Röckel Nina, Wolf Sebastian, Kost Benedikt, Rausch Thomas, Greiner Steffen
Heidelberg Institute for Plant Sciences, INF 360, 69120 Heidelberg, Germany.
Plant J. 2008 Jan;53(1):133-43. doi: 10.1111/j.1365-313X.2007.03325.x. Epub 2007 Oct 29.
In dicots, pectins are the major structural determinant of the cell wall at the pollen tube tip. Recently, immunological studies revealed that esterified pectins are prevalent at the apex of growing pollen tubes, where the cell wall needs to be expandable. In contrast, lateral regions of the cell wall contain mostly de-esterified pectins, which can be cross-linked to rigid gels by Ca(2+) ions. In pollen tubes, several pectin methylesterases (PMEs), enzymes that de-esterify pectins, are co-expressed with different PME inhibitors (PMEIs). This raises the possibility that interactions between PMEs and PMEIs play a key role in the regulation of cell-wall stability at the pollen tube tip. Our data establish that the PME isoform AtPPME1 (At1g69940) and the PMEI isoform AtPMEI2 (At3g17220), which are both specifically expressed in Arabidopsis pollen, physically interact, and that AtPMEI2 inactivates AtPPME1 in vitro. Furthermore, transient expression in tobacco pollen tubes revealed a growth-promoting activity of AtPMEI2, and a growth-inhibiting effect of AtPPME1. Interestingly, AtPPME1:YFP accumulated to similar levels throughout the cell wall of tobacco pollen tubes, including the tip region, whereas AtPMEI2:YFP was exclusively detected at the apex. In contrast to AtPPME1, AtPMEI2 localized to Brefeldin A-induced compartments, and was found in FYVE-induced endosomal aggregates. Our data strongly suggest that the polarized accumulation of PMEI isoforms at the pollen tube apex, which depends at least in part on local PMEI endocytosis at the flanks of the tip, regulates cell-wall stability by locally inhibiting PME activity.
在双子叶植物中，果胶是花粉管顶端细胞壁的主要结构决定因素。最近，免疫学研究表明，酯化果胶在生长中的花粉管顶端普遍存在，而此处的细胞壁需要具有可扩展性。相比之下，细胞壁的侧面区域主要含有去酯化果胶，这些果胶可通过钙离子交联形成刚性凝胶。在花粉管中，几种果胶甲酯酶（PMEs），即使果胶去酯化的酶，与不同的果胶甲酯酶抑制剂（PMEIs）共同表达。这增加了PMEs与PMEIs之间的相互作用在调节花粉管顶端细胞壁稳定性中起关键作用的可能性。我们的数据表明，在拟南芥花粉中特异性表达的PME同工型AtPPME1（At1g69940）和PMEI同工型AtPMEI2（At3g17220）发生物理相互作用，并且AtPMEI2在体外使AtPPME1失活。此外，在烟草花粉管中的瞬时表达揭示了AtPMEI2的促生长活性和AtPPME1的生长抑制作用。有趣的是，AtPPME1:YFP在烟草花粉管的整个细胞壁中，包括顶端区域，积累到相似水平，而AtPMEI2:YFP仅在顶端被检测到。与AtPPME1不同，AtPMEI2定位于布雷菲德菌素A诱导的区室，并在含FYVE结构域的内体聚集体中被发现。我们的数据强烈表明，PMEI同工型在花粉管顶端的极化积累，这至少部分取决于顶端侧面局部的PMEI内吞作用，通过局部抑制PME活性来调节细胞壁稳定性。
