Nodulation phenotypes of Pisum sativum mutants

Nitrogen-fixing bacteria, collectively referred to as rhizobia, are able to trigger the organogenesis of novel organs on legumes, called nodules. The initiation and development of nodules requires a complex signal exchange involving both plant and bacterial compounds. Phytohormones have been implicated in this process, although in many cases direct evidence is lacking. In the work reported here, the root and nodulation phenotypes of various mutant lines of Pisum sativum L. are characterized, including those having alterations in their phytohormone levels and/or perception, and a homeotic mutant. Root systems having similar or elevated GA levels compared with that of their wild type developed wild type numbers of nodules, whereas those deficient in gibberellins or brassinosteroids exhibited reduced nodulation. Gibberellin application or grafting to a wild type root or shoot restored the nodule number of a gibberellin-deficient mutant to that of its wild type. In contrast, the shoot controlled the number of nodules that formed in graft combinations of a brassinosteroid-deficient mutant and its wild type. Interestingly, a strong correlation between nodule and lateral root numbers was observed in all lines assessed, consistent with a possible overlap in the early developmental pathways of the two organs. Double mutants possessing the na mutation, which results in severe GAdeficiency, and the sln mutation, which results in elevated seedling GA levels, displayed abnormal nodules and a reduced capacity to autoregulate their nodule numbers. Constitutive GA signalling mutants also produced significantly fewer nodules than their wild type. However, these nodules were normal in appearance, and significantly greater in number compared with that of na plants, regardless of whether or not they also possessed the na mutation. This indicates that intact GA signalling pathways are required for nodule development. Additional double mutants were created by crossing na with one of three independent mutations, nod3, sym28, and sym29/nark/har-1, that result in a plants inability to regulate its nodule number. Double mutant segregates from each of these crosses formed significantly more nodules than na, but these structures maintained the aberrant na nodule morphology. A significant increase in nodule numbers was also observed on na following treatment with an ethylene biosynthesis inhibitor, but these nodules were also aberrant. These findings suggest that GAs are required for late nodule development and that ethylene has a role in nodule initiation. The histology of na nodules further supported a role for gibberellins late in nodule development as the cells of the infected zone failed to enlarge. The nodulation phenotype of the homeotic mutant, cochleata, which has stipules replaced by alternative leaf components, abnormal flowers and reduced fertility, was also investigated. Although the root system dry weight, root lengths and nodule numbers of cochleata were similar to those of its wild type, the nodulation phenotype of the mutant was unique. The nodules typically dichotomously branched and multiple callus and root structures emerged from their meristems. These nodule-roots incorporated a peripheral vascular bundle of the nodule into their own central vascular cylinder and both the nodules and roots of the hybrid structures appeared functional.