Activation and activity of plasma membrane H+-ATPase: key events in germinating Vicia faba seeds

The regulation of plasma membrane H+-ATPase was considered in imbibing Vicia faba seeds, a distinctive feature of which is germination by cell elongation, whereas the mitotic activity starts later. The enzyme activation is known to precede germination because it provides H+ ion efflux from the cytoplasm to cell walls which favours their modification and loosening, being the prerequisites of cell elongation commencement. The presence of an enzyme in imbibing embryo axes was confirmed immunochemically. H+ ion efflux was recorded with a pH-meter as acidification of ambient solution by the embryonic axes for 5 min. The activation of the enzyme and its subsequent activity are regulated in different ways. Enzyme activation is hydration-driven, it starts when water content increases up to the threshold level of 55% (fresh weight basis). This value was confirmed by imbibition in the presence of the osmoticum polyethylene glycol 6000, at various osmotic potentials. The activation does not depend on indolylacetic or abscisic acid treatment. Hydration-triggered activation of the enzyme favours rapid seed germination and its correspondence to the soil water potential. Enzyme activity after its activation is inhibited by 60–70% by 10−5–10−7 M abscisic acid, whereas indolylacetic acid exerted no effect. The regulation of plasma membrane H+-ATPase activity is presumably accomplished by the interaction of the enzyme with 14-3-3 proteins and endogenous fusicoccin, present in imbibing axes.