Quantitative analysis of changes in the phosphoproteome of maize induced by the plant hormone salicylic acid

Phytohormone salicylic acid (SA) plays an important role in regulating various physiological and biochemical processes. Our previous study identified several protein kinases responsive to SA, suggesting that phosphorylation events play an important role in the plant response to SA. In this study, we characterized the phosphoproteome of maize in response to SA using isotope tags for relative and absolute quantification (iTRAQ) technology and TiO2 enrichment method. Based on LC-MS/MS analysis, we found a total of 858 phosphoproteins among 1495 phosphopeptides. Among them, 291 phosphopeptides corresponding to 244 phosphoproteins were found to be significantly changed after SA treatment. The phosphoproteins identified are involved in a wide range of biological processes, which indicate that the response to SA encompasses a reformatting of major cellular processes. Furthermore, some of the phosphoproteins which were not previously known to be involved with SA were found to have significantly changed phosphorylation levels. Many of these changes are phosphorylation decreases, indicating that other currently unknown SA signaling pathways that result in decreased phosphorylation of downstream targets must be involved. Our study represents the first attempt at global phosphoproteome profiling in response to SA, and provides a better understanding of the molecular mechanisms regulated by SA.