The EU-funded MOLECULAR CLOCK (Sumoylation: A regulatory mechanism for circadian clock function) project has tackled the subject of how a plant perceives environmental changes and responds to those changes by adapting its development. The researchers looked at a blue light photoreceptor CRYPTOCHROME 2 (CRY2) in Arabidopsis that is stable in the dark but then photodegrades in blue light. Degradation of CRY2 is preceded by post-translational modification. First, blue light exposure triggers phosphorylation of the protein, which is followed by ubiquitination of the photoreceptor that targets the protein for degradation by the proteasome. MOLECULAR CLOCK scientists investigated a newly discovered post-translational modification of CRY2, sumoylation. Sumoylation involves the covalent attachment of small ubiquitin-like modifier (SUMO) to the target protein. The researchers confirmed its sumoyation of CRY2 and identified one of the major target residues. Looking at the biological significance of the process, results showed that sumoylation did not affect binding of its cofactor flavin adenine dinucleotide (FAD), which enables CRY2 to perceive blue light. Moreover, sumoylation did not change the subcellular localization of CRY2. Importantly, stability tests revealed that addition of SUMO is involved in the fine-tuning of CRY2 degradation during light perception. Biological impact of sumoylation includes induction of slight differences in the hypocotyl length during seedling development. Flowering time was delayed in cry2 mutants and complementation of this phenotype was observed by expressing wild type and mutant forms of CRY2. However, there were significant differences between experiments, which suggest that there are many more variables in action other than light signals. Some complemented lines with mutant versions of CRY2 showed leaf arrangement irregularities and their flower buds presented defects at anther formation. Research results of MOLECULAR CLOCK are very relevant in today's world of food shortages as a result of increasing population. Light input signal has a profound effect on plant growth and development, and molecular details of key regulators will help to design plants more adaptable to climate changes. Moreover, CRY2 is currently the basis of optogenetic tools used to control gene expression through light exposure. Refinement of these tools could bring about further improvements in crop yield.
Plants, molecular clock, crops, circadian clock, sumoylation, CRY2, ubiquitin