Pleiotropy of pea RisfixC supernodulation mutation is symbiosis-independent

Abstract

Mutations affecting the development of root symbiosis between legume plants (Fabaceae) and nodule bacteria (rhizobia) are often associated with pleiotropy. This might either primarily be caused by the mutation or develops as a physiological consequence of a changed nodule structure, number and activity. Three pleiotropic traits were revealed in the pea (Pisum sativum L.) mutant RisfixC which is of supernodulation/nitrate-tolerant symbiosis (Nts) type. They comprise shortened internodes, reduced shoot dry mass accumulation and increased nitrogen content in the root tissues when compared to the wild type. The changes were expressed in the same degree in asymbiotic and nodulated plants when the effect of symbiotic nitrogen on plant growth was abolished with a saturating nitrate level. Consequently, the pleiotropic traits are inherently associated with the mutation. In RisfixC, the pleiotropy coincided with the presumed absence of the systemic feedback factor regulating nodule number. However, no differences were detected in the comparison of nonnodulating mutant Risnod27 (sym8) with the wild type and of inoculated with noninoculated wild-type plants although these pairs also differ in the presence of the systemic factor. Therefore, the pathway leading from the RisfixC mutant product to pleiotropic changes appears to be independent of systemic nodule number regulation. Implications for the genetic improvement of growth and yield parameters of supernodulating breeding lines are discussed.