A family of repetitive proline-rich proteins interact with acidic pectins and play distinct roles in legume root cell walls affecting cortical and vascular structure. A proline-rich protein (PRP) family, composed of tandemly repeated Pro-Hyp-Val-X-Lys pentapeptide motifs, is found primarily in the Leguminosae. Four distinct size classes within this family are encoded by seven tightly linked genes: MtPRP1, MtPRP2 and MtPRP3, and four nearly identical MtPRP4 genes. Promoter fusions to β-glucuronidase showed strong expression in the stele of hairy roots for all 4 PRP genes tested, with additional expression in the cortex for PRP1, PRP2 and PRP4. All except MtPRP4 are strongly expressed in non-tumorous roots, and secreted and ionically bound to root cell walls. These PRPs are absent from root epidermal cell walls, and PRP accumulation is highly localized within the walls of root cortical and vascular tissues. Within xylem tissue, PRPs are deposited in secondary thickenings where it is spatially exclusive to lignin. In newly differentiating xylem, PRPs are deposited in the regularly spaced paired-pits and pit membranes that hydraulically connect neighboring xylem elements. Hairpin-RNA knock-down constructs reducing PRP expression in Medicago truncatula hairy root tumors disrupted cortical and vascular patterning. Immunoblots showed that the knockdown tumors had potentially compensating increases in the non-targeted PRPs, all of which cross-react with the anti-PRP antibodies. However, PRP3 knockdown differed from knockdown of PRP1 and PRP2 in that it greatly reduced viability of hairy root tumors. We hypothesize that repetitive PRPs interact with acidic pectins to form block-copolymer gels that can play distinct roles in legume root cell walls.

中文翻译：

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蒺藜苜蓿根部的皮层组织完整性需要定位于中层的 PRP。

一个富含脯氨酸的重复蛋白家族与酸性果胶相互作用，并在影响皮质和血管结构的豆科植物根细胞壁中发挥不同的作用。富含脯氨酸的蛋白质 (PRP) 家族由串联重复的 Pro-Hyp-Val-X-Lys 五肽基序组成，主要存在于豆科植物中。该家族中的四个不同大小类别由七个紧密相关的基因编码：MtPRP1、MtPRP2 和 MtPRP3，以及四个几乎相同的 MtPRP4 基因。与 β-葡萄糖醛酸酶融合的启动子在毛状根的石碑中显示出对所有 4 个 PRP 基因的强烈表达，在皮层中对 PRP1、PRP2 和 PRP4 有额外的表达。除 MtPRP4 外，所有细胞均在非肿瘤根中强烈表达，并分泌并与根细胞壁离子结合。这些 PRP 不存在于根表皮细胞壁中，PRP 的积累高度集中在根皮层和血管组织的壁内。在木质部组织内，PRP 沉积在二次增厚中，在空间上它是木质素独有的。在新分化的木质部中，PRP 沉积在规则间隔的成对凹坑和凹坑膜中，这些凹坑膜通过液压连接相邻的木质部元素。减少蒺藜苜蓿毛状根肿瘤中 PRP 表达的发夹 RNA 敲低结构破坏了皮质和血管模式。免疫印迹显示，敲低的肿瘤可能会补偿非靶向 PRP 的增加，所有这些都与抗 PRP 抗体发生交叉反应。然而，PRP3 敲低与 PRP1 和 PRP2 敲低的不同之处在于它大大降低了毛根肿瘤的活力。