Crosstalk between abiotic ultraviolet-B stress and biotic (flg22) stress signalling in Arabidopsis prevents flavonol accumulation in favor of pathogen defence compound production

Plants respond to both abiotic and biotic stresses with alterations in the expression of genes required to produce protective metabolites. Sometimes plants can be challenged with different stresses simultaneously and as they cannot evade from this situation, priorities have to be set to deal with the most urgent threat. The abiotic stress ultraviolet-B (UV-B) light induces the production of UV-protective flavonols in Arabidopsis Col-0 cell suspension cultures and this accumulation is attenuated by concurrent application of the bacterial elicitor flg22 (simulating biotic stress). This inhibition correlates with strong suppression of the flavonol biosynthesis genes. In parallel, flg22 induces the production of defence-related compounds, such as the phytoalexins, camalexin and scopoletin, as well as lignin, a structural barrier thought to restrict pathogen spread. This correlated positively with flg22-mediated expression of enzymes for lignin, scopoletin and camalexin production. As flavonols, lignin and scopoletin are all derived from phenylalanine, it appears that the plant focuses the metabolism on production of scopoletin and lignin at the expense of flavonol production. Furthermore, it appears that this crosstalk involves antagonistic regulation of two opposing MYB transcription factors, the positive regulator of the flavonol pathway MYB12 (UV-B-induced and flg22-suppressed) and the negative regulator MYB4 (UV-B- and flg22-induced).