MYB308-mediated transcriptional activation of plasma membrane H -ATPase 6 promotes iron uptake in citrus.

Iron-deficiency chlorosis is a common nutritional disorder in crops grown on alkaline or calcareous soils. Although the acclimation mechanism to iron deficiency has been investigated, the genetic regulation of iron acquisition is still unclear. Here, by comparing the iron uptake process between the iron-poor-soil-tolerant citrus species Zhique (ZQ) and the iron-poor-soil-sensitive citrus species trifoliate orange (TO), we discovered that enhanced root H efflux is crucial for the tolerance to iron deficiency in ZQ. The H efflux is mainly regulated by a plasma membrane-localized H-ATPase, HA6, the expression of which is upregulated in plants grown in soil with low iron content, and significantly higher in the roots of ZQ than TO. Overexpression of the gene in the mutant, defective in iron uptake, recovered the wild-type phenotype. In parallel, overexpression of the gene in TO significantly increased iron content of plants. Moreover, an iron deficiency-induced transcription factor, MYB308, was revealed to bind the promoter and activate the expression of in ZQ in yeast one-hybrid, electrophoretic mobility shift, and dual-luciferase assays. Overexpression of in ZQ roots significantly increased the expression level of the gene. However, MYB308 cannot bind or activate the promoter in TO due to the sequence variation of the corresponding MYB308 binding motif. Taking these results together, we propose that the MYB308 could activate to promote root H efflux and iron uptake, and that the distinctive MYB308- transcriptional module may be, at least in part, responsible for the iron deficiency tolerance in citrus.