Auxin application to maize plants at flowering increases abundance and activity of plasma membrane H+-ATPase in developing maize kernels

BackgroundThe kernel number of maize (Zea mays L.) at maturity is mainly determined at the time around pollination. Kernel abortion frequently occurs during this period, leading to grain yield depressions. Plasma membrane (PM) H+-ATPase was identified as a key enzyme responsible for supply of assimilates to the developing maize kernels shortly after pollination.AimsThis study aimed at stimulating PM H+-ATPase activity in the kernels by in vivo application of the auxin indole-3-acetic acid (IAA) to maize plants at flowering, leading to an improved hexose uptake and finally to a better kernel set.MethodsMaize plants were cultivated under well-watered conditions using the container technique. IAA was applied to unstressed maize plants twice, 2 days before controlled pollination and at pollination (application rate per plant: 1.9 mL of 1.5 mM IAA). The developing kernels were harvested 2 days after pollination, and PM vesicles were isolated and purified using two-phase partitioning.ResultsThe in vitro hydrolytic activity of the PM H+-ATPase was significantly stimulated by 22% due to in vivo IAA application (control: 0.99 ± 0.05, IAA treatment: 1.21 ± 0.03* μmol inorganic phosphate mg-1 protein min-1). Vmax was significantly increased by IAA treatment, whereas Km was reduced. The maximal pH gradient (ΔA492) at the PM was increased by 10% (control: 0.071 ± 0.002, IAA treatment: 0.078 ± 0.002*). IAA caused a significant increase of PM H+-ATPase abundance in the vesicles. Concentrations of sucrose and hexoses as well as acid invertase activity in the kernels were unaffected by IAA treatment. However, at maturity kernel numbers per cob were significantly decreased causing grain yield reductions of 19%.ConclusionsIncreased PM H+-ATPase activity could not be translated into grain yield improvements. Probably the auxin application occurred too early during kernel development. As cytokinins play a key role during pollination, auxin application at this stage may have disturbed the phytohormone balance, causing disruption of cell division and a rather early onset of cell extension due to increased IAA concentrations. In further studies, it should be tested if applications of cytokinin at flowering or of IAA at a later growth stage have positive impacts on kernel set.