Phosphate deprivation counteracts the cell death caused by loss-of-function of MEDIATOR 18 in Arabidopsis root meristems.

Mutation of MEDIATOR 18 leads to death of highly proliferating cells within the Arabidopsis root apical meristem, which impairs root growth. Phosphate (Pi) is a macronutrient required to support mitotic activity in meristems, and its deficiency causes root growth inhibition; thus, we hypothesized that Pi availability could influence cell viability as well. With this in mind, in vitro experiments were performed varying the Pi concentration (0, 1, 10, and 250 µM) in the growth medium of Arabidopsis WT seedlings and med18-1 mutants to analyze meristem integrity and root hair development and correlate it with gene expression of selected promoter-reporter gene fusions. We found that WT (Col-0) seedlings entered the already reported determinate root growth program that terminates mitosis and differentiates primary root meristems at low (0, 1, and 10 µM) Pi concentrations. Unexpectedly, in marked contrast to the WT, med18-1 null mutant seedlings had healthy meristems under low Pi availability, and the cell death occurred only at high Pi (250 µM Pi). Root hair density and length were greater in med18-1 mutants than WT at all Pi concentrations tested. Gene expression analyses for cell cycle, auxin, and damage response as well as detection of hydrogen peroxide indicated that MED18 promotes the transit from cell division into differentiation of primary root tips induced by Pi starvation but protects the root meristem from genotoxic stress upon zeocin application. These results uncover an unexpected finding in which the lack of an essential macronutrient decreases the genotoxic pressure to highly proliferating plant cells.