Exploring plant hormonal signalling through chemical perturbation

Abstract

Plant growth and development is tightly regulated by a set of plant hormones that includes abscisic acid (ABA) and gibberellic acid (GA). Understanding how this is achieved is challenging due to a complex interplay between the various signalling pathways involved. A chemical genetics approach was used in this study to explore this hormonal crosstalk with a focus on plant growth. A small library of analogues, modified from the calmodulin inhibitor W7, was generated and the effect of these compounds was tested in root growth assays. One particular compound, eW5, was identified as providing enhanced and prolonged root growth even when plants were subsequently removed from the compound. Further analysis suggested that the phenotype induced upon eW5 application was due to modification of DNA methylation. Therefore it was hypothesized that eW5 might affect gene expression, which was tested using a RNA-seq experiment. Results from this suggested that eW5 regulates hormone signalling pathways, with a particular positive correlation to the GA signalling pathway observed.
Interestingly, eW5 binds to the ABA receptor PYR1, thus potentially functioning as an ABA antagonist and also promotes DELLA (a negative regulator of plant growth) degradation, in a similar fashion to that observed with GA. Further work was performed to investigate the effect of eW5 on ABA and GA independent pathways. In ABA signalling, eW5 showed inhibition of some ABA responses such as stomatal opening, however no recovery in PP2C phosphatase activity suggests that it does not promote growth by inhibiting ABA perception. In addition, due to the specific GA-mediated response in hypocotyl growth, the eW5 effect was further explored in this particular process. With regard to eW5’s potential role in GA signalling, it was found to enhance sensitivity to GA that leads to DELLA protein degradation and growth promotion. Moreover, it was suggested that the promotion of eW5 in stomatal opening occurs through GA signalling.
Having established the positive effect of eW5 on root and hypocotyl growth and stomatal opening, a further small library of analogues of eW5 was generated to further explore its mode-of-action. The position of sulfonamide was identified as a potential site that is responsible for hypocotyl growth promotion.