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Engineering citrus canker and Huanglongbing resistance by overexpressing of glutamate decarboxylase in citrus

Shujian Zhang: New Mexico Consortium

<div>Gamma-Aminobutyric acid (GABA) is a ubiquitous four-carbon, nonprotein amino acid synthesized by glutamate decarboxylase (GAD) in bacteria, animals and plants. Plant-derived GABA has been shown to modulate quorum sensing of <em>Agrobacterium tumefaciens</em> C58 in tobacco (Chevrot <em>et al</em>., 2006), and to repress the expression of the type III secretion system of <em>Pseudomonas syringae</em> pv. <em>tomato</em> DC3000 in <em>Arabidopsis</em> (Ward <em>et al.,</em> 2010). Overexpression of GAD in tobacco plants with elevated GABA levels was sufficient to impart enhanced resistance to an insect, a nematode and a bacterial pathogen (McLean <em>et al.,</em> 2003; MacGregor <em>et al.,</em> 2003;Chevrot <em>et al.,</em> 2006). Citrus canker and Huanglongbing are two destructive bacterial diseases in citrus production worldwide. They are caused by <em>Xanthomonas citri</em> subsp. <em>citri</em> (Xcc) and <em>Candidatus</em> Libe­­ribacter spp., respectively. Here, a gene expression survey of citrus GAD orthologs was conducted in ‘Valencia’ sweet orange, ‘Cleopatra’ mandarin, ‘Carrizo’ citrange, rough lemon and citron. The expression levels of two citrus GAD genes were significantly higher in all Liberibacter-infected trees compared with uninfected trees regardless genotype, indicating the involvement of GABA in citrus-Liberibacter interactions. GABA at various concentrations didn’t directly inhibit Xcc growth in medium. However, GABA eliminated Xcc-mediated hypersensitive response (HR) in the non-host <em>Nicotiana benthamiana </em>and promoted bacterial growth, suggesting a suppression effect of GABA on Xcc type III secretion. Full-length cDNA of two GAD homologs from ‘Valencia’ sweet orange was cloned and genetically transformed into Carrizo citrange. GAD-overexpressing plants have been identified and are being characterized for increased canker and HLB resistance.</div>