Arabidopsis plasma membrane H⁺-ATPase genes AHA2 and AHA7 have distinct and overlapping roles in the modulation of root tip H⁺ efflux in response to low-phosphorus stress

Phosphorus deficiency in soil is one of the major limiting factors for plant growth. Plasma membrane H⁺-ATPase (PM H⁺-ATPase) plays an important role in the plant response to low-phosphorus stress (LP). However, few details are known regarding the action of PM H+-ATPase in mediating root proton (H⁺) flux and root growth under LP. In this study, we investigated the involvement and function of different Arabidopsis PM H⁺-ATPase genes in root H⁺ flux in response to LP. First, we examined the expressions of all Arabidopsis PM H+-ATPase gene family members (AHA1- AHA11) under LP. Expression of AHA2 and AHA7 in roots was enhanced under this condition. When the two genes were deficient in their respective Arabidopsis mutant plants, root growth and responses of the mutants to LP were highly inhibited compared with the wild-type plant. AHA2-deficient plants exhibited reduced primary root elongation and lower H⁺ efflux in the root elongation zone. AHA7-deficient plants exhibited reduced root hair density and lower H+ efflux in the root hair zone. The modulation of H+ efflux by AHA2 or AHA7 was affected by the action of 14-3-3 proteins and/or auxin regulatory pathways in the context of root growth and response to LP. Our results suggest that under LP conditions, AHA2 acts mainly to modulate primary root elongation by mediating H⁺ efflux in the root elongation zone, whereas AHA7 plays an important role in root hair formation by mediating H+ efflux in the root hair zone.