Nodulation and Root Development in the Model Legume Lotus Japonicus

豆科植物与各种革兰氏阴性土壤细菌(通常称为根瘤菌)之间的固氮共生是一种重要的农业现象。此外，根瘤的形成和细菌对细胞内二氮的还原过程代表了植物基础研究的一个非常有趣的发展过程。根瘤个体发育以及豆科植物对根瘤菌侵染所形成的固氮根瘤的数量主要受植物遗传程序的控制。潜在的分子机制还没有被很好地理解，并形成了本提案中的研究主题。利用遗传学方法，PIS实验室从二倍体模式豆类植物莲花中鉴定出一个基因(HAR1)，它是与共生细菌正常互动所必需的。在根瘤菌存在的情况下，突变植株表现出一种改变的共生表型，其特征是形成大量的根瘤，几乎覆盖了整个根长。后者与根和茎的生长几乎完全受到抑制和突变植物的活力迅速恶化相吻合。有趣的是，同样的突变也被发现显著改变了未接种植物的根发育程序。因此，HAR1基因产物很可能在根的形态发生和寄主植物调节共生发育的机制中发挥作用。所提出的实验将利用分子遗传学方法(1)克隆和鉴定日本狼尾草HAR1基因，以及(2)通过遗传抑制分析寻找促进根和共生发育的途径(S)中的其他基因。希望这些分析的结果将有助于更好地理解共生植物-微生物相互作用的分子基础，并最终有助于将共生固氮扩展到目前非节生的农艺植物，如谷物等重要植物所需的基础科学知识。

The nitrogen fixing symbiosis between members of the legume family (Fabaceae) and diverse Gram- negative soil bacteria, commonly known as rhizobia, is an agriculturally important phenomenon. Moreover, the process of nodule formation and intracellular reduction of di-nitrogen by the bacteria represents a very interesting developmental process for basic studies on plants. Nodule ontogeny, as well as the number of nitrogen fixing nodules formed on legume plant roots in response to rhizobial infection is predominantly controlled by a plant genetic program. The underlying molecular mechanism is not well understood and forms the topic of investigation in this proposal. Using genetic approach, the PIs laboratory identified a gene (Har1), from the diploid model legume Lotus japonicus, that is required for normal interaction with symbiotic bacteria. In the presence of rhizobia, the mutant plants show an altered symbiotic phenotype, characterized by the formation of excessive numbers of nodules which cover almost the entire root length. The latter coincides with nearly total inhibition of root- and stem growth and rapid deterioration of mutant plant vitality. Interestingly, the same mutation was found also to significant modify the root developmental program of uninoculated plants. Thus, it is likely that the Har1 gene product functions in both root morphogenesis and the mechanism by which the host plant regulates symbiotic development. The experiments proposed will utilize molecular genetic approaches to (1) clone and characterize the L. japonicus Har1 gene, and (2) identify additional genes in a pathway(s) for root and symbiotic development by genetic suppressor analysis. It is hoped that the results of these analyses will result in a better understanding of the molecular basis of symbiotic plant-microbe interactions and may eventually contribute to the pool of basic scientific knowledge that will be required for the extension of symbiotic nitrogen fixation to presently non-nodulated agronomicall important plants, such as cereals.