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One of the directions in plant biotechnological research is the use of callus cultures in vitro. The callus is an integrated system that is formed both exogenously (as a result of the proliferation of surface cells of various tissues of the plant organism) and endogenously (in the depth of these tissues). Initially, callus consists of homogeneous cells gradually transformed into a system of groups of heterogeneous cells with specific morphogenetic potencies. These potencies are realized by various ways of morphogenesis in vitro, including those leading to the formation of full-fledged plants-regenerants. In this review, using cereals as an example, we analyze the literature and our data on the use of calli in vitro as experimental systems for studying the plant stress-resistance to abiotic factors, especially to the drought factor. The advantages and limitations of using callus cultures in vitro are considered. The prospects of studying the mechanisms of action of abiotic stressors and resistance to them at the cellular and tissue levels under model conditions of callus culture in vitro are shown. Attention is paid to the assessment of the drought resistance of cereals under selective conditions simulating drought in vitro by such a parameter as the growth activity of calli. The issue of studying anti-stress effects in callus cultures in vitro is analyzed on the example of brassinosteroids. The prospects of using the model system “immature embryo in vivo–embryonic morphogenic callus in vitro” for rapid assessment of the effect of anti-stress plant growth regulators are considered. It is emphasized that the basis for using calli as the model system is both the significant role of the cell in all morphogenetic events of the plant organism in vivo and in vitro and the similarity of the responses of plants in vivo, calli in vitro, regenerants of callus origin in vitro and ex vitro.

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The use of in vitro callus cultures as experimental model systems allows us to get closer to understanding the patterns and features of morphogenesis in intact plants. In this regard, the problem of realizing the morphogenetic potential of callus cells due to their pluri- and totipotency properties is of great interest. To solve this problem, it is important to use the histological approach, which involves studying the structures of developing tissues, organs and organisms in their interactions and relationships. This review article analyzes data devoted to the study of the histological features of formed primary morphogenic calli (formation of morphogenetic centers and superficial meristematic zones), as well as the in vitro morphogenesis pathways in calli that lead to the formation of regenerants (de novo organogenesis and in vitro somatic embryogenesis). The terminology used is considered. Some questions for discussion are raised. The opinion is expressed that histological (structural) studies should be considered as a methodologic basis for further investigation of various morphogenetic scenarios in in vitro callus cultures, especially in economically valuable plants and for biotechnological purposes.