树种中的维管形成层是分生组织细胞的圆柱形结构域,负责向内产生次生木质部(也称为木材)和向外产生次生韧皮部。杨树(杨树)WUSCHEL(WUS)-相关的HOMEOBOX(WOX)家族成员,PtrWUSa和PtrWOX13b,先前显示在杨树茎的维管形成层和分化木质部细胞中表达,但它们的功能仍然未知。这里,我们调查了PtrWUSa的角色,PtrWOX13b及其在维管组织和木材形成中的紧密同源物。表达分析显示PtrWUSa和PtrWOX13b,他们的近亲,PtrWUSb,PtrWUS4a/b和PtrWOX13a/c,在杨树茎的维管形成层和分化木质部细胞中也有表达。PtrWUSa在发育中的韧皮纤维中也表现出高水平的表达。在转基因杨树中与优势EAR抑制域(PtrWUSa-DR)融合的PtrWUSa的表达导致茎扭曲和叶片卷曲的植物生长迟缓,并严重破坏了血管组织。在PtrWUSa-DR茎中,在维管形成层和韧皮部纤维之间的韧皮部区域发生了细胞的急剧增殖,它们形成了异位的血管组织或韧皮部纤维样巩膜细胞的岛。在PtrWUSa-DR叶柄和中静脉中也观察到类似的细胞增殖。另一方面,PtrWOX4a-DR的过度表达导致皮质区血管束的异位形成,PtrWOX13a-DR和PtrWOX13b-DR的过表达导致木材形成减少,而不影响转基因杨树植物的维管组织。一起,这些发现表明PtrWUSa和PtrWOX13a/b在调节血管组织和木材形成中的关键作用,这进一步加深了我们对WOX基因在调节树种维管形成层活性中的功能的理解。
Vascular
cambium in tree species is a cylindrical domain of meristematic cells that are responsible for producing secondary xylem (also called wood) inward and secondary phloem outward. The poplar (Populus trichocarpa) WUSCHEL (WUS)-RELATED HOMEOBOX (WOX) family members, PtrWUSa and PtrWOX13b, were previously shown to be expressed in vascular
cambium and differentiating xylem cells in poplar stems, but their functions remain unknown. Here, we investigated roles of PtrWUSa, PtrWOX13b and their close homologs in vascular organization and wood formation. Expression analysis showed that like PtrWUSa and PtrWOX13b, their close homologs, PtrWUSb, PtrWUS4a/b and PtrWOX13a/c, were also expressed in vascular
cambium and differentiating xylem cells in poplar stems. PtrWUSa also exhibited a high level of expression in developing phloem fibers. Expression of PtrWUSa fused with the dominant EAR repression domain (PtrWUSa-DR) in transgenic poplar caused retarded growth of plants with twisted stems and curled leaves and a severe disruption of vascular organization. In PtrWUSa-DR stems, a drastic proliferation of cells occurred in the phloem region between vascular
cambium and phloem fibers and they formed islands of ectopic vascular tissues or phloem fiber-like sclerenchyma cells. A similar proliferation of cells was also observed in PtrWUSa-DR leaf petioles and midveins. On the other hand, overexpression of PtrWOX4a-DR caused ectopic formation of vascular bundles in the cortical region, and overexpression of PtrWOX13a-DR and PtrWOX13b-DR led to a reduction in wood formation without affecting vascular organization in transgenic poplar plants. Together, these findings indicate crucial roles of PtrWUSa and PtrWOX13a/b in regulating vascular organization and wood formation, which furthers our understanding of the functions of WOX genes in regulating vascular
cambium activity in tree species.