Function of AP2/ERF transcription factors in phytochrome signalling

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In order to adapt growth and development to the environment, plants constantly monitor the light conditions in their surrounding using dedicated photoreceptors. Phytochromes are photoreceptors sensitive to the red and far-red range of the light spectrum. Upon sensing light, phytochromes accumulate in the nucleus and regulate the expression of hundreds of genes that control seed germination, seedling establishment, induction of flowering, and many other aspects of growth and development. Seed germination and seed dormancy are important ecological and agronomic traits controlled by light and the phythormones abscisic acid (ABA) and gibberellin (GA). ERF55 and ERF58, two closely related AP2/ERF transcription factors that we identified as novel phytochrome interacting proteins, are expressed in seeds and we found that light-dependent seed germination is enhanced in the erf55 erf58 mutant. Light-activated phytochromes bind ERF55/ERF58 and displace them from promoters of germination-repressing genes such as PIF1, SOM, ABI5, and genes coding for ABA anabolic or GA catabolic enzymes, thereby promoting germination. Based on preliminary data, we hypothesise that ERF55 and ERF58 also play a role in setting the depth of dormancy in response to the light environment perceived by the developing seeds. In the first work package of the proposed project, we aim to test this hypothesis and use DAP-seq to identify target genes of ERF55 and ERF58 with a function in regulation of dormancy. ERF55 and ERF58 belong to subgroup A6 of the AP2/ERF transcription factor family. All members of this subgroup interact with phytochromes but – except for ERF55 and ERF58 – are expressed in seedlings or adult plants and not in seeds. Therefore, in the second work package, we aim to identify the functions of these AP2/ERFs in light signalling in seedlings and adult plants, using CRISPR/Cas9 generated erf higher order mutants and high-throughput screening for potential target genes by DAP-seq. Phytochromes bind to the highly conserved DNA-binding domain of AP2/ERFs, suggesting that binding to – and possibly regulation by – phytochromes may be widespread among AP2/ERFs. Several randomly picked AP2/ERFs from different subgroups indeed interacted with phytochromes. In the third work package, we therefore want to screen the entire AP2/ERF family for members interacting with phytochromes. In summary, we have identified members of the AP2/ERF transcription family as novel interactors of phytochromes. In the proposed project, we want to investigate their function in phytochrome-mediated light responses and elucidate the underlying molecular mechanisms.