Stimulation of plasma membrane H+-ATPase by auxins or fusicoccin and its relation to maize kernel setting, grain yield, and harvest index

Abstract

Increase in kernel setting and decrease of vegetative shoot biomass have been suggested as two possibilities for improvement of maize harvest index. The kernel number at maturity is mainly determined at the time around pollination, when, particularly under stress conditions, kernel abortion occurs leading to grain yield depressions. Plasma membrane (PM) H+-ATPase activity of vesicles derived from maize kernels shortly after pollination was shown to be significantly inhibited when plants were grown under drought or salinity. It is therefore hypothesized that with application of the phytohormone auxin or of fusicoccin, PM H+-ATPase activity can be stimulated in the developing maize kernels, leading to an improved hexose uptake and thus to a better kernel set. In this review, we first focus on the auxin indole-3-acetic acid (IAA) and give an overview of the main studies on auxin-induced elongation growth with respect to stimulation of PM H+-ATPase. Particular attention is paid to the IAA action in developing maize kernels, including interactions with cytokinins. The second part of our review addresses fusicoccin, its impact on PM H+-ATPase, determined with various experimental approaches. With application of IAA at anthesis a stimulation of PM H+-ATPase could be achieved, yet it did not result in a better kernel set at maturity. IAA was probably applied too early, disturbing the balance between cytokinins and auxins in the kernels. Applied at the appropriate growth stages, auxins and also cytokinins are promising measures to improve maize kernel development and grain yield, thus finally increasing the harvest index.