Crosstalk among stress responses in plants: pathogen defense overrides UV protection through an inversely regulated ACE/ACE type of light-responsive gene promoter unit.

Plants often have to cope with two or more environmental hazards simultaneously. Such coincidences require instantaneous decisions on relative severity and consequential crosstalk between the respective signaling cascades. Among the frequently encountered threats are pathogen infections and UV irradiation, both of which trigger specifically targeted defense responses by means of changes in gene transcription rates. In Petroselinum crispum, pathogen defense has been shown to be associated with extensive metabolic reprogramming, including strong repression of the UV-protective flavonoid biosynthetic pathway. Here we show that one of the involved genes, encoding acyl-CoA oxidase, responds positively to UV light and negatively to a pathogen-derived elicitor through an inversely regulated promoter unit consisting of two almost identical ACGT-containing elements (ACEs). This unit, when either introduced into an unrelated promoter or generated by mutation of a differently composed unit, confers the same type of response pattern on the recipient genes, confirming its general functionality at a convergence site of two largely distinct signaling pathways. Similarly large, rapid, and partly inverse effects of UV light and elicitor were observed for several mRNAs encoding common plant regulatory factors (CPRFs) that exhibit distinct dimerization and DNA-binding properties. This striking coincidence suggests a major role of common plant regulatory factors in mediating the apparent switch in the function of ACGT-containing elements from positive UV light to negative elicitor or pathogen responsiveness.