The role of SMXL7 in plant vessel formation

持续的生长和组织形成是植物发育的特征，对于使不同的身体结构与不断变化的环境条件保持一致很重要。形成层驱动的双子叶植物地上部和根部的径向生长是这种生长模式的一个重要特征，也是生物量生产和二氧化碳长期封存的关键。形成层干细胞(CSCs)通常向内提供木质(即木质部)细胞，向外提供韧皮部(即韧皮部)细胞。在木质部和韧皮部组织中，不同类型的细胞执行高度专门化的功能，如木质部导管分子运输水分或韧皮部筛子分子运输糖。然而，调控CSC相关细胞命运决定以应对应激环境条件的机制在很大程度上是未知的。在这里，我们建议在PHD项目的背景下通过识别调节血管要素形成的Strigolacone(SL)信号通路的基因靶标来解决这一知识差距。我们最近发现了SL信号通路对拟南芥导管分子形成的影响。主要的观察结果是，SL途径受损的植物，放射状生长器官内导管分子的密度和大小增加，而SL的应用则相反。基于这些观察，我们提出了SL信号通路在调节植物的结构可塑性和水分运输能力中的作用，以响应水分有效性的波动。Max21-like 6(SMXL6)、SMXL7和SMXL8蛋白是转录调节因子，在SL信号的启动过程中被泛素化，随后被降解。然而，SL信号如何整合到发育程序中导致导管分子的形成，即在发育的木质部细胞中，哪些基因是SMXL蛋白以依赖于SL的方式靶向的，目前还不清楚。然而，这一知识对于估计环境整合对植物维管发育的意义以及在更广泛的物种范围内表征当前和未来气候条件下的环境-植物相互作用是关键。在这里，我们将利用拟南芥中可用的遗传工具来生成这些知识。我们的目标将通过以下方式实现：i)以全基因组的方式阐明SMXL7染色质结合谱，ii)在形成层环境中鉴定SMXL7依赖的基因，以及iii)使用遗传和生化手段验证所识别的靶点在血管形成方面的相关性。

Continuous growth and tissue formation are characteristic for plant development and important for aligning distinct body structures with changing environmental conditions. Cambium-driven radial growth of shoots and roots of dicotyledonous species is an important feature of this growth mode, and key for biomass production and for the long-term sequestration of CO2. Cambium stem cells (CSCs) proliferate usually providing wood (i.e. xylem) cells inwards and bast (i.e. phloem) cells outwards. Within xylem and phloem tissues, various cell types fulfil highly specialized functions like water transport by xylem vessel elements or sugar transport by phloem sieve elements. However, mechanisms regulating CSC-associated cell fate decisions to cope with stressful environmental conditions are largely unknown. Here, we propose to tackle this gap of knowledge in the context of a PhD project by identifying gene targets of the strigolactone (SL) signaling pathway regulating vessel element formation. We recently uncovered the impact of the SL signaling pathway on the formation of vessel elements in Arabidopsis thaliana. The central observations are that plants in which the SL pathway is impaired, density and size of vessel elements within radially growing organs are increased, whereas SL application has the contrary effect. Based on these observations, we propose a role of the SL-signaling pathway in mediating structural plasticity and water transport capacities in plants in response to fluctuating water availabilities. SUPPRESSOR OF MAX2 1-LIKE 6 (SMXL6), SMXL7 and SMXL8 proteins are transcriptional regulators which are ubiquitinated and subsequently degraded upon the initiation of SL signaling. However, how SL signaling is integrated into developmental programs leading to the formation of vessel elements, i.e. which genes are targeted by SMXL proteins in an SL-dependent manner in developing xylem cells is unclear. This knowledge, however, is key to estimate the significance of environmental integration into plant vascular development and to characterize environment-plant interaction under current and future climatic conditions in a broader spectrum of species. Here, we will generate this knowledge by taking advantage of genetic tools available in Arabidopsis thaliana. Our aim will be achieved by i) elucidation of the SMXL7 chromatin binding profile in a genome-wide fashion, ii) identification of SMXL7-dependent genes in a cambium context and, iii) verification of the relevance of identified targets in the context of vessel formation using genetic and biochemical means.