer

Introduction / Context:Plant growth regulators coordinate cell division and expansion. Auxin-driven acid growth and wall loosening constitute a primary mechanism for cell enlargement, central to callus proliferation and organogenesis stages when balanced with other hormones. Given Data / Assumptions: Focus is on cell growth via elongation and wall extensibility, not meristem identity. Auxins trigger proton pump activity and wall-loosening processes. Concept / Approach:Auxins stimulate H+-ATPases, acidifying the cell wall space, activating expansins and other wall-modifying proteins. This leads to wall plasticity and turgor-driven expansion. While gibberellins and brassinosteroids also influence growth, auxin is the primary regulator for elongation in many contexts and is the standard first answer in tissue culture basics. Step-by-Step Solution: Recall the “acid growth” hypothesis linked to auxin.Relate this to increased cell wall extensibility and elongation.Select Auxins as the principal regulator of cell elongation growth in culture. Verification / Alternative check:Physiology texts consistently attribute cell wall loosening and elongation to auxin action, especially in coleoptiles and stem segments. Why Other Options Are Wrong: Cytokinins: promote cell division and shoots, not primary wall loosening. Gibberellins and brassinosteroids affect elongation but are not the principal first-line regulator in basic culture contexts. ABA: generally growth-inhibitory/stress-related. Common Pitfalls:Equating organogenesis control (cytokinin-rich) with basic cell expansion; overlooking that multiple hormones can modulate growth but auxin is the classic driver of elongation. Final Answer:Auxins

Question 10

Use of very high sugar concentrations (approximately 40–100 g/L) in plant tissue culture: In which situations are such elevated carbohydrate levels purposefully applied?

Accepted Answer

Introduction / Context:Carbohydrate concentration in plant culture media affects osmotic potential, growth rate, morphogenesis, and metabolite pathways. While 20–30 g/L sucrose is common, some applications deliberately use higher concentrations for specific physiological and metabolic outcomes. Given Data / Assumptions: Typical basal media use ~30 g/L sucrose. Higher levels (40–100 g/L) are exceptional and goal-driven. Two common goals: secondary metabolite enhancement and osmotic manipulation during regeneration or stress conditioning. Concept / Approach:Elevated sugars can impose osmotic stress that alters water relations and can direct morphogenesis or improve stress tolerance during critical transitions. High sugar also modifies carbon flux and may upregulate or stabilize pathways tied to secondary metabolite accumulation. Step-by-Step Solution: Identify contexts that benefit from hyperosmotic media: pre- or post-regeneration treatments, embryo maturation, or shoot quality improvement.Recognize specialized production systems where carbon supply and osmotic status enhance metabolite yields.Select “Both (a) and (b).” Verification / Alternative check:Reports on somatic embryo maturation and metabolite-focused cultures document efficacy of higher sucrose concentrations for these aims. Why Other Options Are Wrong: Routine maintenance: usually 20–30 g/L; higher levels are not standard. Single-purpose options omit legitimate uses. Common Pitfalls:Prolonged exposure to hyperosmotic conditions can reduce growth or cause stress responses; adjust duration and concentration carefully. Final Answer:Both (a) and (b)

Question 11

In plant tissue culture, what is meant by a “nurse” or conditioned medium, and how is it typically obtained?

Accepted Answer

Introduction / Context:Conditioned (or “nurse”) medium is a practical tool to improve growth or regeneration of difficult explants by providing secreted factors from actively growing cells. It can enhance viability, reduce lag phases, and improve morphogenesis in sensitive cultures. Given Data / Assumptions: Fast-growing cell suspensions release vitamins, peptides, phytohormone-like compounds, and other metabolites into the medium. This “conditioned” medium can be transferred to new cultures that benefit from these factors. Concept / Approach:The concept mirrors feeder-layer strategies in animal cell culture. Instead of co-culture, the supernatant medium itself is used, carrying diffusible growth-promoting molecules that are difficult to define or synthesize individually. Step-by-Step Solution: Grow a vigorous suspension culture under optimal conditions.Remove and clarify the spent medium (supernatant) that now contains secreted factors.Use this “nurse” medium wholly or partially to supplement target cultures that need support. Verification / Alternative check:Numerous regeneration protocols report improved outcomes when a fraction of conditioned medium is included, especially for recalcitrant species or stages. Why Other Options Are Wrong: Generic “growth factor-rich” descriptions lack the defining feature (derivation from an active culture). Antibiotic-containing medium is unrelated to conditioning. Common Pitfalls:Using over-aged or nutrient-exhausted medium; failing to sterile-filter before use; ignoring batch variability. Final Answer:The liquid medium removed from a vigorously growing suspension culture and used to support other cultures

