Novak Stacey, Kalbakji Nataly, Upthegrove Kylie, Neher Wesley, Jones Jay, de León Jazmin
Department of Biology, University of La Verne, La Verne, CA, United States.
Southern California College of Optometry at Marshall B. Ketchum, Fullerton, CA, United States.
Front Plant Sci. 2018 Aug 17;9:1215. doi: 10.3389/fpls.2018.01215. eCollection 2018.
Polar auxin transport (PAT) is facilitated by polar localization of PIN-FORMED (PIN) efflux carriers, which direct auxin flow and regulate developmental events. Brassinosteroids (BRs) and auxin work synergistically to promote growth, and in root geotropisms this cross-talk involves BR-directed polarization of PIN through the mobilization of F-actin. However, the role of BR in PAT during shoot growth, hair formation, and embryogenesis has not been well studied. Orchid seed are mature at a point in development that is analogous to the globular-stage of embryogenesis in typical angiosperms. Thus, this system provided a unique opportunity to study the effects of BR on PAT during embryogenesis-like events, including meristem/first leaf formation and protocorm/stem development, which is followed by protocorm hair formation. In this work, the degree to which BRs rescued embryo-like protocorms from the impact of PAT-disrupting agents, such as PAT inhibitors or high auxin levels, was determined based on growth responses. This study first established that auxin and BRs work together synergistically to promote seedling elongation in . Repressed seedling growth caused by the PAT-disrupting agents was alleviated with eBL, suggesting that BRs enhance PAT in embryogenesis-like stages of young protocorms. However, similar responses were not evident in seed embryos. Results from this study also suggested that BRs may enhance orchid protocorm elongation by regulating auxin transport through an F-actin-mediated mechanism. With regard to protocorm hairs, increased eBL levels inhibited formation, whereas reduced BR biosynthesis altered hair patterning, and prevented outgrowth of auxin-stimulated hairs. Moreover, PAT inhibitors and repression of BR biosynthesis caused hair bud formation without hair outgrowth, suggesting a role for BR in PAT during protocorm hair development.
极性生长素运输（PAT）由PIN形成（PIN）流出载体的极性定位促进，这些载体引导生长素流动并调节发育事件。油菜素类固醇（BRs）和生长素协同作用促进生长，在根向地性中，这种相互作用涉及通过F-肌动蛋白的动员使PIN发生BR导向的极化。然而，BR在茎生长、毛状体形成和胚胎发生过程中的PAT作用尚未得到充分研究。兰花种子在发育的某个阶段成熟，这类似于典型被子植物胚胎发生的球形阶段。因此，该系统为研究BR在胚胎发生样事件（包括分生组织/第一片叶形成和原球茎/茎发育，随后是原球茎毛状体形成）中对PAT的影响提供了独特的机会。在这项工作中，根据生长反应确定了BRs从PAT破坏剂（如PAT抑制剂或高生长素水平）的影响中拯救类胚原球茎的程度。本研究首先证实生长素和BRs协同作用促进幼苗伸长。PAT破坏剂引起的幼苗生长受抑制可被表油菜素内酯（eBL）缓解，这表明BRs在年轻原球茎的胚胎发生样阶段增强了PAT。然而，在种子胚中未观察到类似反应。本研究结果还表明，BRs可能通过F-肌动蛋白介导的机制调节生长素运输来增强兰花原球茎的伸长。关于原球茎毛状体，eBL水平升高会抑制其形成，而BR生物合成减少会改变毛状体模式，并阻止生长素刺激的毛状体生长。此外，PAT抑制剂和BR生物合成的抑制导致毛芽形成但无毛状体生长，这表明BR在原球茎毛状体发育过程中的PAT中发挥作用。
