MYB6/bHLH13-AbSUS2 involved in sugar metabolism regulates root hair initiation of Abies beshanzuensis.

Root hair is regarded as a pivotal complementary survival tactic for mycorrhizal plant like Abies beshanzuensis when symbiosis is disrupted. Relatively little is known about the mechanism underlying root hair morphogenesis in plant species that are strongly dependent on mycorrhizal symbiosis. Many of these species are endangered, and this knowledge is critical for ensuring their survival. Here, a MYB6/bHLH13-sucrose synthase 2 (AbSUS2) module was newly identified and characterized in A. beshanzuensis using bioinformatics, histochemistry, molecular biology, and transgenesis. Functional, expression pattern, and localization analysis showed that AbSUS2 participated in sucrose synthesis and was involved in root hair initiation in A. beshanzuensis. Additionally, the major enzymatic product of AbSUS2 was found to suppress root hair initiation in vitro. Our data further showed that a complex involving the transcription factors AbMYB6 and AbbHLH13 directly interacted with the promoter of AbSUS2 and strengthened its expression, thereby inhibiting root hair initiation in response to exogenous sucrose. Our findings offer novel insights into how root hair morphogenesis is regulated in mycorrhizal plants and also provide a new strategy for the preservation of endangered mycorrhizal plant species.