Main conclusionThe review provides information on the mechanisms underlying the development of spontaneous and pathogen-induced tumors in higher plants. The activation of meristem-specific regulators in plant tumors of various origins suggests the meristem-like nature of abnormal plant hyperplasia.

Studying aberrations of normal developmental processes like plant tumour initiation provides a great opportunity for understanding of systemic control of plant cells differentiation. Most well-studied example of plant tumour is the crown gall caused by Agrobacterium tumefaciens; much less studied are spontaneous tumours developing in plants with certain genotype, regardless of agrobacterial infection. In the present study, we used RNA-seq to analyse the changes in gene expression at the early stages of development of A. tumefaciens-induced tumours in radish seedlings. In our studies we used two closely related radish inbred lines contrasting in their ability to form spontaneous tumours. RNA-seq and qPCR analysis showed significant changes in the expression levels for numerous genes involved in different metabolic and developmental processes, and Gene Ontology analysis indicated many biological processes probably involved in the crown gall development in radish. For example, a unique result of our research is upregulation of genes related to photomorphogenesis and downregulation of senescence-associated genes in the young crown gall tumours. Differences in gene expression between the analysed radish lines included upregulation of pathways related to chromatin remodelling and protein targeting to chloroplast in the tumorous line along with downregulation of pathways of chaperone-mediated complex assembly and anthocyanin biosynthesis.

This review highlights the years of research on the genetics of in vitro regeneration in higher plants conducted at the Department of Genetics and Biotechnology of Saint Petersburg State University. The genetic collection of radish (Raphanus sativus) created at the department by selfing of individual plants from three cultivars was used as a model in these studies. Some radish inbred lines from the genetic collection form spontaneous tumors in the roots and are also used to study mechanisms of tumor growth in higher plants. It was revealed that radish lines that differed in the ability to form tumors also contrastingly differed in the reaction of their explants to auxin and cytokinin in vitro, which reflects a difference in the levels of these hormones in the tissues of related tumorous and non-tumorous radish lines. Moreover, high concentrations of cytokinins in cultural medium induced tumor formation in the regenerated plants of tumorous radish lines. The presence of meristematic zones in spontaneous tumors in radish lines, as well as in crown gall tumors induced by Agrobacterium tumefaciens and cytokinin-induced tumors made it possible to reveal the role of the main meristem regulators, such as KNOX and WOX family transcription factors and the CLAVATA system, in both the process of tumor growth and regeneration in plants. Analysis of the expression of meristem-specific genes during the development of spontaneous and induced tumors in radish as well as in regenerated radish plants confirmed this assumption.