Faculty of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki, 305-8572, Japan.
Bioproduction Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2-17-2-1 Tsukisamu-Higashi, Toyohira, Sapporo, 062-8517, Japan.
J Plant Res. 2019 Sep;132(5):655-665. doi: 10.1007/s10265-019-01123-9. Epub 2019 Jul 9.
XSP25, previously shown to be the most abundant hydrophilic protein in xylem sap of Populus nigra in winter, belongs to a secretory protein family in which the arrangement of basic and acidic amino acids is conserved between dicotyledonous and monocotyledonous species. Its gene expression was observed at the same level in roots and shoots under long-day conditions, but highly induced under short-day conditions and at low temperatures in roots, especially in endodermis and xylem parenchyma in the root hair region of Populus trichocarpa, and its protein level was high in dormant buds, but not in roots or branches. Addition of recombinant PtXSP25 protein mitigated the denaturation of lactate dehydrogenase by drying, but showed only a slight effect on that caused by freeze-thaw cycling. Recombinant PtXSP25 protein also showed ice recrystallization inhibition activity to reduce the size of ice crystals, but had no antifreezing activity. We suggest that PtXSP25 protein produced in shoots and/or in roots under short-day conditions and at non-freezing low temperatures followed by translocation via xylem sap to shoot apoplast may protect the integrity of the plasma membrane and cell wall functions from freezing and drying damage in winter environmental conditions.
XSP25 先前被证明是黑杨冬季木质部汁液中含量最丰富的亲水性蛋白，属于分泌蛋白家族，其碱性和酸性氨基酸的排列在双子叶植物和单子叶植物中是保守的。在长日照条件下，其基因在根和茎中的表达水平相同，但在短日照条件和低温下，尤其是在杨树根毛区的内皮层和木质部薄壁组织中，其基因表达水平高度诱导，其蛋白水平在休眠芽中较高，但在根或枝条中较低。添加重组 PtXSP25 蛋白可减轻干燥导致的乳酸脱氢酶变性，但对冻融循环引起的变性只有轻微影响。重组 PtXSP25 蛋白还具有抑制冰晶重结晶的活性，可减小冰晶的大小，但没有抗冻活性。我们认为，在短日照条件和非冻结低温下，由茎和/或根产生的 PtXSP25 蛋白随后通过木质部汁液转移到茎的质外体，可能会保护质膜和细胞壁功能的完整性，防止冬季环境条件下的冰冻和干燥损伤。
