ROLE OF PHYTOHORMONES IN THE CONTROL OF SYMBIOTIC NODULE DEVELOPMENT IN LEGUME PLANTS. I. CYTOKININS

The influence of cytokinins on nitrogen-fixing nodule development in legume plants, as well as the molecular mechanisms of this effect and interaction with components of signaling cascade activated by nodulation inductors, the bacterial signals Nod factors, are discussed in the review. Positive role of cytokinins was first shown in the experiments with their exogenous application to plants that resulted in spontaneous nodule formation on the roots of legume plants (K.R. Libbenga, P.A.A. Harkers, 1973). The experiments with bacterial strains defective in Nod factor biosynthesis, but producing the trans-zeatin, confirmed the assumption that cytokinins are involved in the control of nodule formation (J.B. Cooper, S.R. Long, 1994). As a result the nodule-like structures are developed expressing the symbiosis-specific early nodulin genes. At the present stage of research the identification of legume mutants defective in the genes encoding receptors to cytokinins allowed to provide evidence for the involvement of cytokinins in nodulation. The inhibition of nodule development was found in Medicago truncatula mutants defective in the cytokinin receptor gene CRE1 (cytokinin response 1) and LHK1 (Lotus histidine kinase 1) in Lotus japonicus (S. Gonzalez-Rizzo et al., 2006; J.D. Murray et al., 2007). Thus inhibition of plant susceptibility to cytokinins affected both the infection thread development and nodule formation. In contrast, the strengthening of the LHK1 and CRE1 gene function in L. japonicus and M. truncatula using a recombination approach, resulted in nodule-like structure formation in the absence of rhizobia (L. Tirichine et al., 2007; E. Ovchinnikova et al., 2011). The pathways of cytokinin biosynthesis and activation have been considered, as well as their perception and signal transduction. Cytokinin accumulation may be connected with the gene expression induction, that control the biosynthesis/activation of these hormones, but the molecular mechanisms of this activation remains to be seen. Analysis showed that cytokinins are involved in signal transduction from Nod factor after the stage controlled by one of the key regulators of the signaling pathway, the calcium calmodulin-dependent kinase. This suggests that activation of the receptor to cytokinins is dependent on Nod factors. The expression of the genes encoding transcription factors NSP2, ERN1 and NIN was significantly reduced in the mutants cre1 and lhk1, that allowed to conclude the activation of the receptor to cytokinins precedes the involvement of these transcription factors in signal transduction (L. Tirichine et al., 2007; J. Plet et al., 2011). The analysis has shown that cytokinins are involved in the control of early stages of organogenesis and infection development, but they are also important for nodule differentiation. In addition to local control of nodulation, cytokinins take participation in system control, i.e. in the autoregulation of nodulation. Therefore the cytokinins may play various roles in nodule development depending on their spatial and temporal activation.

• Dolgikh A.V.
• Dolgikh E.A.
• Kirienko A.N.
• Leppyanen I.V.

• Искусство. Декоративно-прикладное искусство. Фотография. Музыка. Игры. Спорт

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