Genetic Regulation by NLA and MicroRNA827 for Maintaining Nitrate-Dependent Phosphate Homeostasis in Arabidopsis

Abstract

Plants need abundant nitrogen and phosphorus for higher yield. Improving plant genetics for higher nitrogen and phosphorus use efficiency would save potentially billions of dollars annually on fertilizers and reduce global environmental pollution. This will require knowledge of molecular regulators for maintaining homeostasis of these nutrients in plants. Previously, we reported that the NITROGEN LIMITATION ADAPTATION (NLA) gene is involved in adaptive responses to low-nitrogen conditions in Arabidopsis, where nla mutant plants display abrupt early senescence. To understand the molecular mechanisms underlying NLA function, two suppressors of the nla mutation were isolated that recover the nla mutant phenotype to wild type. Map-based cloning identified these suppressors as the phosphate (Pi) transport-related genes PHF1 and PHT1.1. In addition, NLA expression is shown to be regulated by the low-Pi induced microRNA miR827. Pi analysis revealed that the early senescence in nla mutant plants was due to Pi toxicity. These plants accumulated over five times the normal Pi content in shoots specifically under low nitrate and high Pi but not under high nitrate conditions. Also the Pi overaccumulator pho2 mutant shows Pi toxicity in a nitrate-dependent manner similar to the nla mutant. Further, the nitrate and Pi levels are shown to have an antagonistic crosstalk as displayed by their differential effects on flowering time. The results demonstrate that NLA and miR827 have pivotal roles in regulating Pi homeostasis in plants in a nitrate-dependent fashion. Higher crop yields require increased use of fertilizers, especially for the prime macronutrients nitrogen and phosphorus. Increasing nitrogen and phosphorus use efficiency in plants would decrease crop production cost and reduce environmental pollution. In an attempt to isolate the regulatory genes for nitrogen and phosphorus homeostasis in plants, we identified the NLA gene as having a role in plant adaptation under low-nitrogen conditions. In the current work, detailed genetic and molecular analysis for the functionality of this gene revealed that NLA has a key role in the maintenance of phosphate (Pi) homeostasis in plants in a nitrate-dependent fashion. Further, Pi has an antagonistic crosstalk with nitrate, not only with regards to its accumulation, but also in its differential effects on flowering time. Interestingly, the antagonistic genetic interaction of Pi is with nitrate, but not with ammonium.