Review: ABA, flavonols, and the evolvability of land plants.

There is evidence that the ABA signaling pathway has greatly contributed to increase the complexity of land plants, thereby sustaining their ability to adapt in an ever-changing environment. The regulatory functions of the ABA signaling pathway go well beyond the movements of stomata and the dormancy of seeds. For instance, the ABA signaling regulates the flavonoid biosynthesis, consistent with the high integration of ABA and light signaling pathways, which occurs at the level of key signaling components, such as the bZIP transcription factors HY5 and ABI5. Here we focus on the regulation of 'colorless' (UV-absorbing) flavonol biosynthesis by the ABA signaling and, about how flavonols may regulate, in turn, the ABA signaling network. We discuss very recent findings that quercetin regulates the ABA signaling pathway, and hypothesize this might occur at the level of second messenger and perhaps of primary signaling components as well. We critically review old and recent suggestions of the primary roles played by flavonols, the ancient class of flavonoids already present in bryophytes, in the evolution of terrestrial plants. Our reasoning strongly supports the view that the ABA-flavonol relationship may represent a robust trait of land plants, and might have contributed to their adaptation on land.