School of Life Sciences, Centre for Cell and Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China.
Graduate School of Science and Technology, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan.
Plant Physiol. 2024 Oct 1;196(2):1180-1195. doi: 10.1093/plphys/kiae402.
The root cap secretes mucilage and sheds border cells (border-like cells, BLCs) in Arabidopsis (Arabidopsis thaliana). These mucilage and root cap-derived cells form a defensive barrier against soil pathogens. BEARSKIN1 (BRN1) and BRN2 are 2 homologous NAM, ATAF1/2, and CUC2 (NAC) family transcription factors of Arabidopsis, and mucilage secretion is inhibited in the brn1/2 double mutant. BRN1 and BRN2 are also involved in the expression of a pectin-digesting enzyme, POLYGALACTURONASE (RCPG), that facilitates BLC shedding. To further explore the connection between mucilage secretion and BLC shedding, we examined mucilage production in Arabidopsis lines displaying altered BLC detachment. Inactivation of BRN2 blocked mucilage synthesis and secretion, while inactivation of BRN1 and RCPG did not. Interestingly, RCPG sorted into mucilage-carrying vesicles budding from the Golgi and inhibited mucilage secretion in brn2-delayed BLC detachment. The root cap of a germinating seedling is initially covered with a cuticle, which is replaced by mucilage from BLCs as the seedling begins to shed these cells. Ectopic expression of RCPG in germinating seedlings caused early BLC formation and accelerated the cuticle-to-mucilage transition, indicating that RCPG expression and mucilage secretion are co-regulated. Furthermore, brn2 roots exhibited slower growth and increased cell death when subjected to salt or osmotic stress. Our research suggests that BRN2-mediated mucilage secretion contributes to BLC release to build an extracellular defense zone surrounding the root cap.
根冠分泌粘液，并在拟南芥中脱落边缘细胞（类似边缘的细胞，BLC）。这些粘液和根冠衍生细胞形成了抵御土壤病原体的防御屏障。BEARSKIN1（BRN1）和 BRN2 是拟南芥中 2 个同源的 NAM、ATAF1/2 和 CUC2（NAC）家族转录因子，BRN1/2 双突变体中粘液分泌受到抑制。BRN1 和 BRN2 也参与果胶分解酶 POLYGALACTURONASE（RCPG）的表达，促进 BLC 脱落。为了进一步探讨粘液分泌和 BLC 脱落之间的联系，我们检查了显示 BLC 脱离改变的拟南芥系中粘液的产生。BRN2 的失活阻断了粘液的合成和分泌，而 BRN1 和 RCPG 的失活则没有。有趣的是，RCPG 分选到从高尔基体出芽的携带粘液的囊泡中，并抑制了 brn2 延迟 BLC 脱落时的粘液分泌。萌发幼苗的根冠最初被一层角质层覆盖，随着幼苗开始脱落这些细胞，角质层被 BLC 的粘液取代。在萌发的幼苗中异位表达 RCPG 导致早期 BLC 形成，并加速了角质层到粘液的转变，表明 RCPG 的表达和粘液分泌是共同调节的。此外，BRN2 介导的粘液分泌会导致 brn2 根在受到盐或渗透胁迫时生长缓慢和细胞死亡增加。我们的研究表明，BRN2 介导的粘液分泌有助于 BLC 的释放，以在根冠周围建立一个细胞外防御区。
