Understanding the Origins of Nodulation by Studying Rhizobia-Root Interactions in Basal Legumes

根瘤菌与豆科植物之间的固氮互惠作用是植物与细菌之间研究最多的相互作用之一。根瘤菌的两个主要类群，1)α -变形菌，即根瘤菌科（α -根瘤菌），2)某些β -变形菌，伯克霍尔德菌和铜菌（β -根瘤菌），栖息在豆科根瘤中，将惰性大气氮转化为氨，氨被植物吸收为蛋白质和核酸。令人惊讶的是，人们对其与根瘤菌的相互作用知之甚少，特别是对豆科三个亚类中最基础的凯撒类豆科植物。虽然超过80%的含豆科和凤蝶科豆科植物结瘤，但不到30%的豆科植物结瘤。因此，它们代表了从非结瘤到结瘤豆科植物的过渡，可以帮助梳理出结瘤形成的进化起源。本项目将研究两种Caesalpinioid豆科植物：1)Gleditsia triacanthos，细菌进入根部并固定氮，而不形成根瘤，这种互惠关系可能早于根瘤的进化；2)Chamaecrista fasciculata，为数不多的结瘤的Caesalpinioid豆科植物之一。将进行实验以确定细菌如何与基础豆科植物建立这些联系，并确定已知对高级豆科植物中结瘤形成至关重要的基因的同源物。在确定基因同源物后，将对其进行突变以测试其功能。在一名本科生的帮助下，建立了一个实现RNAi诱变的毛状根转化系统，这是研究基生豆科植物基因功能的重大进展。该项目将为学生培训提供其他机会。豆科植物是亚热带和热带地区的主要植物群，是重要的粮食/林业作物，为氮限制土壤补充氮。对这些基生豆科植物的研究不仅有助于我们进一步了解结瘤的基因，而且还能使学生了解基生植物对亚热带和热带土壤氮经济的价值；由于人口增长和气候变化，它们受到了威胁。土壤退化可能导致这些对环境至关重要的豆科植物的灭绝。

The nitrogen-fixing mutualism between rhizobia and legumes is one of the best-studied interactions between plants and bacteria. Two major groups of rhizobia, 1) alpha-proteobacteria, i.e. the family Rhizobiaceae (alpha-rhizobia), and 2) certain beta-proteobacteria, Burkholderia and Cupriavidus (beta-rhizobia), inhabit legume root nodules, converting inert atmospheric nitrogen into ammonia, which is assimilated by plants into proteins and nucleic acids. Surprisingly, little is known about the interactions with beta-rhizobia, especially for the Caesalpinioid legumes, the most basal of the three sub-classes of the legume family. Although more than 80% of Mimosoid and Papilionoid legumes nodulate, less than 30% of the Caesalpinioid species form nodules. Thus, they represent a transition from non-nodulating to nodulating legumes and can help tease out the evolutionary origins of nodule formation. This project will investigate two Caesalpinioid legumes: 1) Gleditsia triacanthos, in which bacteria enter roots and fix nitrogen without nodule formation, a mutualism that probably predates the evolution of nodules, and 2) Chamaecrista fasciculata, one of the few caesalpinioid legumes that nodulates. Experiments will be carried out to determine how the bacteria establish these associations with basal legumes and identify orthologs of genes known to be critical for nodule formation in the advanced legumes. After identifying the gene orthologs, they will be mutated to test their function. A hairy root transformation system for implementing RNAi mutagenesis has been established with the help of an undergraduate student, and is a major advance for studying gene function in basal legumes. The project will provide other opportunities for student training. Caesalpinioid legumes comprise the major flora of the subtropics and tropics, serving as important food/forestry crops and replenishing nitrogen to N-limited soils. Studying these basal legumes will bring us closer not only to learning more about the genes responsible for nodulation, but also to educating students about the value of basal plants to the nitrogen economy of subtropical and tropical soils; these are threatened because of increasing human populations and climate change. Soil degradation could lead to the extinction of these environmentally important legumes.