SYMBIOTIC NODULE SENESCENCE IN LEGUMES: MOLECULAR-GENETIC AND CELLULAR ASPECTS (review)

Senescence is the natural stage in development of symbiotic nodule. As a result of senescence, reutilization of different nutrients from nodule to the other plant organs occurs. Generally senescence in legumes is triggered after flowering finishing, although the first traits of senescence can be observed very early during nodule development. A delay of the triggering of senescence program will allow to prolong the active nitrogen-fixating period and therefore to increase the amount of symbiotrophic nitrogen in plants and, finally, to elevate legume productivity. That is why no wonder that in the recent years the senescence of nitrogen-fixing nodules is actively studied. In this review the main developmental stages of nitrogen-fixing symbiotic nodule of legumes, particularities of symbiotic nodule development of determinate and indeterminate types are considered. In legumes with indeterminate nodules, the symbiosomes are not long-leaving as the infected tissues are permanently renewing due to apical meristem. There are two subsequent stages identified in an indeterminate nodule senescent. First a bacteroid degradation and the death of some infected cells occur, and then both symbiosomes and all infected cells are destroyed. In determinate nodules, the senescence initiated in the central part of a nodule, then extends to the peripheral zone. In this review morphological characters of nodule senescence at ulatrstructural level are analyzed. The role of cysteine and threonine proteases is discussed. Reutilization of nitrogen and other products of protein degradation are probably the most important during senescence. There are the evidences that in the root nodules of legumes the cysteine proteases are involved into nodule functions, adaptation of the host plant cells to physiological stresses, and the nodule senescence control. By a large-scale analysis of nodule transcriptome of Medicago truncatula Gaertn. several groups of genes expressed at successive stages of the senescence of indeterminate nodule are revealed. In this review the role of phytohormones, such as ethylene, abscisic acid, jasmonic acid, gibberellins and nitrogen monooxide in senescence of symbiotic nodule is considered. Nevertheless, until recent days our knowledge about hormonal control of nodule senescence is still incomplete. The oxidative stress, accompanying the process of nodule senescence is discussed. On the nodule aging, the concentrations of peroxides, protein carbonyls, modified DNA nucleotides and catalytic Fe increase. Iron activates lipids peroxidation in a peribacteroid space, resulting in degradation of the peribacteroid membrane in senescent nodules. The concentrations of oxidized glutathione and homoglutathione rise significantly during the nodule development, and the reduced forms decrease under senescence, indicating an oxidative stress in the senescing nodules. In this review the role of genes, encoding proteins involved in transport of wide-range of molecules, and genes, whose products are involved in regulatory and signal functions in cell; differences between stress-induced senescence and natural senescence are considered. Using model legumes, Lotus japonicus (Regel) K. Larsen and M. truncatula, several genes were cloned the mutations of which caused early senescence. It is emphasized that these genes encode different proteins involved into functioning of a symbiotic nodule. Until now, two transcription factors in M. truncatula are described, which are involved into nodule senescence. An induced senescence is more rapid, comparing to natural senescence, and manifests the signs of an oxidative stress and programmed cell death.