7.3 Plant Growth Regulators and Phototropism

Plant growth regulators are signalling molecules produced in low concentrations by many tissues rather than by a dedicated endocrine system. They often act locally, although some move through vascular tissue. Auxins, gibberellins and cytokinins commonly promote aspects of growth; abscisic acid contributes to dormancy and stress responses; ethylene is a gaseous regulator with roles including fruit ripening and senescence. Each can promote one process and inhibit another depending on tissue, stage and concentration.

Auxin is produced particularly in shoot apices, young leaves and developing tissues. Its distribution is directional because auxin influx and efflux carriers are placed asymmetrically in cell membranes. Carrier localization, not diffusion alone, produces polar transport from cell to cell. Auxin and cytokinin also interact: their relative concentrations influence cell division and whether cultured plant tissue develops shoots or roots.

Positive phototropism is shoot growth toward a light source. Blue-light photoreceptors called phototropins become activated more strongly on the illuminated side of a shoot tip. Signalling changes auxin-carrier distribution so auxin accumulates more on the shaded side. In shoots, that unequal auxin distribution produces greater cell elongation on the shaded side, curving the organ toward light. The lit side does not need to shrink.

The acid-growth model links auxin to expansion. Auxin signalling activates proton pumps in the plasma membrane, moving H⁺ into the cell wall and lowering apoplast pH. Wall-loosening proteins become active, interactions among wall components weaken, and turgor pressure drives expansion. The wall must loosen while remaining strong enough to contain the protoplast. Auxin also changes gene expression, supporting longer-term growth responses.

Classic seedling experiments compare intact tips, removed tips, opaque caps and permeable barriers. If an opaque cap prevents bending while the growth region below remains able to elongate, the tip is implicated in light perception. A permeable connection between tip and lower tissue supports movement of a chemical signal. Good experiments include dark controls, uniform-light controls, adequate replication and measurements of curvature or side-specific elongation rather than a simple 'bent/not bent' judgement.

Ripening fruit produces ethylene, and ethylene stimulates enzyme expression associated with softening, pigment change, aroma and sugar metabolism. Ripening can stimulate still more ethylene production, creating positive feedback that synchronizes a rapid transition. The loop ends when substrates, responsive tissue or developmental context change; positive feedback does not imply infinite ethylene production.

Commercial suppliers exploit this system by transporting some fruit while unripe and exposing it to controlled ethylene near its destination. Temperature, humidity and ventilation must also be controlled because the regulator acts within a living respiratory system. The application demonstrates a general principle: understanding signalling permits timing to be changed without inventing a new developmental pathway.