Abstract
Gummosis, one of the most detrimental diseases to the peach industry worldwide, can be induced by Lasiodiplodia theobromae. Ethylene (ET) is known to trigger the production of gum exudates, but the mechanism underlying fungus-induced gummosis remains unclear. In this study, L. theobromae infection triggered the accumulation of ET and jasmonic acid (JA) but not salicylic acid (SA) in a susceptible peach variety. Gaseous ET and its biosynthetic precursor increased gum formation, whereas ET inhibitors repressed it. SA and methyl-jasmonate treatments did not influence gum formation. RNA-seq analysis indicated that L. theobromae infection and ET treatment induced a shared subset of 1808 differentially expressed genes, which were enriched in the category "starch and sucrose, UDP-sugars metabolism". Metabolic and transcriptional profiling identified a pronounced role of ET in promoting the transformation of primary sugars (sucrose, fructose, and glucose) into UDP-sugars, which are substrates of gum polysaccharide biosynthesis. Furthermore, ethylene insensitive3-like1 (EIL1), a key transcription factor in the ET pathway, could directly target the promoters of the UDP-sugar biosynthetic genes UXS1a, UXE, RGP and MPI and activate their transcription, as revealed by firefly luciferase and yeast one-hybrid assays. On the other hand, the supply of SA and inhibitors of ET and JA decreased the lesion size. ET treatment reduced JA levels and the transcription of the JA biosynthetic gene OPR but increased the SA content and the expression of its biosynthetic gene PAL. Overall, we suggest that endogenous and exogenous ET aggravate gummosis disease by transactivating UDP-sugar metabolic genes through EIL1 and modulating JA and SA biosynthesis in L. theobromae-infected peach shoots. Our findings shed light on the molecular mechanism by which ET regulates plant defense responses in peach during L. theobromae infection.
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