The KH-domain genes FLK and HOS5 integrate flowering and stress responses in Arabidopsis thaliana.

Plant reproductive success largely relies on flowering under favorable conditions. However, stress factors have forced plants to acquire adaptive strategies to coordinate floral timing and stress responses through key genetic regulators. RNA-binding proteins with K-homology (KH) domains are emerging as versatile regulators of an increasing number of plant developmental processes, including flowering and stress acclimation. In Arabidopsis thaliana, the KH-domain genes FLOWERING LOCUS K (FLK) and HIGH OSMOTIC STRESS GENE EXPRESSION 5 (HOS5) are associated with transcription and co-transcriptional operations. FLK facilitates floral transition by repressing FLOWERING LOCUS C (FLC), the central flowering inhibitor, while both KH-domain genes have been shown to be involved in abiotic stress and pathogen defense. Our genetic and molecular data identify HOS5 as a novel flowering regulator that acts in concert with FLK to repress FLC. Our transcriptomic results reveal that, in addition, FLK and HOS5 cooperatively repress numerous stress-responsive genes. Consistent with this, flk hos5 double mutant plants exhibit elevated levels of stress-induced gene activities and enhanced resistance to abiotic stress and pathogenic fungi. The coordinated repression of FLC and stress-induced genes, together with the interaction at the molecular level, suggests that FLK and HOS5 participate in a co-transcriptional regulatory hub key for orchestrating flowering time and environmental adaptation responses. This study aims to better define the role of KH-domain genes and provides candidates for their exploitation in crop biotechnology.