The Cytokinin Receptors in Rice Have Specific Roles in Vegetative and Reproductive Development
E. SHOUSE, T. HEALY, E. JENNINGS-TALLERDAY, J. KIEBER
Department of Biology, University of North Carolina, 120 South Rd, Chapel Hill, NC 27599

The diverse functions of the phytohormone cytokinin and its signaling pathway have been well studied in the model organism Arabidopsis thaliana. Although several studies have observed direct connections between cytokinin levels and yield, less is known about cytokinin signaling in the model monocot Oryza sativa, or rice. To elucidate the function of cytokinin in rice, we utilized CRISPR to generate a suite of cytokinin signaling mutants. In rice, four Histidine Kinases (HKs) serve as cytokinin receptors: HK3, HK4, HK5, and HK6. We phenotyped mutant rice lines containing all different combinations of null HK alleles to elucidate the specific functions of each receptor. With respect to vegetative development, HK3 and HK5 are most important for shoot elongation, whereas HK4 and HK6 are most important for cytokinin perception in the roots. Furthermore, HK6 alone displayed defects in trichome development. The redundancy of these pairs of HKs was also true for panicle development, as hk3 and hk5 mutants showed defects in secondary branch formation, whereas hk4 and hk6 mutants showed the strongest reduction in spikelet filling. Overall, the HK receptors show a combination of specificity and redundancy to control development at all stages of growth.