Abstract
The phytohormone cytokinin plays a critical role in regulating growth and development throughout the life cycle of the plant. The primary transcriptional response to cytokinin is mediated by the action of the type-B response regulators (RRs), with much of our understanding for their functional roles being derived from studies in the dicot Arabidopsis. To examine the roles played by type-B RRs in a monocot, we employed gain-of-function and loss-of-function mutations to characterize RR22 function in rice. Ectopic overexpression of RR22 in rice results in an enhanced cytokinin response based on molecular and physiological assays. Phenotypes associated with enhanced activity of RR22 include effects on leaf and root growth, inflorescence architecture, and trichome formation. Analysis of four Tos17 insertion alleles of RR22 revealed effects on inflorescence architecture, trichomes, and development of the stigma brush involved in pollen capture. Both loss‐ and gain-of-function RR22 alleles affected the number of leaf silica-cell files, which provide mechanical stability and improve resistance to pathogens. Taken together, these results indicate that a delicate balance of cytokinin transcriptional activity is necessary for optimal growth and development in rice.
Highlights
The phytohormone cytokinin plays a critical role in regulating growth and development throughout the life cycle of the plant, including the regulation of cell proliferation, shoot and root architecture, seed yield, senescence, and stress responses (Sakakibara, 2006; Hwang et al, 2012; Kieber and Schaller, 2014; Jameson and Song, 2016)
To functionally characterize the role of a type-B RR in rice, we focused on RR22 based on its relatively high expression level among the rice type-B RRs, prior indication that RR22 plays a role in mediating cytokinin signaling, and the availability of insertion alleles through public resources (Kim et al, 2012; Tsai et al, 2012; Yamburenko et al, 2017; Worthen et al, 2019)
We recently employed a CRISPR/Cas9 gene editing approach to target the four most abundant type-B RRs of rice (RR21, RR22, RR23, and RR24), results from that analysis confirming a role in cytokinin signaling and revealing various roles in the growth and development (Worthen et al, 2019)
Summary
The phytohormone cytokinin plays a critical role in regulating growth and development throughout the life cycle of the plant, including the regulation of cell proliferation, shoot and root architecture, seed yield, senescence, and stress responses (Sakakibara, 2006; Hwang et al, 2012; Kieber and Schaller, 2014; Jameson and Song, 2016). Among the primary-response genes are a second family of RRs, the type-A RRs, which function as negative feedback regulators to desensitize the plant to cytokinin (To et al, 2004; Kieber and Schaller, 2014). As in most plants, these signaling elements are encoded by multi-gene families, there being 13-type-B RRs and 13 type-A RRs in the rice genome (Pareek et al, 2006; Du et al, 2007; Schaller et al, 2007; Tsai et al, 2012)
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