Regulation of Flowering Time by the RNA-Binding Proteins AtGRP7 and AtGRP8

The timing of floral initiation is a tightly controlled process in plants. The circadian clock regulated glycine-rich RNA-binding protein AtGRP7, a known regulator of splicing, was previously shown to regulate flowering time mainly by affecting the MADS-box repressor FLC. Loss of AtGRP7 leads to elevated FLC expression and late flowering in the atgrp7-1 mutant. Here, we analyze genetic interactions of AtGRP7 with key regulators of the autonomous and the thermosensory pathway of floral induction. RNAi mediated reduction of the level of the paralogous AtGRP8 in atgrp7-1 further delays floral transition compared to atgrp7-1. AtGRP7 acts in parallel to FCA, FPA and FLK in the branch of the autonomous pathway comprised of RNA-binding proteins. It acts in the same branch as FLD, and AtGRP7 loss-of-function mutants show elevated levels of di-methylated lysine 4 of histone H3, a mark for active transcription. In addition to its role in the autonomous pathway, AtGRP7 acts in the thermosensory pathway of flowering time control by regulating alternative splicing of the floral repressor FLM. Overexpression of AtGRP7 selectively favors formation of the repressive isoform FLM-beta. Our results suggest that the RNA-binding proteins AtGRP7 and AtGRP8 influence MADS-Box transcription factors in at least two different pathways of flowering time control. This highlights the importance of RNA-binding proteins to fine-tune the integration of varying cues into flowering time control and further strengthens the view that the different pathways, although genetically separable, constitute a tightly interwoven network to ensure plant reproductive success under changing environmental conditions. © The Author(s) 2019. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.