FHY3 promotes shoot branching and stress tolerance in Arabidopsis in an AXR1-dependent manner

The transposase-related transcription factor FAR-RED ELONGATED HYPOCOTYL3 (FHY3) promotes seedling de-etiolation in far-red light, which is perceived by phytochrome A (phyA). In this role, FHY3 indirectly mediates the nuclear import of light-activated phyA, which triggers downstream transcriptional responses. Here, we present genetic evidence for additional roles of FHY3 in plant development and growth. New fhy3 alleles were isolated as suppressors of max2-1 (more axillary branching2-1), a strigolactone-insensitive mutant characterised by highly branched shoots. Branching suppression by fhy3, in both wild-type and max2-1 backgrounds, resulted from inhibition of axillary bud outgrowth. Additional roles in axillary meristem initiation were revealed in the revoluta (rev) fhy3 double mutant, with fhy3 enhancing rev mutant defects in axillary shoot meristem formation, as well as in floral meristem maintenance. fhy3 also affected embryonic and floral patterning with low penetrance, and displayed oxidative stress-related phenotypes of retarded leaf growth and of cell death. The fhy3 phenotypes of axillary bud outgrowth suppression and of stress-induced leaf growth retardation both required the AUXIN-RESISTANT1 gene, and are independent of phyA. Consistent with the recent discovery that FHY3 regulates many Arabidopsis promoters, our results suggest much wider roles for FHY3 in growth and development, either in concert with, or beyond, light signalling.