Question 12

Plant tissue culture additive — role of silver thiosulfate (STS)  In plant cell and micropropagation media, why is silver thiosulfate commonly added to the culture vessel or medium?

Accepted Answer

Introduction / Context:Silver thiosulfate (STS) is a widely used supplement in plant tissue culture and micropropagation. In sealed vessels, explants often accumulate ethylene, a gaseous plant hormone that can inhibit growth, induce abscission, or cause vitrification. Understanding why STS is used helps practitioners prevent culture decline and improve morphogenesis. Given Data / Assumptions: Cultures are maintained in closed or semi-closed containers. Ethylene accumulates readily in limited headspace. STS releases Ag+ ions complexed with thiosulfate. Goal: identify the principal reason for adding STS. Concept / Approach: Ethylene perception in plants occurs at membrane-bound receptors that require a transition metal cofactor. Silver ions (Ag+) competitively inhibit ethylene binding at these receptors, effectively blocking ethylene signaling. In sealed culture systems, suppressing ethylene action prevents callus browning, leaf abscission, shoot fasciation, and other stress symptoms that compromise regeneration. Step-by-Step Solution: Identify the primary culture problem: ethylene accumulation in sealed vessels.Recall STS chemistry: Ag+ (as a thiosulfate complex) interacts with ethylene receptors.Predict the physiological result: reduced ethylene responses → healthier shoots/embryos.Select the option that states STS prevents harmful ethylene action/accumulation effects. Verification / Alternative check: Protocols for ornamentals, potato, and woody plants frequently include STS dips or medium supplements to reduce ethylene response, improving shoot elongation and reducing leaf drop. Why Other Options Are Wrong: pH maintenance is typically achieved with buffers (e.g., MES) and by adjusting salts, not by STS. Phenolic removal is aided by activated charcoal or antioxidants, not STS. Increasing dissolved oxygen chemically is not STS’s role. Common Pitfalls: Confusing inhibition of ethylene biosynthesis with inhibition of ethylene action. STS primarily blocks perception/action, yet functionally it suppresses ethylene’s effects in culture. Final Answer: To block ethylene (C2H4) action and thereby prevent harmful ethylene build-up in closed cultures

Question 13

Plant growth regulators — which hormone class directly promotes cell division in culture?

Accepted Answer

Introduction / Context:Successful plant tissue culture relies on balancing plant growth regulators (PGRs). While several hormones influence growth, one class is classically associated with stimulating cytokinesis (cell division), shoot organogenesis, and bud proliferation. Given Data / Assumptions: We refer to standard in vitro conditions (e.g., MS medium). Common PGRs include auxins, cytokinins, gibberellins, brassinosteroids, and abscisic acid. We seek the hormone class that directly promotes cell division. Concept / Approach: Cytokinins (e.g., BAP/BA, kinetin, TDZ, zeatin) promote DNA synthesis and cytokinesis, driving shoot initiation and proliferation. Auxins (e.g., IAA, NAA, IBA, 2,4-D) promote cell expansion, callusing, and rooting; high auxin with low cytokinin favors roots or callus, whereas higher cytokinin with lower auxin favors shoots. Gibberellins mainly elongate stems and break dormancy; brassinosteroids modulate growth and stress responses; abscisic acid typically promotes maturation or stress tolerance and inhibits division. Step-by-Step Solution: Identify the process of interest: cytokinesis/cell division in vitro.Match PGR to function: cytokinins stimulate cell division and shoot organogenesis.Confirm by classical Skoog and Miller ratio concept (cytokinin:auxin balance).Choose “Cytokinins”. Verification / Alternative check: Textbook organogenesis rules: high cytokinin/low auxin → shoots; high auxin/low cytokinin → roots/callus, confirming cytokinins drive cell division for shoot formation. Why Other Options Are Wrong: Auxins primarily influence elongation, rooting, and callus induction. Gibberellins elongate internodes but do not primarily drive mitosis in culture. Brassinosteroids are growth modulators, not the main division driver. Abscisic acid inhibits division and promotes maturation. Common Pitfalls: Assuming “more growth” equals “more division.” Some PGRs elongate cells without increasing mitotic events. Final Answer: Cytokinins

