Recombinant protein targeting in plants: In reported systems expressing human serum albumin (HSA) in transgenic plants, where is HSA typically secreted when a signal peptide directs it to the secretory pathway?

Introduction / Context:
Plants can produce complex recombinant proteins such as human serum albumin (HSA). By fusing an N-terminal signal peptide to the HSA coding sequence, the protein enters the endomembrane system and can be secreted to the apoplast. Knowing the compartment helps with extraction strategy and protein stability considerations in molecular farming.

Given Data / Assumptions:

• Construct includes a signal peptide for secretion.
• Expression is under a suitable promoter in plant cells (e.g., leaves).
• No plastid transit peptide is provided unless explicitly engineered.

Concept / Approach:
Secretory signal peptides route nascent polypeptides into the endoplasmic reticulum, then through the Golgi to the cell surface, releasing the protein into the apoplast. Leaves are commonly used expression organs in transgenic plants or transient systems (e.g., agroinfiltration), and secreted HSA accumulates extracellularly, simplifying purification by apoplastic wash fluids or extraction from leaf tissue.

Step-by-Step Solution:

Verification / Alternative check:
Published plant recombinant protein studies report secreted HSA in leaves and apoplastic fluids when secretory signals are used; chloroplast targeting requires a distinct transit peptide, and vacuolar retention requires specific sorting signals.

Why Other Options Are Wrong:

• Roots or stems only: secretion is not organ-exclusive; leaves are widely used.
• Not secreted: contradicts the presence of a signal peptide and measured apoplastic accumulation.
• Chloroplast lumen: requires a plastid transit peptide, not a secretory signal.

Common Pitfalls:
Confusing secretion signals (apoplast) with targeting signals for chloroplasts, mitochondria, or vacuoles; each requires different leader peptides.

Final Answer:
Transgenic leaf tissues (apoplast/extracellular space)