Cytokinin Perception Drives Development Across the Entire Lifespan of Rice and is Required for Shoot Formation

Rice (Oryza sativa) is both a critical cereal crop, providing 19% of the calories consumed by humans, and an excellent model system for monocots, which include other cereal crops such as maize, wheat, and barley. The phytohormone cytokinin has diverse functions in plants, including balancing root and shoot growth, regulating cell division, and responding to environmental stressors, roles that have been largely elucidated in dicots. We are investigating the role of cytokinin in growth and development of rice, in which it has been shown to positively regulate grain yield. We utilized CRISPR-Cas9 to generate a suite of cytokinin receptor mutant lines in rice with null mutations in the four putative cytokinin receptors (Histidine Kinases 3,4,5,6) in all single and multiple combinations. The HKs play diverse roles in rice growth and development, some of which are distinct from the roles found in the dicot Arabidopsis. The rice HKs acted partially redundantly in most processes in rice, though different HKs played more prominent roles in other processes. This was especially true in reproductive development, where certain HKs promoted specific aspects of panicle architecture and spikelet morphology. Furthermore, unlike in Arabidopsis, cytokinin signaling is essential for shoot formation in rice, as the hk3,4,5,6 quadruple mutant is completely shootless due to a lack of development of a shoot apical meristem. Overall, this work elucidates the many roles cytokinin perception has throughout the lifespan of rice, some of which diverge from what is known in Arabidopsis, highlighting the need to study alternative model systems.