IAA transport during the phototropic responses of intact Zea and Avena coleoptiles

Abstract

Transport of indolyl-3-acetic acid (IAA) was studied during the phototropic responses of intact shoots and detached coleoptiles of Zea mays L. and Avena sativa L. The use of a high specific activity [5-(3)H]IAA and glass micropipettes enabled asymmetric application of the IAA to be made to individual coleoptiles with minimal tissue damage.A unilateral stimulus of 2.59×10(-11) einstein cm(-2) of blue light, probably in the dose range of the first positive phototropic response, caused significant net lateral movement of radioactivity from [5-(3)H]IAA away from the illuminated side of intact shoots and detached coleoptile apices of both Avena and Zea. The magnitude of the net lateral movement was 15.3% in Zea seedlings and 12.3% in Avena seedlings. Chromatographic analyses indicated that the movement of radioactivity reflected that of IAA. A phototropic stimulus of 1.24×10(-7) einstein cm(-2), which was probably in the second positive dose range, caused significant lateral movement of radioactivity in intact shoots and detached coleoptiles of Zea but not of Avena.In intact Zea seedlings, neither phototropic dosage affected the longitudinal transport of IAA. In intact Avena seedlings, first positive stimulation inhibited longitudinal transport only when the IAA was applied to the illuminated side of the coleoptile, but second positive stimulation inhibited basipetal movement of IAA regardless of the side of application.Exposing the intact seedlings to red light before phototropic stimulation abolished lateral transport after a first positive stimulus in Zea and in Avena.Phototropic stimulation can thus induce a lateral transport of IAA towards the shaded side of the coleoptiles of both Zea and Avena seedlings, and can affect longitudinal movement of IAA in the coleoptile of Avena. However, since phototropic curvature was observed under certain conditions in the absence of either of these effects, the extent to which they are involved in the induction of asymmetric growth in a stimulated coleoptile has yet to be resolved.

Phototropism: Bending towards Enlightenment

“…Related to this model, Naqvi (1972) proposed that light-induced production of abscisic acid on the illuminated flank of the seedling causes unequal polar auxin transport. To test the idea that unequal polar auxin transport gives rise to phototropism, Gardner et al (1974) followed radiolabeled auxin that had been asymmetrically applied to coleoptiles. Although they were not able to completely exclude a role for light-mediated inhibition of polar auxin transport, their results further demonstrated that unilateral blue light induced lateral auxin transport.…”

Critical study of coleoptile elongation controlled by IAA and ABA I. Growth kinetics and distribution

“…1) caused by IAA (treatment in situ). It is well established that the movement of endogenous auxin is basipetal in coleoptiles (8) and acropetal in roots (20), consequently polar (3, 9, II). For exogenous IAA, the present data indicated a similar situation (Fig.…”

How do plant growth substances work?

“…Curry (1969) measured these and reported that the rate of growth of the dark side was 4-5 fold higher than the lighted side. Although Curry (1969) pointed out that these were much higher than the auxin differences which had been measured-very much so when considering the 0-15% lateral redistribution of Gardner et al (1974)-he failed to draw the obvious conclusion that they could not account for unequal growth. The problem is exacerbated when, as will be considered later, the evidence favours a logarithmic dose-response relation between auxin and extension, and auxin and curvature.…”

“…(We shall return to this paper later.) In a critical and penetrating reassessment of these experiments, Gardner et al (1974) solved many of the difficulties in earlier work. They micro-injected labelled indoleacetic acid asymmetrically into the tip of intact coleoptiles.…”