Defense-related callose synthase PMR4 promotes root hair callose deposition and adaptation to phosphate deficiency in Arabidopsis thaliana

Plants acquire phosphorus (P) primarily as inorganic phosphate (Pi) from the soil. Under Pi deficiency, plants induce an array of physiological and morphological responses, termed phosphate starvation response (PSR), thereby increasing Pi acquisition and use efficiency. However, the mechanisms by which plants adapt to Pi deficiency remain to be elucidated. Here, we report that deposition of a β-1,3-glucan polymer called callose is induced in Arabidopsis thaliana root hairs under Pi deficiency, in a manner independent of PSR-regulating PHR1/PHL1 transcription factors and LPR1/LPR2 ferroxidases. Genetic studies revealed PMR4 (GSL5) callose synthase being required for the callose deposition in Pi-depleted root hairs. Loss of PMR4 also reduces Pi acquisition in shoots and plant growth under low Pi conditions. The defects are not recovered by simultaneous disruption of SID2, mediating defense-associated salicylic acid (SA) biosynthesis, excluding SA defense activation from the cause of the observed pmr4 phenotypes. Grafting experiments and characterization of plants expressing PMR4 specifically in root hair cells suggest that a PMR4 pool in the cell type contributes to shoot growth under Pi deficiency. Our findings thus suggest an important role for PMR4 in plant adaptation to Pi deficiency.