Question 14

Identify the non-auxin — which of the following is not classified as an auxin used in plant tissue culture?

Accepted Answer

Introduction / Context:Auxins and cytokinins are the cornerstone hormones of plant tissue culture. Being able to distinguish which compounds belong to which class is essential for designing media that drive callus, root, or shoot formation. Given Data / Assumptions: Common auxins: IAA, NAA, IBA, 2,4-D. Common cytokinins: zeatin, BAP/BA, kinetin, TDZ. We must select the compound that is not an auxin. Concept / Approach: IAA, NAA, IBA, and 2,4-D are all widely used auxins that promote cell elongation, callusing, and rooting. Zeatin, however, is a naturally occurring cytokinin that promotes cell division and shoot morphogenesis, not an auxin. Step-by-Step Solution: List known auxins in culture: IAA, NAA, IBA, 2,4-D.Identify zeatin as a cytokinin.Select the non-auxin: zeatin.Confirm with typical media recipes (e.g., BA/zeatin for shoots; auxins for roots/callus). Verification / Alternative check: Standard references and MS-based protocols classify zeatin as a cytokinin; it is never grouped among auxins. Why Other Options Are Wrong: IAA, NAA, IBA, and 2,4-D are bona fide auxins with clear auxinic effects on tissues. Common Pitfalls: Confusing “plant growth regulator” with a specific hormone class; remember zeatin is cytokinin. Final Answer: Zeatin

Question 15

Carbon sources in plant tissue culture — which carbohydrate is most commonly used in media formulations?

Accepted Answer

Introduction / Context:Plant cells in vitro are typically heterotrophic and require an exogenous carbon source. Media such as MS, B5, and WPM almost always include a carbohydrate; choosing the right one affects osmotic potential, callus growth, and morphogenesis. Given Data / Assumptions: Question asks for the most common carbon source in standard culture media. Typical concentrations are around 20–30 g/L. Alternatives like glucose and fructose are sometimes used but less common. Concept / Approach: Sucrose is the dominant carbohydrate in plant tissue culture. It is inexpensive, readily available, and, upon autoclaving or in culture, can hydrolyze to glucose and fructose, which cells readily take up. Its osmotic properties and compatibility with a wide range of explants make it the default choice. Step-by-Step Solution: Survey standard recipes: MS medium typically uses 30 g/L sucrose.Consider alternatives: glucose/fructose can work but may be less stable or cost-effective.Select the most widely used option: sucrose.Note: some specialized protocols use maltose (e.g., anther culture), but not as the general default. Verification / Alternative check: Protocol collections and manufacturer datasheets list sucrose as the standard carbon source across most species and explant types. Why Other Options Are Wrong: Glucose and fructose are used occasionally but are not the general default. Maltose is more niche. Lactose is rarely used in plant culture. Common Pitfalls: Assuming glucose is preferred because it is a primary metabolite; in practice, sucrose’s handling and osmotic traits make it superior. Final Answer: Sucrose

Question 16

Controlling ethylene in vitro — which additive is used to inhibit ethylene effects during culture?

