In northeast China, bean is one of the commonly used vegetable species with the highest nutritional content [22]. Because of the low latitude in northeast China, the growth of spring bean seedlings is easily affected by chilling injury, which severely limits bean production. We investigated the resistance mechanism of common bean to chilling stress by comparing the cold tolerance of early type variety 120 with the that of the cold-sensitive variety 093.
Photosynthetic indices of 093 were greater under non-chilling stress, but the indices of the two varieties declined to about the same level under chilling stress. The decrease in photosynthetic indices could be linked to biofilm system damage caused by low temperature, which reduces the flexibility of the cell membrane, impacting photosynthetic reactions on the thylakoid membrane [23]. MDA, a result of membrane lipid peroxidation, can be used to assess cell membrane damage [24]. Under chilling stress, the MDA content of 093 increased significantly, while that of 120 was almost changed, which was verified by the fact that the photosynthetic index of 093 changed more dramatically. Transcriptome results showed that Rieske FeS protein (petC) expression of the b6f complex was downregulated in both 093 and 120 photosynthetic systems under chilling stress. Studies have shown that petC of the Arabidopsis thaliana cytochromic b6f complex was overexpressed. The petC gene can increase electron transfer efficiency [25], whereas its deletion leads to the blockage of electron flow, and eventually plant death [26]. The change in the petC expression level may be one of the reasons leading to the decline in photosynthetic indices (Fig. 10).
In addition to the common reactions of lipids to cold, colnelenic acid (CnA) accumulation was found in 093 and 120 under chilling stress. Some studies have shown that CnA accumulates in potato leaves after fungal infection, indicating that it plays a major role in potato resistance to fungal infection [27]. CnA is also believed to play a role in chilling stress resistance in common bean, but additional studies are warranted to confirm this belief. In addition to accumulation of lipids and their derivatives, the accumulation of different amino acids was dramatically upregulated during chilling stress. Moreover, alkaline amino acids were found to be more enriched in 093 than in 120, while most acidic amino acids were enriched in 120. Researchers have not focused on different acidic and basic amino acid buildup in 093 and 120. Proline is essential for plant resilience to chilling stress because it plays various roles such as an osmotic regulator, a protein and plasma membrane stabilizer, an osmotic pressure-related gene inducer, a reactive oxygen scavenger, and a stable source of nitrogen and carbon during late recovery growth [28]. In this experiment, proline content in 093 and 120 increased significantly under chilling stress. Lipid peroxide damage in primary and secondary lysosome membranes, causing the body to dissolve enzyme containing released as hydrolysis acid phosphatase, to cells and mitochondrial membrane damage important organelles, methionine through various ways to Fight the damage [29]. Under chilling stress, methionine content in 120 was clearly enhanced, whereas that in 093 was not enriched. One of the reasons for the differences in chilling stress tolerance between 093 and 120 could be differential methionine enrichment.
Flavonoids, a class of secondary plant phenols, have significant antioxidant and chelating properties [30]. The protective effects of most flavonoids in biological systems are attributable to their ability to transfer electrons, scavenge free radicals, and chelate metals[31] to activate antioxidant enzymes [32], reduce α-tocopherol free radicals [33], and inhibit oxidases, respectively. In particular, flavonoids play a defensive role in abiotic stress. Quercetin is a flavonoid compound with strong antioxidant capacity. Under chilling stress, quercetin was significantly accumulated in 093 and 120. Several flavonoids such as naringenin-7-O-glucoside were only upregulated in 120, and the differential expression of these flavonoids may be the reason for the different tolerance of 093 and 120 to chilling stress. Enrichment analysis of DEGs showed that several flavonoid metabolism-related genes were upregulated under chilling stress, such as dihydroflavanol oxidreductase (phvul.009G04460), chalone isomerase (phvul.009G143100), and flavonoid 3-hydroxylase (phvul.003G261900) (Fig. 8d).
In the chilling stress response, plant endogenous hormones change. showed that after 1 day of low temperature domestication, ABA content in winter wheat increased rapidly, the cytokinin level with biological activity decreased, the GA level with abiotic activity increased, and the content of the ethylene precursor amino cyclopropanecarboxylic acid increased. After 3–7 days of acclimation, the content of ABA, cytokinin, and GA decreased, whereas the content of salicylic acid and JA began to increase [34]. After hormone treatment, tea plant [35] and A. thaliana [36] exhibited enhanced low temperature tolerance. Studies have shown that ABA plays a key role in plant response to chilling stress [37–39]. In 093 and 120, the expression of a short-chain alcohol dehydrogenase (ABA2) (phvul.005g031600), which is a cytosolic short-chain dehydrogenase/reductase involved in the conversion of xanthoxin to ABA-aldehyde, was upregulated during ABA biosynthesis under chilling stress. Moreover, the expression of 8-hydroxylase (phvul.003g278400), which is a protein with ABA 8'-hydroxylase activity and is involved in ABA catabolism, was downregulated during chilling stress. This indicated that compared with 093, ABA anabolism in 120 was inhibited and ABA catabolism was promoted (Fig. 11), thereby keeping the ABA content in 120 lower. We also found that the expression level of ent-Kaurene oxidase, which is involved in GA metabolism, was higher in 093 (Fig. 12). This protein is a member of the CYP701A cytochrome p450 family that is involved in later steps of the GA biosynthesis pathway. JAs are synthesized from αlinolenic acid in the chloroplast membrane as a substrate [40]. Several genes in the JA biosynthesis pathway were differentially expressed in 093 and 120 (phvul.010G135800, phvul.005G156900, phvul.007g055600, phvul.002G228700, phvul.002G043000). The expression levels of these genes were all higher in 093 than in 120 (Fig. 13). The differential expression of different hormones in 093 and 120 may affect the tolerance of common bean to low temperature stress.