Does Potassium (K+) Contribute to High-Nitrate (NO3−) Weakening of a Plant’s Defense System against Necrotrophic Fungi?

Abstract

In this opinion article, we have analyzed the relevancy of a hypothesis which is based on the idea that in Arabidopsis thaliana jasmonic acid, a (JA)-mediated defense system against necrotrophic fungi is weakened when NO3− supply is high. Such a hypothesis is based on the fact that when NO3− supply is high, it induces an increase in the amount of bioactive ABA which induces the sequestration of the phosphatase ABI2 (PP2C) into the PYR/PYL/RCAR receptor. Consequently, the Ca sensors CBL1/9-CIPK23 are not dephosphorylated by ABI2, thus remaining able to phosphorylate targets such as AtNPF6.3 and AtKAT1, which are NO3− and K+ transporters, respectively. Therefore, the impact of phosphorylation on the regulation of these two transporters, could (1) reduce NO3− influx as in its phosphorylated state AtNPF6.3 shifts to low capacity state and (2) increase K+ influx, as in its phosphorylated state KAT1 becomes more active. It is also well known that in roots, K+ loading in the xylem and its transport to the shoot is activated in the presence of NO3−. As such, the enrichment of plant tissues in K+ can impair a jasmonic acid (JA) regulatory pathway and the induction of the corresponding biomarkers. The latter are known to be up-regulated under K+ deficiency and inhibited when K+ is resupplied. We therefore suggest that increased K+ uptake and tissue content induced by high NO3− supply modifies the JA regulatory pathway, resulting in a weakened JA-mediated plant’s defense system against necrotrophic fungi.