Physiological and transcriptional responses of Alternaria alternata induced abnormal leaf senescence in Pyrus pyrifolia

Abstract In the Yangtze Valley of China, Pyrus pyrifolia is known by the name sand pear and often suffers abnormal, early leaf senescence and premature defoliation, which causes serious yield loss and poor fruit quality. In this study, the relationship of Alternaria alternata with abnormally senesced leaves (ASLs) was analyzed by physiological and molecular method. Using data from the P. pyrifolia cultivars ‘Huanghua’ and ‘Suisho’, we found that A. alternata inoculation caused early leaf senescence. Restricted foliar nutrient redistributions were observed in ASLs, such as nitrogen (N) and phosphorus (P). Higher abscisic acid (ABA) levels were induced in the tissues of the leaf mesophyll, veins and stalks of ASLs, and higher concentrations of indole-3-acetic acid (IAA) and H2O2 were induced in the tissues of leaf mesophylls and veins of ASLs, while an opposite trend was observed in the tissues of leaf veins. Quantitative real-time PCR (qRT-PCR) consistently demonstrated that ASLs upregulated the expression of ABA biosynthesis genes (e.g., PyNCED3, PyZEP and PyAAO3), and the catabolism gene (PyABA8′OH), whereas PyBG and PyDXS showed opposite trends. Based on the leaf senescence database (LSD) 3.0, 42 senescence-associated genes (SAGs) that induced differential expression by A. alternata inoculations were enriched for P. pyrifolia cv. ‘Hongfeng’, which has high susceptibility to A. alternata and readily exhibits early leaf senescence. These results showed that A. alternata infection is one of major factors for abnormal leaf senescence and ABA signaling involved in this pathogenic process.