Origin, morphology, and anatomy of fasciation in plants cultured in vivo and in vitro

Fasciation (or cristation) is a variation in the morphology of plants, characterized by the development of various widened and flattened organs. According to origin, fasciations are classified as physiological or genetic but comparatively little is known on their epigenetic or genetic nature at the molecular level. Physiological fasciations are caused by natural environmental factors or artificial treatments including exogenously applied growth regulators. CLAVATA genes (CLV1, CLV2, and CLV3) have been shown to be the main genetic factors associated with fasciation. Despite the great variety of fasciation-induction factors, fasciations have similar features of development during the first few weeks, i.e., increased mitotic activity and size of the apical meristem and an altered arrangement of cells in the meristematic zones, often leading to an increased number of organs and changes in the plastochron. The enhanced activity of apical meristem and cambium results in a significantly increased circumference of the stem and enlarged proportions of pith and cortical parenchyma, associated with a delayed differentiation of the vascular tissues. An elliptical or irregular shape of the cross section of a fasciated organ corresponds to a similar shape of the vascular cylinder. Later stages of the ontogenic development of fasciations are species-specific, may depend on the origin of fasciation, and in some cases may lead to deviations from the normal structure of the epidermis, shape of leaves, as well as altered development of axillary buds. Studying the causes and patterns of development of fasciations could provide a better understanding of the growth processes in the vegetative apex. Further anatomical and physiological research should focus on the structure and activity of meristems of fasciated shoots, as well as on their transcriptome analysis, in order to better understand the pattern of fasciation development.