Figure 13.

Hypothetical model of the cross talk between melatonin and different phytohormones of Valencia sweet orange leaves. In this model, we propose that infection with Ca. L. asiaticus bacterium and infestation with D. citri vector, as well as exogenous melatonin supplementation, might induce the biosynthesis of phytomelatonin and several phytohormone groups, including SAs, tJA, ABA, and auxins. Briefly, exogenous melatonin plays a dual role in the activation of both the SA-mediated pathway against pathogen infection and the JA-mediated pathway against herbivory, which are the most common defensive pathways in vascular plants. Besides these well-known pathways, our findings show that melatonin is involved in the activation of both ABA and auxin pathways. ABA and auxins play a key role in citrus defense response against HLB. The activation of four phytohormone pathways leads to complex cross talk among them, which interact synergistically or antagonistically together, and link back to the SA-mediated pathway and the JA-mediated pathway. Solid lines with arrows indicate well-established/confirmed pathways, dashed lines with whiskers signify negative reactions, and dotted lines represent hypothetical mechanisms or uncharacterized elements. DAHP, 3-Deoxy-D-arabino-heptulosonic acid 7-phosphate; TCA, tricarboxylic acid.