Abstract
NRT1.1, a nitrate transceptor, plays an important role in nitrate binding, sensing and nitrate dependent lateral root (LR) morphology. However, little is known about NRT1.1- mediated nitrate signaling transduction through plasma membrane (PM)-localized proteins. Through in-depth phosphoproteome profiling using membranes of Arabidopsis roots, we identified receptor kinase QSK1 and plasma membrane H+- ATPase AHA2 as potential downstream components of NRT1.1 signaling in a mild low nitrate (LN)-dependent manner. QSK1, as a functional kinase and molecular link, physically interacts with NRT1.1 and AHA2 at LN, and specifically phosphorylates AHA2 at S899. Importantly, we found that LN, not HN, induces formation of NRT1.1- QSK1-AHA2 complex in order to repress the proton efflux into the apoplast by increased phosphorylation of AHA2 at S899. Loss of either NRT1.1 or QSK1 thus results in a higher T947/S899 phosphorylation ratio on AHA2, leading to enhanced pump activity and longer LRs under LN. Our results uncover a regulatory mechanism in which NRT1.1, under LN conditions, recruits coreceptor QSK1 into a complex to transduce LN sensing to the PM H+-ATPase AHA2, controlling the phosphorylation ratio of activating and inhibitory phosphorylation sites on AHA2. This then results in altered proton pump activity, apoplast acidification, and regulation of NRT1.1-mediated LR growth.