8.2.4  Overall models for control of tropisms

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The pioneering studies on auxin responses in coleoptiles have undoubtedly influenced present-day models, yet vigorous debate among researchers continues on the wider importance or otherwise of auxin in tropisms, especially where sensing and responding cells are the same (Trewavas et al. 1992). Some researchers have attempted to generate a single model to explain all the types of differential growth that are represented by tropisms. Early researchers, including Charles Darwin, measured responses by angle of curvature either towards or away from the stimulus. However, detailed kinetic analysis has revealed that, perhaps surprisingly, there are at least four versions of growth differential. Some involve growth acceleration and some, deceleration (Table 8.7; Firn and Digby 1980). It is hard to envisage a single growth-regulating chemical, whether auxin or not, being laterally redistributed and causing sometimes net growth promotion, sometimes net growth inhibition and sometimes no change at all in growth rate on one side of the organ (Franssen et al. 1982). Coleoptile tips are very sensitive to light and may initiate a basipetal wave of growth-regulating chemical, but it is difficult to reconcile this notion with the observations that (a) all growing regions of oat coleoptiles initiate a response at the same time (Figure 8.14a) and (b) virtually the same response can occur even when the coleoptile is covered with a black cap (Figure 8.14b). Overall, greater progress has been made on the signal perception systems for light and gravity than on how the signals are translated into altered growth patterns.


Table 8.7


Figure 8.14 Differential growth during phototropic response of oat (Avena sativa) coleoptiles. Curvature is due to growth in all zones of the coleoptile stopping simultaneously on the illuminated side, but continuing unchanged on the shaded side. Zone 1 is nearest the apex. (a) Intact coleoptiles; (b) the response remains the same in intact coleoptiles with tip covered by a black cap, rotating on a horizontal clinostat at 1.2 rpm.

(Based on Franssen et al. 1982; reproduced with permission of Springer-Verlag)