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Stem cells (SCs) are undifferentiated cells of multicellular organisms that can divide, self-renew, and differentiate. Despite the differences in their properties, the general principles of the existence of SCs can be distinguished in all multicellular organisms. In plants, SCs are found in meristems, structures that ensure the continuous growth of a plant and provide material for the formation of various specialized tissues. There are numerous types of meristems: shoot apical meristem (SAM) and root apical meristem (RAM), and lateral meristems (LMs) (procambium, cambium, and pericycle), as well as the so-called irregular meristems, developing under certain conditions (callus, meristems of symbiotic nodules, spontaneous and pathogen-induced tumors, etc.). Specific mechanisms of regulation, which are based on the interaction of plant hormones and the major groups of transcription factors, were identified for each meristem. The activity of meristems is determined by two opposing processes: proliferation and self-renewal of SCs in the central part of the meristem and differentiation of specialized cells in the periphery. WOX-CLAVATA systems are a regulatory component conservative for different meristems, which ensures the consistency of the composition of the meristem, as well as the balance of SC proliferation and differentiation. In this review, we consider the similarities and differences between the principles of the organization of SC niches in plants and animals, as well as in a variety of meristems of higher plants; special attention will be paid to the role of WOX-CLAVATA systems in maintaining meristems and their interaction with other meristem regulators.

The gene encoding the transcription factor LEAFY was identified in the genome of the mycoheterotrophic plant, pinesap Monotropa hypopitys. In the transcriptomes of roots, bracts, and flowers of flowering pinesaps, the MhyLFY gene expression was absent. These data suggest the conservativeness of the LFY-dependent mechanism of flower meristem identity and flower formation in heterotrophic species with some differences associated to the specificity of development and the structure of such plants. The pinesap flowering under the control of the transcription factor MhyLFY may be initiated either in an embryonic inflorescence during spring dormancy release of adventitious root buds or in an inflorescence of a growing reproductive stem after photoperiodic induction.

The paper described a new mutation that causes the development of multiple meristematic foci as part of shoot apical meristem, which can give rise to new stem axes or cause stem fasciation. The wus-1 mutation represses development of additional apical meristem in fas5 mutant, indicating to the sequential action of the genes in the formation of the shoot apical meristem and FAS5 gene participation in spatial restriction of the WUS gene expression. This function gene FAS5 performs independently of other negative regulators of WUS gene--namely genes CLV, as demonstrated by additive phenotype of double mutants fas5 clv2-1 and fas5 clv3-2. Besides the effect on the development of the shoot apical meristem fas5 mutation causes a change in the shape and number of leaves, accelerates the plant transition to the reproductive stage and leads to the development of cell neoplasms on the stem (buds, stigmatic tissues and ovule-like structures). The mutation also causes changes in apical meristems and leaf cell morphology indicating the activation in cells of DNA endoreduplication. Pleiotropic effect of the fas5 mutation on different stages of ontogeny and different organs suggests that the FAS5 gene plays a complex regulatory role at all stages of the A. thaliana shoot development, and affects many direct or indirect target genes.

Identification of new genes involved in the control of flower initiation and development, is an important problem of the plant developmental genetics. Central approach to solve it is the study of mutants with changes in these characters. The effect of pleiotropic mutation fasciata5 on the transition to the reproductive stage and flower development was studied. By analyzing double mutants we identified interactions of FASCIATA5 gene with LEAFY, APETALA1 and APETALA2, which control the floral meristem identity. The results indicate an important role of gene FASCIATA5 in upregulation of these genes.

In recent years, the role of WOX genes encoding homeodomain transcription factors in the development of the apical meristem of shoots and roots has been actively investigated. However, the role of WOX genes in the control of the cell proliferation in other meristem types is poorly studied. In our work, we have studied the role of the WOX5 gene in the development of the meristem in nitrogen-fixing nodules developing on the roots of legumes in a symbiosis with rhizobia. We have shown that the WOX5 gene is involved in the development of the nodule meristem in legumes, have quantitatively evaluated the gene’s expression at different nodule formation stages, and have studied the localization of its expression using a construct containing the WOX5 promoter fused with a reporter gene. The role of the WOX5 transcription factor in the nodule organogenesis and its possible interaction with the hormonal system in the course of the nodule development has been discussed.

A new morphogenetic mutation of the shoot, FLORAL MERISTEM UNDETERMINATION, was found in Papaver somniferum L. with monocarpic shoot. The expression of the DFM (determination of floral meristem) gene, which limits the proliferative activity of stem cells in the floral meristem, was affected. The mutation displayed spontaneous phenotypic instability in ontogeny, variation in the mutant character expression on different flowers of the same plant in the same genotypic environment. The mutation phenotype varied from no expression or formation of individual phyllomes in the center of the primary ovary to formation of a new flower and a new capsule with viable seeds.