Department of Biology, 129 E. Jefferson St. Iowa City, Iowa, 52242-1324, USA.
BMC Plant Biol. 2022 Apr 25;22(1):210. doi: 10.1186/s12870-022-03590-0.
Plants have the lifelong ability to generate new organs due to the persistent functioning of stem cells. In seed plants, groups of stem cells are housed in the shoot apical meristem (SAM), root apical meristem (RAM), and vascular cambium (VC). In ferns, a single shoot stem cell, the apical cell, is located in the SAM, whereas each root initiates from a single shoot-derived root initial. WUSCHEL-RELATED HOMEOBOX (WOX) family transcription factors play important roles to maintain stem-cell identity. WOX genes are grouped phylogenetically into three clades. The T3WOX/modern clade has expanded greatly in angiosperms, with members functioning in multiple meristems and complex developmental programs. The model fern Ceratopteris richardii has only one well-supported T3WOX/modern WOX gene, CrWUL. Its orthologs in Arabidopsis, AtWUS, AtWOX5, and AtWOX4, function in the SAM, RAM, and VC, respectively. Identifying the function of CrWUL will provide insights on the progenitor function and the diversification of the modern WOX genes in seed plants.
To investigate the role of CrWUL in the fern, we examined the expression and function of CrWUL and found it expresses during early root development and in vasculature but not in the SAM. Knockdown of CrWUL by RNAi produced plants with fewer roots and fewer phloem cells. When expressed in Arabidopsis cambium, CrWUL was able to complement AtWOX4 function in an atwox4 mutant, suggesting that the WOX function in VC is conserved between ferns and angiosperms. Additionally, the proposed progenitor of T3WOX genes from Selaginella kraussiana is expressed in the vasculature but not in the shoot apical meristem. In contrast to the sporophyte, the expression of CrWUL in the gametophyte exhibits a more general expression pattern and when knocked down, offered little discernable phenotypes.
The results presented here support the occurrence of co-option of the T3WOX/modern clade gene from the gametophyte to function in vasculature and root development in the sporophyte. The function in vasculature is likely to have existed in the progenitor of lycophyte T3WOX/modern clade genes and this function predates its SAM function found in many seed plants.
由于干细胞的持续功能，植物具有生成新器官的终身能力。在种子植物中，干细胞群位于茎尖分生组织（SAM）、根尖分生组织（RAM）和维管束形成层（VC）中。在蕨类植物中，单个茎干细胞，即顶端细胞，位于 SAM 中，而每个根则由单个茎衍生的根原基起始。WUSCHEL 相关同源盒（WOX）家族转录因子在维持干细胞身份方面发挥着重要作用。WOX 基因在系统发育上分为三个分支。T3WOX/现代分支在被子植物中大大扩张，其成员在多个分生组织和复杂的发育程序中发挥作用。模式蕨类植物凤尾蕨属植物只有一个支持良好的 T3WOX/现代 WOX 基因，CrWUL。它在拟南芥中的同源物 AtWUS、AtWOX5 和 AtWOX4 分别在 SAM、RAM 和 VC 中发挥作用。鉴定 CrWUL 的功能将为种子植物中现代 WOX 基因的祖细胞功能和多样化提供见解。
为了研究 CrWUL 在蕨类植物中的作用，我们检查了 CrWUL 的表达和功能，发现它在早期根发育和脉管系统中表达，但不在 SAM 中表达。通过 RNAi 敲低 CrWUL 会导致植物根较少且韧皮部细胞较少。当在拟南芥形成层中表达时，CrWUL 能够在 atwox4 突变体中补充 AtWOX4 的功能，这表明 VC 中的 WOX 功能在蕨类植物和被子植物之间是保守的。此外，来自卷柏的 T3WOX 基因的祖先是在脉管系统中表达的，但不在茎尖分生组织中表达。与孢子体相比，配子体中 CrWUL 的表达表现出更普遍的表达模式，并且当敲低时，几乎没有明显的表型。
这里呈现的结果支持从配子体中选择 T3WOX/现代分支基因在孢子体中发挥作用的情况，在孢子体中，该基因在维管束和根发育中发挥作用。这种在脉管系统中的作用可能存在于石松类 T3WOX/现代分支基因的祖细胞中，并且这种作用早于其在许多种子植物中发现的 SAM 功能。
