Abstract
The auxin early response gene Gretchen Hagen3 (GH3) plays dual roles in plant development and responses to biotic or abiotic stress. It functions in regulating hormone homeostasis through the conjugation of free auxin to amino acids. In citrus, GH3.1 and GH3.1L play important roles in responding to Xanthomonas citri subsp. citri (Xcc). Here, in Wanjingcheng orange (Citrus sinensis Osbeck), the overexpression of CsGH3.1 and CsGH3.1L caused increased branching and drooping dwarfism, as well as smaller, thinner and upward curling leaves compared with wild-type. Hormone determinations showed that overexpressing CsGH3.1 and CsGH3.1L decreased the free auxin contents and accelerated the Xcc-induced decline of free auxin levels in transgenic plants. A resistance analysis showed that transgenic plants had reduced susceptibility to citrus canker, and a transcriptomic analysis revealed that hormone signal transduction-related pathways were significantly affected by the overexpression of CsGH3.1 and CsGH3.1L. A MapMan analysis further showed that overexpressing either of these two genes significantly downregulated the expression levels of the annotated auxin/indole-3-acetic acid family genes and significantly upregulated biotic stress-related functions and pathways. Salicylic acid, jasmonic acid, abscisic acid, ethylene and zeatin levels in transgenic plants displayed obvious changes compared with wild-type. In particular, the salicylic acid and ethylene levels involved in plant resistance responses markedly increased in transgenic plants. Thus, the overexpression of CsGH3.1 and CsGH3.1L reduces plant susceptibility to citrus canker by repressing auxin signaling and enhancing defense responses. Our study demonstrates auxin homeostasis’ potential in engineering disease resistance in citrus.
Highlights
Citrus canker, caused by Xanthomonas citri subsp. citri (Xcc), is an important disease of citrus
Our work first showed that the overexpression of CsGH3.1 and CsGH3.1L decreased free Indole-3-acetic acid (IAA) levels
Naphthalene acetic acid treatments significantly induced the expression of CsGH3.1 in citrus leaves [26]
Summary
Citrus canker, caused by Xanthomonas citri subsp. citri (Xcc), is an important disease of citrus. Citrus canker, caused by Xanthomonas citri subsp. Citri (Xcc), is an important disease of citrus. Xcc affects various citrus species and most of the economically important cultivars, including orange, grapefruit, lime, lemon, pomelo and citrus rootstock [1]. The canker’s development includes the initial appearance of oily looking spots, usually on the abaxial leaf surface, outbursts of white or yellow spongy pustules and the formation of brown corky cankers [2]. Pustule formation (excessive cell division) in the infected tissues plays a vital role in citrus canker development and pathogen spread [1, 3,4,5]. Understanding the molecular mechanisms involved in responding to pathogen-induced pustule formation in citrus could stimulate renewed efforts to develop more effective and economical control methods of citrus canker management
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