Abstract
Analyses of natural variation and the use of mutagenesis and molecular-biological approaches have revealed 50 symbiotic regulatory genes in pea (Pisum sativum L.). Studies of genomic synteny using model legumes, such as Medicago truncatula Gaertn. and Lotus japonicus (Regel) K. Larsen, have identified the sequences of 15 symbiotic regulatory genes in pea. These genes encode receptor kinases, an ion channel, a calcium/calmodulin-dependent protein kinase, transcription factors, a metal transporter, and an enzyme. This review summarizes and describes mutant alleles, their phenotypic manifestations, and the functions of all identified symbiotic regulatory genes in pea. Some examples of gene interactions are also given. In the review, all mutant alleles in genes with identified sequences are designated and still-unidentified symbiotic regulatory genes of great interest are considered. The identification of these genes will help elucidate additional components involved in infection thread growth, nodule primordium development, bacteroid differentiation and maintenance, and the autoregulation of nodulation. The significance of symbiotic mutants of pea as extremely fruitful genetic models for studying nodule development and for comparative cell biology studies of legume nodules is clearly demonstrated. Finally, it is noted that many more sequences of symbiotic regulatory genes remain to be identified. Transcriptomics approaches and genome-wide sequencing could help address this challenge.
Highlights
Symbiotic nitrogen fixation has attracted the attention of scientists for more than 100 years.This interest is explained by the exceptional importance of the symbiotic nitrogen fixation process, for agricultural production, and for the fundamentals of biology
The initiation of cortical cell divisions in the root of these mutants was only observed at 23 days after inoculation (DAI), while this occurred at 3 DAI in wild-type plants
These results indicate that brassinosteroids act as positive regulators of nodulation, regardless of the autoregulation of nodulation (AON) system [166]
Summary
Symbiotic nitrogen fixation has attracted the attention of scientists for more than 100 years. The year 2019 marks the 20th anniversary of the identification of the sequence of the first symbiotic gene Nodule inception (NIN) in the model legume Lotus japonicus (Regel) K. There are several benefits to using such models in molecular-biological studies They have small genomes, and protocols for effective genetic transformation have been developed for them [12,13]. This has led to the identification of specific sequences of symbiotic genes in model legumes [11]. Analyses of genomic synteny and microsynteny using model legumes have helped identify symbiotic genes of important crops, including pea [14,15]. To accelerate the identification of new sequences of such genes, it seems promising to use ‘omics’ technologies
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