Sitrit Y, Hadfield K A, Bennett A B, Bradford K J, Downie A B
Department of Vegetable Crops, One Shields Avenue, University of California, Davis, California 95616-8631, USA.
Plant Physiol. 1999 Oct;121(2):419-28. doi: 10.1104/pp.121.2.419.
Radicle protrusion from tomato (Lycopersicon esculentum Mill.) seeds to complete germination requires weakening of the endosperm tissue opposite the radicle tip. In common with other cell wall disassembly processes in plants, polygalacturonases (PGs) may be involved. Only calcium-dependent exo-PG activity was detected in tomato seed protein extracts. Chromatographic profiles of a partially acid-hydrolyzed fraction of polygalacturonic acid further digested with seed extract were consistent with the presence of only calcium-dependent exo-PG activity. In addition, a transcript encoding a previously unknown PG was detected prior to the completion of germination. The mRNA, produced from a gene (LeXPG1) estimated by Southern analysis to be represented once in the genome, was also present in flowers (anthers) and in lower amounts in roots and stems. LeXPG1 mRNA abundance was low during seed development, increased during imbibition, and was even greater in seeds that had completed germination. Expression of LeXPG1 during germination predominates in the endosperm cap and radicle tip, and in the radicle appears as a distinct band possibly associated with vascular tissue differentiation. We suggest that PG is involved in cell wall loosening of the endosperm necessary for radicle protrusion from tomato seeds and in subsequent embryo and seedling growth.
番茄（Lycopersicon esculentum Mill.）种子的胚根突出直至完成萌发需要胚根尖端相对的胚乳组织弱化。与植物中其他细胞壁分解过程一样，多聚半乳糖醛酸酶（PGs）可能参与其中。在番茄种子蛋白提取物中仅检测到钙依赖性外切PG活性。用种子提取物进一步消化的部分酸水解的聚半乳糖醛酸级分的色谱图谱与仅存在钙依赖性外切PG活性一致。此外，在萌发完成之前检测到一种编码先前未知PG的转录本。通过Southern分析估计在基因组中仅出现一次的基因（LeXPG1）产生的mRNA也存在于花（花药）中，在根和茎中的含量较低。LeXPG1 mRNA丰度在种子发育期间较低，在吸胀期间增加，在完成萌发的种子中更高。LeXPG1在萌发期间的表达主要在胚乳帽和胚根尖端，并且在胚根中表现为可能与维管组织分化相关的明显条带。我们认为PG参与了番茄种子胚根突出所需的胚乳细胞壁松弛以及随后的胚胎和幼苗生长。