Accepted Answer

Introduction / Context:Ethylene accumulation in closed vessels can suppress morphogenesis and induce physiological disorders in plant cultures. Various additives are used to mitigate this, either by reducing biosynthesis, adsorbing molecules, or blocking perception. Given Data / Assumptions: The question seeks a culture additive that effectively inhibits ethylene's impact. Practical tissue culture usage prioritizes reliable suppression of ethylene responses. We compare organic acids, adsorbents, and silver complexes. Concept / Approach: Silver thiosulfate (STS) releases Ag+ that binds ethylene receptors, blocking ethylene perception and thereby neutralizing ethylene's inhibitory effects on growth and regeneration. While some chemicals (e.g., AVG or AOA) inhibit ethylene biosynthesis, STS is widely used in vitro because it robustly suppresses ethylene action. Activated charcoal can adsorb hormones and phenolics but is nonspecific and can remove beneficial compounds; organic acids do not specifically inhibit ethylene pathways. Step-by-Step Solution: Identify the cause: ethylene accumulation harms cultures.Select the agent with targeted anti-ethylene action: STS blocks receptor perception.Reject nonspecific adsorbents and unrelated acids.Answer: Silver thiosulfate. Verification / Alternative check: Protocols for cut flowers and micropropagation commonly use STS dips or medium supplements to reduce ethylene sensitivity and prevent abscission or vitrification. Why Other Options Are Wrong: Citric and succinic acids are pH/antioxidant agents; they do not target ethylene. Activated charcoal is nonspecific and can remove PGRs. Cobalt salts can affect ACC oxidase but are less commonly used due to toxicity and inconsistency in culture. Common Pitfalls: Equating “inhibits ethylene biosynthesis” with “inhibits ethylene effects.” In practice, STS is chosen to suppress ethylene action reliably in vitro. Final Answer: Silver thiosulfate (STS)

Question 17

Typical sucrose levels — what concentration of sucrose is commonly used in plant tissue culture media?

Accepted Answer

Introduction / Context:Carbohydrate concentration in plant tissue culture affects osmotic potential, growth rate, and morphogenesis. Sucrose is the standard carbon source, and its concentration is tuned for most explants to balance energy supply and osmotic conditions. Given Data / Assumptions: Media such as MS typically list sucrose at 30 g/L. Explants are heterotrophic in vitro and need an external carbon supply. We want the common concentration range used broadly across protocols. Concept / Approach: The majority of general protocols use 2–3% (w/v) sucrose, corresponding to 20–30 g/L. Lower concentrations may starve tissues; much higher levels increase osmotic stress, cause vitrification, or inhibit regeneration unless specifically required (e.g., for embryo maturation). Step-by-Step Solution: Reference typical recipes: MS medium with 30 g/L sucrose.Consider physiologic balance: energy supply vs. osmotic potential.Select the range that matches standard usage: 20–30 g/L.Note exceptions (e.g., 40–50 g/L) for specialized applications. Verification / Alternative check: Handbooks and supplier datasheets consistently recommend 2–3% sucrose as a starting point for most species. Why Other Options Are Wrong: 10–15 g/L is often insufficient; 40–100 g/L is unusually high and used only in specific, controlled contexts. Common Pitfalls: Assuming more sugar means faster growth; overly high sucrose can impair morphogenesis. Final Answer: 20–30 g/L (about 2–3% w/v)

Question 18

Nurse/conditioned medium — identify the incorrect statement about its preparation and use

Accepted Answer

Introduction / Context:Nurse or conditioned medium is a proven strategy to improve survival and regeneration of delicate cells such as protoplasts. It harnesses extracellular factors secreted by fast-growing cells. Recognizing correct handling is critical to maintain activity. Given Data / Assumptions: Conditioned medium is harvested from actively dividing suspension cultures. Secreted molecules may be heat-labile proteins or small metabolites. Sterility must be ensured without degrading active factors. Concept / Approach: Because many components of conditioned media are heat-labile, autoclaving would denature or destroy them. Therefore, the accepted practice is to collect the medium aseptically and sterilize it by membrane filtration (e.g., 0.22 μm), not by autoclaving. It is then used alone or mixed with fresh medium to aid difficult cultures. Step-by-Step Solution: Define nurse medium: supernatant from fast-growing cultures.Identify key property: presence of heat-labile growth factors.Determine correct sterilization: filter sterilization, not autoclaving.Select the statement claiming autoclaving is required as the incorrect one. Verification / Alternative check: Standard protocols for protoplast culture recommend filter-sterilized conditioned medium (or co-culture with feeder layers) to maintain bioactivity. Why Other Options Are Wrong: Options a, b, c, and e match accepted definitions and practices; they accurately describe source, content, application, and sterilization method (filtration). Common Pitfalls: Autoclaving “to be safe” compromises the very factors that make conditioned medium effective. Final Answer: It is removed aseptically and then autoclaved before use to ensure sterility

