Functional Study of GbSMXL8-Mediated Strigolactone Signaling Pathway in Regulating Cotton Fiber Elongation and Plant Growth

The novel plant hormone strigolactones (SL) are involved significantly in plant growth and development. Its key members <i>SMXL6</i>, <i>7</i>, <i>8</i> can modulate SL signal reception and response negatively and can regulate plant branching remarkably. There are relatively scarce studies of cotton SMXL gene family, and this study was carried out to clarify the role of <i>GbSMXL8</i> in cotton fiber development. Phylogenetic analysis identified 48 cotton SMXL genes, which were divided into SMXL-I (<i>SMXL 1</i>, <i>2</i>), SMXL-II (<i>SMXL 3</i>) and SMXL-III (<i>SMXL6</i>, <i>7</i>, <i>8</i>) groups. The results of the cis-element analysis indicated that the SMXL gene could respond to hormones and the environment to modulate cotton growth process. A candidate gene <i>GbSMXL8</i> was screened out based on the expression difference in extreme varieties of <i>Gossypium barbadense</i>. Tissue-specific analysis indicated that <i>GbSMXL8</i> was mainly expressed in roots, 20D, 25D, and 35D and was involved in SL signaling pathways. In vitro ovule culture experiments showed that exogenous SLs (GR24) could promote the fiber elongation of <i>G. barbadense</i>, and <i>GbSMXL8</i> expression was increased after GR24 treatment, indicating that <i>GbSMXL8</i> was specifically responsive to GR24 in regulating fiber growth. <i>GbSMXL8</i> knockout resulted in creased length and number of epidermal hairs and the length of fiber, indicating the interference role of <i>GbSMXL8</i> gene with the development of cotton fiber. The <i>GbSMXL8</i> transgenic plant was detected with a higher chlorophyll content and photosynthetic rate than those of the control plant, producing a direct impact on plant growth, yield, and biomass accumulation. <i>GbSMXL8</i> gene knockout could increase plant height, accelerate growth rate, and lengthen fiber length. Intervening <i>GbSMXL8</i> may mediate cotton growth, plant type formation and fiber elongation. In conclusion, the present study uncovers the function of <i>GbSMXL8</i>-mediated SL signal in cotton, providing theoretical insight for future breeding of new cotton varieties.