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Histological and ultrastructural nodule organization of the pea (Pisum sativum) mutant sgefix–-5 in the Sym33 gene encoding the transcription factor PsCYCLOPS/PsIPD3

Background. The transcription factor CYCLOPS/IPD3 is a key activator of the organogenesis of symbiotic nodules. Its participation in the development of infection threads and symbiosomes is also shown. In pea, three mutant alleles were identified for this gene (sym33-1 sym33-3). The phenotypic manifestations of the sym33-3 allele of the SGEFix-2 mutant, characterized by a leaky phenotype (the formation of two types of nodules: white and pinkish) were the most studied. The sym33-2 allele in the mutant SGEFix-5 was described as a strong allele, however, its phenotypic manifestations have not been studied in detail. 
Materials and methods. In this study, the histological and ultrastructural nodule organization of the SGEFix-5 mutant was analyzed using confocal laser scanning microscopy and transmission electron microscopy. 
Results. In the nodules locked infection threads were observed, from which no bacteria release into the cytoplasm of the plant cell occurs. In this case, in some infection threads, bacteria were degraded, which may indicate the activation of strong defense reactions in the nodules of the SGEFix-5 mutant. 
Conclusions. The sym33-2 allele in the mutant SGEFix-5 is a strong allele, which triggers the severe defense reactions, when rhizobia are already perceived as pathogens in infection threads.