Question 19

Directing morphogenesis — which growth regulator chiefly stimulates embryo/shoot development in vitro?

Accepted Answer

Introduction / Context:In organogenesis and somatic embryogenesis, plant growth regulator balance determines developmental outcomes. Identifying the hormone class most associated with shoot/embryo promotion guides medium design for regeneration protocols. Given Data / Assumptions: Auxin–cytokinin balance is the classical framework (Skoog–Miller concept). We focus on stimulating embryo or shoot formation. Common cytokinins include BA/BAP, kinetin, zeatin, TDZ. Concept / Approach: Relatively higher cytokinin to auxin ratios favor shoot organogenesis and often support somatic embryo development or maturation in combination with other cues. Auxins are essential for callus induction and rooting; gibberellins promote elongation; brassinosteroids are modulators; abscisic acid is used later to mature somatic embryos. Step-by-Step Solution: Define target: promote shoots/embryos rather than roots/callus.Apply PGR rule: higher cytokinin supports shoot induction.Select the option naming cytokinins.Acknowledge complementary roles of auxin/ABA in specific stages. Verification / Alternative check: Most shoot regeneration protocols employ BAP or kinetin; somatic embryo protocols often adjust cytokinin levels or use TDZ. Why Other Options Are Wrong: Auxins tend to promote roots or callus; gibberellins elongate; brassinosteroids and ABA are not the principal inducers of shoot/embryo initiation. Common Pitfalls: Overusing auxin during shoot induction can suppress bud formation or cause vitrification. Final Answer: Cytokinins (e.g., BAP, kinetin, zeatin) to drive shoot/embryo development

Question 20

Maintaining pH in plant tissue culture — which approach is standard practice?

Accepted Answer

Introduction / Context:pH stability in culture media affects nutrient availability, enzyme activities, and morphogenesis. Media are typically adjusted (e.g., to pH ~5.6–5.8) before autoclaving, but metabolic processes can shift pH over time, so buffering is often beneficial. Given Data / Assumptions: Common buffers used in plant culture include MES and other Good's buffers. Organic acids listed are metabolic intermediates, not standard buffers for media. Ammonium salts alter pH during uptake and are not stable buffers. Concept / Approach: Synthetic zwitterionic buffers (e.g., MES, MOPS, HEPES) provide reliable buffering capacity in the slightly acidic range appropriate for plant culture. They minimize pH drift caused by ammonium assimilation or exudates. Organic acids are not used as primary buffers; Tris is less ideal at acidic pH but appears in some specialized formulations. Ammonium salts can acidify media as NH4+ is taken up, so they do not “maintain” pH. Step-by-Step Solution: Identify target pH (around 5.6–5.8).Select suitable buffering agents: MES or similar Good’s buffers.Exclude organic acids and ammonium salts as stable buffers.Choose the option naming synthetic buffers. Verification / Alternative check: Media supplier guides and tissue culture manuals recommend MES for pH stabilization in many protocols. Why Other Options Are Wrong: Organic acids are not standard buffers for media; ammonium salts alter pH during assimilation; pH does not remain constant without buffering. Common Pitfalls: Over-buffering can chelate micronutrients or interfere with growth; use appropriate concentrations. Final Answer: Use synthetic buffers (e.g., MES, MOPS, HEPES, or Tris) to stabilize pH

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