Evaluation of cold resistance in hybrid population
Winter freezing injury is an important factor that restricts the growth of grape in the north of our country, which causes the decrease of grape yield and quality and even the economic loss of fruit farmers [19]. Therefore, it is very important for the development of grape industry in the north of our country to select high-quality and cold-resistant grape varieties. Wang evaluated the cold hardiness of V. vinifera hybrid germplasm and found that Chinese grape growing areas could be divided into 4 cold hardiness regions according to the lethal temperature (LT50), the LT50 of different germplasm resources in the same producing area was different [20]. The cold hardiness of hybrid progenies of 'Beihong' and '7-11-49' was comprehensively evaluated by Wang [21]. The results showed that the cold hardiness of hybrid progenies was segregated and there was a phenomenon of superparent inheritance. The cold-resistance phenotype of 'Shuangyou' and ' Italian Riesling ' hybrids showed normal distribution, and the cold-resistance traits were controlled by multiple genes [22]. In this study, the LT50 of the progenies of V. vinifera intraspecific hybrids was determined, and the cold hardiness grades were classified by the subordinate function method. It was found that the cold hardiness of the hybrid population was a continuous partial normal distribution, and it belonged to the super-parent quantitative trait inheritance, this is consistent with previous studies. Selection of varieties with high cold resistance as intermediate materials in cross breeding and introduction of molecular breeding can speed up the breeding process. Therefore, this study screened breeding materials with high cold resistance, combined with molecular breeding, this study provides a basis for mining the low-effective cold-resistance genes in V. vinifera.
Cold resistance and secondary metabolite
Plants can defend against cold through a multi-layered defense strategy, in which secondary metabolite has an important impact on the cold-resistant response of plants [23, 24]. The release of secondary metabolite terpenoids can be induced by conditioned stresses such as light and low temperature, leading to plant defense responses and plant-plant interactions [25]. These compounds have a wide range of functions in plant responses to cold and are also important components of wine aroma [26]. Among them, (E)-8-carboxylinalool, which is catalyzed by the multifunctional single-titritol oxidase CYP76F14, is a key precursor of wine aroma [27, 28]. Carotenoid synthesis pathway of terpenoid Dxs in different colors of mature pepper fruit, Berry revealed a correlation between carotenoid content and Dxs transcriptional levels [29]. Simpson and Kevin studied the major flow control steps in the methyl erythritol 4-phosphate (MEP) pathway, increased Dxs expression levels result in increased abundance of gene transcripts for carotenoid pathway major rate-determining enzymes [30]. Genetically modified crops accumulate different levels of isoprene, which is one of the rate-limiting steps in the catalytic MEP biosynthesis pathway [31]. In this study, CYP76F14 and Dxs genes were differentially expressed in both C and D groups, and the expression level in Group C was significantly higher than that in Group D, therefore, these two genes can be regarded as the key candidate genes for improving cold resistance of V. vinifera.
Cold resistance and lipid metabolism
Low temperature is a key environmental stress factor that restricts plant growth and has a significant impact on its quality and yield [32, 33]. In order to relieve cold stress, a series of physiological and biochemical and complex molecular regulation mechanisms occur in plants [34]. The importance of plant membrane and stored lipid levels in improving cold resistance and cold germination of seeds has been extensively studied in a range of plant species, storage lipid components are an energy source for plant growth [35]. In the study of the regulatory mechanism of cold resistance in gramineous plants, lipids and lipid molecules were found to be the most important metabolites, accounting for 14.26% of the total metabolites [36]. Peroxidase (ACOX) gene encoding acyl-COA oxidase was characterized by Joo [37]. The results showed that this gene is a key rate-limiting enzyme for the fatty acid metabolism of domperidone, fatty acid oxidation is significantly promoted by upregulating the expression of ACOX1 [38, 39]. In this study, ACOX1 and ACOX3 were only expressed in Group D, but not in Group C, which was consistent with previous studies. The expression of ACOX1 and ACOX3 decreased the synthesis of lipids, which affected the cold resistance of plants.
Cold hardiness and carbohydrates
Plants improve winter viability by stimulating photosynthesis and carbohydrate metabolism [40], and soluble carbohydrates play an important role in improving plant cold hardiness [41]. Studies on the molecular mechanisms of cold resistance in banana mutants have shown that mutants with significant cold resistance enhance carbohydrate biosynthesis [24]. Yan revealed the resistance mechanism of tea plants to freezing stress, elucidating the cryoprotective effect of soluble carbohydrates at lower temperatures [23], with increased carbohydrate concentrations at cold-resistant sites in plants, whereas the concentration at cold-resistant sites decreased [42]. 3-dehydroquinate dehydratase/shikimate dehydrogenase (DHQ-SDH) in higher plants is a cold-regulated gene with up-regulated expression under low temperature conditions [43]. In this study, the expression level of DHQ-SDH gene in the cold-resistant group was significantly higher than that in the sensitive group, indicating that this gene could help plants form self-protection under cold stress. ADP glucose pyrophosphatase (GLGC) shows high expression levels in the starch synthesis pathway in rice seeds [44], and overexpression in cassava roots increases starch accumulation and its subsequent hydrolysis to sugars [45, 46]. In this study, the gene expression in the two comparison groups showed significant difference, Group C was significantly higher than Group D, may be used as a candidate gene for cold resistance in V. vinifera.
Cold resistance and transcription factor regulation
Transcription factors are major regulators that control plant responses to external stimuli, and the ability of plants to evolve resistance after exposure to harsh environments [47]. MYB transcription factors play an important role in abiotic tolerance in plants [48], reducing the accumulation of reactive oxygen species in turn increases cold resistance [49–51]. Gene regulation is determined by the combination of transcriptional regulators present on specific cis-regulatory elements at specific times [52], and MADS family transcription factors play important roles in regulating plant growth and signal transduction [53]. Overexpression of MADS in Arabidopsis thaliana enhances adaptation to abiotic stresses (drought, low temperature) [54], the expression changes of MADS transcription factors in winter wheat from cold high altitude areas also showed a similar trend to that of cold resistance [55], but have not been reported in grapes. In this study, 3 transcription factor family genes were identified, which can be used as candidate genes for exploring cold resistance in V. vinifera.