Advances in research on bud mutantation mechanism in grape

Abstract: Grape (Vitis L.) is one of the oldest and economically important fruit crops in the world., and grape fruits are mainly used for fresh consuming (30%), wine making (68%), raisins and juice process-ing (2%). The important grape breeding objectives are early maturity, large fruit, seedless fruit, and rich flavor. There are various grape breeding methods, including hybrid breeding, mutation breeding and bio-technology-based breeding. Many new mutant varieties with novel traits have been generated through bud mutant selection. Therefore, mutant selection is one of the most important breeding methods to ob-tain new cultivars with superior traits in grape. This article focuses on the research progress in the identi-fication of mutants and exploration the mechanism of the bud mutants in grape. Somatic mutants main-ly occur in the cell layer of the apical meristem in grape. The shoot apical meristem (SAM) is composed of two layers: the outer meristem layer (L1), and the second meristem layer (L2). The epidermis of leaves is derived from the L1 layer, L2 layer participates in the regulation of internal organization.Changes of L1 and L2 layer would result in bud mutantation. The prerequisite of bud mutantation is the change of DNA sequence. According to the current research on DNA sequence changes, a SNP could al-ter the expression of a gene, leading to a specific phenotype of cells. DNA methylation is also an impor-tant cause for epigenetics, which alters gene expression without changing the sequence of DNA. The transposon insertions could provide a convenient method to generate tagged null mutants that could be easily identified on a genome-wide scale and are likely to reflect phenotypes arising from common in- dels and point mutants that would result in loss-of-function. The change of DNA sequence would cause morphological characteristics could not achieve a precise result. Thus, various DNA molecular marker technologies based on the DNA sequences, Randomly Amplified Polymorphic DNA (RAPD), Inter-Primer Binding Site (IPBS), Inter-Simple Sequence Repeat (ISSR), Simple Sequence Repeat (SSR), Se-quence Related Amplified Polymorphism (SRAP) have been used to identify sports and their original plants. At the same time, studies have demonstrated that different traits of bud mutant are regulated by different genes, for example, VvMYBA1 and VvMYBA2 transcription factors, could regulate the color of the grape berry, in Vitis vinifera. The glycosylation catalyzed by UDP-glucose, anthocyanidin, flavonoid glucosyltransferase (UFGT) is the key step of anthocyanin biosynthesis. The main reason for the partici-pation of VvmybA1 in the regulation is the insertion of a transposon Gret1 in the coding region of its gene, thereby regulating the expression of genes and participating in the synthesis of anthocyanins, lead-ing to hindering the synthesis of anthocyanins and causing color variations. In the inflorescence and ear variation, the homologous factors MADS-box 1, 2 and 3 of the Arabidopsis floral genes AG, SEP and AGL13 alter their expression patterns during flower development and play a decisive role. The VvT-FL1A gene, a secondary transposon Hatvine1-rrm is inserted in the promoter region to affect the inflo-rescence development at an early stage. Fruit ripening is a complex process involving many highly coor- dinated physiological and biochemical changes. It is found to be involved in the regulation of ethylene and growth during the ripening period. Abnormal expression of the berry and cytokinin genes would de-lay or advance the fruit ripening period. For example, the level of reactive oxygen species in the bud mutant is always higher than that of the original plant. The abnormal expression of the VvMADS9 gene would be the main cause of abnormal berry morphology in the fruit type variation; The VvAGL11 gene would be the main candidate gene for seedless fruits. With the development of high-throughput technol-ogy, it is believed that the mutation mechanism of sports would be more clearer