Salicylic acid accumulation is required for Fe-deficiency responses in Arabidopsis

Several phytohormones and other small molecules have been demonstrated to be involved in iron (Fe) homeostasis. However, how salicylic acid (SA), an essential hormone in plant immunity and defense responses, participates in Fe-deficiency responses in plants is largely unknown. Here, we took advantage of a SA biosynthesis defect mutant phytoalexin deficient 4 (pad4: T-DNA Salk_089936) to explore the possible effects of endogenous SA on the morphological and physiological responses to Fe deprivation. Under a Fe-deficiency treatment, Col-0 showed more severe leaf chlorosis and root growth inhibition compared with the pad4 mutant. The soluble Fe concentrations were significant higher in pad4 than Col-0 under the Fe-deficiency treatment, suggesting that a mutation in the PAD4 gene may alleviate the Fe-deficiency-induced symptoms by regulating the soluble Fe concentrations. Furthermore, a SA signaling maker line (PR1promoter: GUS) was used to investigate how Fe deficiency affects endogenous SA biosynthesis and metabolism. The data showed that Fe deficiency significantly induced SA accumulation in Col-0, and the loss function of PAD4 blocked this process. The requirement of endogenous SA accumulation for Fe-deficiency responses was confirmed using a series of SA biosynthetic mutants and transgenic lines. Overall design: To elucidate the molecular and metabolic mechanisms that affect plant SA responses to Fe deficiency, we performed a comparative RNA sequencing analysis of the whole seedling transcriptomes between Col-0 and the pad4 mutant under +Fe and –Fe treatments. The expression levels of many hormone response genes were altered in pad4 compared with Col-0. Hence, Fe acquisition may be under the control of a complex interplay among hormone signaling pathways.