Genetic mapping of loci affecting embryogenic callus formation and in vitro regeneration in cereals and leguminous crops

Recalcitrance is defined as the inability of plant species or individual genotypes to effectively regenerate and/or to be transformed in in vitro culture, and is the most significant limitation for genome editing of agricultural crops. To develop protocols for genotype-independent transformation and regeneration of cultivated plants, knowledge of the genetic factors that determine recalcitrance in various plant species under in vitro conditions is required. Their search by classical QTL mapping in populations segregating for callus formation efficiency, regeneration, and transformation is considered a complex and labor-intensive process due to a specific nature of the analyzed phenotypes and a strong genotype-environment relationship. The article provides an overview of the methodology, prospects, and most outstanding achievements of “forward” genetics in identifying genetic determinants of recalcitrance in the most popular and at the same time most difficult to work with in vitro cereal and legume crops. Examples of genetic mapping and successful cloning of genes responsible for various aspects of recalcitrance in cereals are discussed. Thus, it was found that the formation of rapidly proliferating type II embryogenic callus in maize is determined by increased expression of the Wox2a gene. The Koshihikari rice variety, popular in Japan, poorly regenerates in vitro due to impaired nitrate metabolism, since it has a low expression level of nitrite reductase (NiR), which converts nitrite into ammonia. Callus browning, which occurs among many plant species and leads to a decrease in regenerative capacity and even to plant death, in rice varieties (Oryza sativa ssp. indica) depends on the expression level of the Browning of Callus1 (BOC1) gene, which encodes the SRO protein (Similar to RCD One), regulating the plant response to oxidative stress. Similar studies on mapping loci for somatic embryogenesis traits in soybean have revealed major QTLs explaining 45 and 26 % of phenotypic variation. Studies on genetic mapping of loci affecting the efficiency of regeneration and embryogenesis in recalcitrant plant species have obvious prospects due to the emergence of annotated reference genomes, high-throughput genotyping and high-resolution genetic maps.

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