Dual function of HYPONASTIC LEAVES 1 during early skotomorphogenic growth in Arabidopsis
Authors: Sacnun Juan Manuel , Crespo Roberta , Palatnik Javier , Rasia Rodolfo , González-Schain Nahuel 
Published: August 2019

MicroRNAs are small RNA molecules with big impact in many eukaryotic biological processes. In plants, their role as regulators of important developmental programs such as leaf size and shape, flower organs or phase transitions, among others, have been evidenced by mutants in specific miRNAs and by mutants in components of their biogenesis. However, we are still far from understanding the scope of this regulatory system so other crucial developmental phases might be influenced by the microRNA pathway.

Skotomorphogenesis is an essential developmental program that takes place after seeds germinate underground in order to display a proper response when seedlings reach the light. In this work, we found that the core components of microRNA pathway, DCL1, HYL1 and SERRATE, promote hypocotyl elongation during skotomorphogenesis. Hook unfolding, another characteristic phenotype displayed by dark-grown seedlings is also regulated by these proteins but, surprisingly, they act in different ways. Thus, HYL1 represses hook unfolding while DCL1 and SE promote it since the hooks of mutants on each component are more or less open than those of wild-type during skotomorphogenesis, respectively. Genetic and physiological analyses on HYL1 mutants provide evidence that repression of hook unfolding is carried out through the HYL1 protein-protein interaction domain. Furthermore, the data indicates that phosphorylated HYL1 is necessary for this function. Molecular and genetic analyses also suggest that HYL1 regulates the activity of the master photomorphogenic regulator HY5 in darkness to ensure a proper early skotomorphogenic growth. In summary, while our data show a role for miRNAs in darkness, it also suggests a microprocessor-independent role of HYL1 as a repressor of hook unfolding assigning a biological function to phosphorylated HYL1. This work uncovers a previously unnoticed link between components of the miRNA biogenesis machinery, the skotomorphogenic growth and hook development in Arabidopsis.

Author summary

Seeds germinating underground display a specific developmental program, termed skotomorphogenesis, to ensure survival of the emerging seedlings until they reach the light. They rapidly elongate the hypocotyl and maintain the cotyledons closed, forming a hook with the hypocotyl in order to protect apical meristematic cells from mechanical damage. Such crucial events for the fate of the seedling are tightly regulated and although some transcriptional regulators and phytohormones are known to be implicated in this regulation, we are still far from a complete understanding of these biological processes. Our work provides new information on the diverse roles in skotomorphogenesis of the core components of microRNA biogenesis in Arabidopsis, HYL1, SE, and DCL1. We show that hypocotyl elongation is promoted by all these components, probably through the action of specific miRNAs. Hook development is also controlled by these components although, remarkably, HYL1 exerts its role in an opposite way to DCL1 and SE. Interestingly, we found that a specific HYL1 domain involved protein-protein interaction is required for this function, instead of other regions of the protein with known roles in the biogenesis of miRNAs. We propose that phosphorylated HYL1 help to maintain the hook closed during early skotomorphogenesis by repressing the activity of HY5, the transcriptional master regulator that triggers light responses.