THE BIOSYNTHESIS REGULATION OF POTATO STEROIDAL GLYCOALKALOIDS

Potato steroidal glycoalkaloids (SGAs) compose a part of plant immunity. Some of their modified variants are toxic to humans. In the course of potato domestication, plants with a lower SGA level were selected. The advent of approaches for manipulation with the regulation of metabolic pathways provides an opportunity to overcome the undesirable direct relationship between the potato resistance to pests and the toxicity of its tubers. However, for such a fine regulation, a deep knowledge of the regulatory network of potato SGA biosynthesis is required. The purpose of this review is to summarize the information on the known SGA biosynthesis genes in plants and the results of the investigation of these genes in potato, as well as to consider the mechanisms of the SGA protective toxic action against pathogens and pests. The SGA biosynthesis is realized via the cytosolic mevalonate pathway and consists of three stages. The first two stages are required for the synthesis of primary metabolites, and lead to cycloartanol and cholesterol, respectively. Twelve enzymes are involved in the biosynthesis, and the half of them are involved in the biosynthesis of phytosterols, which is a branch of the first stage of this metabolic pathway. In the potato leaves with an excess of phytosterols, the synthesis switches to SGAs, increasing the content of the latter. In tubers, with an excess of SGA precursors, they are involved in the synthesis of lanosterol, supporting in this way the stable level of SGA. The importance of structural genes encoding the enzymes of the first two stages of biosynthesis does not allow us to consider them as a target for knockout in order to reduce the level of SGAs. However, information about the tissue-specific mechanisms of switching between the pathways of synthesis of SGA and other compounds having common precursors with SGAs can be used to manipulate the tissue-specific level of steroidal glycoalkaloids. At the third stage (the synthesis of glycoalkaloids from cholesterol), about 20 enzymes participate. In the potato genome, 14 corresponding genes were identified, 8 of which were studied in detail using reverse genetics approaches. As a promising target for reducing SGA levels in tubers, the genes encoding PGA enzymes (belonging to the CYP72 subfamily cytochrome-P450-dependent monooxygenases catalyzing the conversion of hydrocholesterol to trihydrocholesterol) and SGT (SGA glycosyltransferases that catalyze the conversion of solanidine to its toxic glycosylated derivatives α-solanine and α-chaconine) are considered. Cis-regulatory elements in the promoter regions of some glycoalkaloid biosynthesis genes, including elements responsible for tissue-specific expression, are described. The accumulated information provides the base for creating potato genotypes with tissue-specific regulation of SGAs, in which high levels of SGAs in leaves will remain to protect against pathogens and pests and, at the same time, the synthesis of toxic substances in tubers will be suppresse