Endoplasmic reticulum calnexins participate in the primary root growth response to phosphate deficiency

Abstract

AbstractAccumulation of incompletely folded proteins in the endoplasmic reticulum (ER) leads to ER stress, activates ER protein degradation pathways, and upregulates genes involved in protein folding (Unfolded Protein Response; UPR). ER stress has been associated with abiotic stress conditions that affect protein folding, including salt stress. However, the role of ER protein folding in plant responses to nutrient deficiencies is unclear. We analyzed severalArabidopsis thalianamutants affected in ER protein quality control and established that bothCALNEXIN(CNX) genes function in the primary root’s response to phosphate (Pi) deficiency. CNX and calreticulin (CRT) are homologous ER lectins that bind to N-glycosylated proteins to promote their folding. Growth ofcnx1-1andcnx2-2single mutants was similar to that of the wild type under high and low Pi conditions, but thecnx1-1 cnx2-2double mutant showed decreased primary root growth under low Pi conditions due to reduced meristematic cell division. This phenotype was specific to Pi deficiency; the double mutant responded normally to osmotic and salt stress. The root growth phenotype was Fe dependent and was associated with Fe accumulation in the root. Two genes involved in Fe-dependent inhibition of root growth under Pi deficiency, the ferroxidase geneLPR1and P5-type ATPasePDR2, are epistatic toCNX1/CNX2. OverexpressingPDR2failed to complement thecnx1-1 cnx2-2root phenotype.cnx1-1 cnx2-2showed no evidence of UPR activation, indicating a limited effect on ER protein folding. CNX might process a set of N-glycosylated proteins specifically involved in the response to Pi deficiency.One sentence summaryCalnexin, a lectin chaperone engaged in the folding of N-glycosylated proteins in the ER, participates in primary root adaptation to low phosphate conditions.