RhSCL3 and RhSCL33 (SCARECROW-LIKE) Regulate the Adventitious Root Formation of Rosa hybrida Responding Auxin

Adventitious root (AR) formation is critical for the successful establishment and survival of rose (Rosa hybrida) cuttings. SCARECROW-LIKE (SCL) genes are known to regulate plant growth and development; however, their functions in rose AR formation remain poorly understood. In this study, the rooting rate of rose cuttings declined, whereas AR number increased with increasing concentrations of exogenous indole-3-butyric acid (IBA) and N-1-naphthylphthalamic acid (NPA). Most of the 36 identified RhSCL genes were significantly up-regulated during AR formation. Notably, the expression level of RhSCL3 and RhSCL33 was positively associated with AR formation under low concentrations of exogenous IBA and NPA. Both genes were expressed at higher levels in roots than in other organs and encoded nuclear-localized proteins with transcriptional activation activity. Heterologous expression of RhSCL3 and RhSCL33 in tobacco increased primary root length under exogenous IBA treatment, enhanced AR number and length, and promoted dry matter accumulation, accompanied by increased starch degradation and soluble sugar production. Compared with wild-type plants (WT), transgenic plants showed reduced indole-3-acetic acid (IAA) levels, more pronounced changes in gibberellic acid (GA3) content, and higher activities of superoxide dismutase (SOD), peroxidase (POD), polyphenol oxidase (PPO), catalase (CAT), and indole-3-acetic acid oxidase (IAAO). These findings suggest that overexpression of RhSCL3 and RhSCL33 facilitates rooting under low-auxin conditions, and RhSCL33 acting earlier during AR development, and RhSCL3 functioning mainly at later stages. This regulatory module may provide useful genetic targets for improving vegetative propagation efficiency in rose and other horticultural or crop species.