MicroProteins miP1b/BBX30 and miP1a/BBX31 form a positive feedback loop with ABI5 to retard seedling establishment

In plants the switch to autotrophic growth involves germination followed by post-germination seedling establishment. When environmental conditions are not favorable, the stress hormone Abscisic Acid (ABA) signals plants to postpone seedling establishment by inducing the expression of the transcription factor ABI5. The levels of ABI5 determine the efficiency of the ABA mediated post-germination developmental growth arrest. The molecular mechanisms regulating the stability and activity of ABI5 during the transition to light are less known. We found that two microProteins miP1a/ BBX31 and miP1b/BBX30 alongwith ABI5 inhibit post-germination seedling establishment in a partially interdependent manner. MicroProteins are single-domain proteins that interact with multi-domain proteins to modulate their post-translational activity. miP1a/ BBX31 and miP1b/BBX30 physically interact with ABI5 to stabilize it and promote its binding to promoters of downstream genes in light. ABI5 reciprocally induces the expression of BBX30 and BBX31 by directly binding to their promoter. ABI5 and the two microProteins thereby form a positive feedback loop to promote ABA-mediated developmental arrest of seedlings. Our study highlights the functional versatility of microProteins which can act as regulators of crucial developmental transitions in plants as well as other eukaryotes.