Molecular mechanism through which phytochrome b controls leaf blade cell division in arabidopsis thaliana: light control of leaf cell division
Plants are highly malleable organisms that utilise developmental plasticity as a strategy to respond to external challenges which threaten their survival. Adaptive growth is regulated by external cues, including light, which is not only an energy source, but also provides vital information about the habitat. Low ratios of red (R) to far red (FR) radiation (R:FR ratio), detected by phytochromes, indicates the presence of nearby vegetation or crowding, and promote a growth strategy named the shade avoidance syndrome (SAS). Although the SAS is generally beneficial to plant survival in vegetation-rich conditions, it can be an undesirable trait in agriculture as it can compromise yield. Indeed, a prominent feature of SAS is the marked reduction in leaf blade size, which diminishes surface area for light capture and photosynthesis. Although SAS is fairly well studied, there are still large gaps in our understanding of the cellular response and the regulatory molecular mechanisms that underlie the small SAS leaf phenotype. Here, I show that exposure to end-of-day (EOD) FR-rich vegetative shade early during leaf development constrains leaf blade expansion by repressing cell division. RNAseq time course data identified a number of transcription factors that are strongly suppressed by FR during early leaf development. Further, systematic bioinformatics analysis of time-course RNAseq data revealed AN3, a transcriptional co-activator, operates downstream of phyB to modulate leaf blade cell division. I demonstrate that when phyB is active, cell division is promoted by AN3, but following phyB deactivation by EODFR, the basic helix-loop-helix (bHLH) transcription factor (PHYTOCHROME-INTERACTING FACTOR 7), PIF7 physically interacts with AN3 and halts AN3 regulation of cell division genes. I established that EODFR favours an antagonistic substitution-suppression transcriptional module in which PIF7 can evict and substitute AN3 by direct binding to same promoter cis-elements of target genes, which in turn promotes a dynamic molecular switch from the promotion to the repression of gene expression. Furthermore, PIF7 activated by EODFR negatively regulates an3-4 mediated cell compensation phenomenon, in which cell proliferation defects lead to compensated cell size enlargement. This study brings a molecular level, mechanistic understanding of how phyB action is coupled to leaf cell division. This mechanism may be of great significance in the natural environment, because AN3 is a key developmental gene that is conserved throughout the plant kingdom. Furthermore, phylogenetic analysis shows that PIF7 occupies a dominant position in the entire legume family, having been identified as a master regulator of shade avoidance response in nature. Therefore, this work has great potential to apply these molecular manipulations to crop species, increasing productivity by improving shade tolerance without compromising crop robustness and yield.