Centre for Plant Science, School of Environmental and Life Sciences, The University of Newcastle, Callahan NSW 2308, Australia.
College of Horticulture and Forestry Sciences, Huazhong Agricultural University, Wuhan Hubei 430070, China.
J Exp Bot. 2022 Sep 12;73(16):5414-5427. doi: 10.1093/jxb/erac234.
In Arabidopsis, polarized deposition of wall ingrowths in phloem parenchyma (PP) transfer cells (TCs) occurs adjacent to cells of the sieve element/companion cell (SE/CC) complex. However, the spatial relationships between these different cell types in minor veins, where phloem loading occurs, are poorly understood. PP TC development and wall ingrowth localization were compared with those of other phloem cells in leaves of Col-0 and the transgenic lines AtSUC2::AtSTP9-GFP (green fluorescent protein) and AtSWEET11::AtSWEET11-GFP that identify CCs and PP cells, respectively. The development of PP TCs in minor veins, indicated by deposition of wall ingrowths, proceeded basipetally in leaves. However, not all PP cells develop wall ingrowths, and higher levels of deposition occur in abaxial- compared with adaxial-positioned PP TCs. Furthermore, the deposition of wall ingrowths was exclusively initiated on and preferentially covered the PP TC/SE interface, rather than the PP TC/CC interface, and only occurred in PP cells that were adjacent to SEs. Collectively, these results demonstrate a tight association between SEs and wall ingrowth deposition in PP TCs and suggest the existence of two subtypes of PP cells in leaf minor veins. Compared with PP cells, PP TCs showed more abundant accumulation of AtSWEET11-GFP, indicating functional differences in phloem loading between PP and PP TCs.
在拟南芥中，韧皮部薄壁组织（PP）转移细胞（TC）中的壁生长的极化沉积发生在筛分子/伴胞（SE/CC）复合体相邻的细胞旁边。然而，在发生韧皮部装载的小脉中，这些不同细胞类型之间的空间关系还了解甚少。在 Col-0 和转 AtSUC2::AtSTP9-GFP（绿色荧光蛋白）和 AtSWEET11::AtSWEET11-GFP 转基因系叶片中，比较了 PP TC 发育和壁生长定位与其他韧皮部细胞的关系，这两个转基因系分别鉴定了 CC 和 PP 细胞。通过壁生长沉积指示的小脉中 PP TC 的发育在叶片中向基侧进行。然而，并非所有的 PP 细胞都发育出壁生长，并且在背侧定位的 PP TC 中比在腹侧定位的 PP TC 中沉积水平更高。此外，壁生长的沉积仅在 PP TC/SE 界面上启动，并优先覆盖该界面，而不在 PP TC/CC 界面上发生，并且仅发生在与 SE 相邻的 PP 细胞中。总之，这些结果表明 SE 和 PP TC 中壁生长沉积之间存在紧密的关联，并表明在叶片小脉中存在两种亚型的 PP 细胞。与 PP 细胞相比，PP TC 显示出更多的 AtSWEET11-GFP 积累，表明在韧皮部装载中 PP 和 PP TC 之间存在功能差异。
