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Multiple phytohormonal signaling mediates citrus response to the bacterial pathogen Candidatus Liberibacter asiaticus

Yasser Nehela: Citrus research and education center, IFAS, University of Florida

<div>Huanglongbing (HLB) is a destructive disease in citrus worldwide. HLB is caused by <em>Candidatus</em> Liberibacter asiaticus (<em>C</em>Las) and transmitted by <span>Asian citrus psyllid, <em>Diaphorina citri</em>. Upon </span>pathogen infection, citrus plants activate their defense responses which are mainly regulated by phytohormones. Herein, we investigated the effect of <em>C</em>Las-infection on the phytohormonal profile of Valencia sweet orange (<em>Citrus sinensis</em>) leaves using GC-MS running in the selective ion monitoring mode (SIM-mode). Overall, we were able to quantify 13 different phytohormones belonging to six groups. Interestingly, <em>C</em>Las-infection alters the levels of all auxins, salicylates, abscisic acid, and their precursors (tryptophan, phenylalanine, and zeaxanthin, respectively), but did not affect the <em>trans</em>-jasmonic acid, gibberellins and cytokinins groups. In addition, the transcript levels of 52 phytohormones’ biosynthetic genes were investigated. The expression levels of genes involved in the biosynthesis of auxins, salicylates, and abscisic acid were upregulated after the <em>C</em>Las-infection, while the expression levels of <em>trans</em>-jasmonic acid-, gibberellins-, and cytokinins-biosynthetic genes remained the same. Both the GC-MS-SIM and gene expression results suggest that, in addition to salicylic acid (SA), both auxins and abscisic acid are implicated in activation of SA-mediated pathway against <em>C</em>Las infection. The crosstalk between these groups is the subject of ongoing research.</div>