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)?

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:

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