Pod color is a major economic trait of snap beans (Phaseolus vulgaris L.), among which the pod with a purple stripe is more attractive to people. A stable purple mutant with purple stripes on the pods was obtained by artificial mutagenesis with the high generation snap bean inbred line ‘A18-1’. In order to reveal the genetic factors and pathways responsible for the purple appearance in snap bean, we performed transcriptome and metabolome analyses using the green stem and yellow pod cultivar ‘A18-1’ and its purple mutant ‘pv-pur’ via ⁶⁰Co-γ radiation. Transcriptome analysis showed that three genes in the anthocyanin biosynthetic pathway were differentially expressed, among which the expression level of F3′5′H (Phvul.006G018800) was increased in the mutant ‘pv-pur’, while expression of F3′H (Phvul.004G021200) and ANS (Phvul.002G152700) was downregulated. Anthocyanin-targeted metabonomics analysis showed significant differences in the contents of 10 metabolites between the wild type and mutant plants. Combined analysis of transcriptome and metabolomics showed that one differential metabolite, delphinidin, was related to the differential expression of Phvul.006G024700, Phvul.002G152700, and Phvul.006G018800. Based on the levels of six anthocyanins in wild type and mutant plants, we speculative that the purple appearance of the mutant ‘pv-pur’ is caused by the increased expression of F3′5′H (Phvul.006G018800), the key enzyme in the transformation from dihydroflavanol (DHK) to dihydromyricetone (DHM) in the anthocyanin biosynthetic pathway. The results lay a foundation for further studies on the molecular mechanism of anthocyanin synthesis in snap bean, and provide a framework for breeding different colors of snap bean.

Background and aims: Snap bean is a widely planted legume vegetable crop in the world, with tender pods as its edible organ. In the actual production process, excessive fiber content in bean pods can lead to a deterioration in their taste after cooking, seriously affecting their edible quality. In order to better study the mechanism of fiber formation in bean pod walls, we selected a bean pod fiber mutant (bfm) from the bean mutant library. Methods: Fiber content determination, cytological observation, and transcriptome and metabolome analysis were performed on the pod walls of mutant bfm and its wild-type at different developmental stages. Key results: The results showed that the crude fiber content of the bfm pod wall tissue was significantly higher than that of the wild type during the mature commercial pod stage, and cellulose may be the main factor causing the increase in fiber content in the bean pod wall. During the mature commercial pod stage, the number of cells in the pod wall tissue of bfm increased significantly compared to the wild type, with an increase in phloem fibers and thicker cell walls. It is speculated that this situation led to changes in fiber content in the mutant bfm. The combined analysis of transcriptome and metabolome showed that differentially expressed genes and metabolites were enriched in metabolic pathways and secondary metabolite biosynthesis pathways. Several metabolites, including glycine, L-glutamine, D-arabinitol, and D-ribose, were associated with the expression of Phvul.007G077800 (CTL) and Phvul.003G089600 (KOR). Conclusions: These genes and metabolites may participate in coordinated metabolic pathways that influence the availability of substrates and energy required for cellulose biosynthesis, thereby potentially contributing to cellulose accumulation in the pod wall. This study provides theoretical research on the mechanism of fiber synthesis in bean pod walls, and also to provide some reference for bean breeding improvement and application practice.

The yellow-to-green mutation in plant leaf color can be easily identified and used as a marker trait for seed purity identification, improved variety breeding, hybrid purity identification, and hybrid production. In this study, the leaves of yellow-to-green mutant ytg-1 of snap bean were selected as the experimental material, and the physiological mechanism underlying the leaf color change was studied. The mechanism was observed to belong to the lack of recovery type in the total chlorophyll stage. The decrease in chlorophyll content was due to the inhibition of the synthesis of Proto IX to Mg Proto IX. Genetic analysis revealed that a single recessive gene, pvytg-1, controlled the mutation trait. The gene was located in the 80-kb region of chromosome 10. Overall, six genes were observed within this interval, and based on gene functional annotation, Phvul.010G041700 was identified as the candidate gene. Sequencing and identification of Phvul.010G041700 revealed a single base insertion in ytg-1 compared with the wild-type, resulting in premature termination of the gene. The results of this study will facilitate the breeding and genetic improvement of snap bean in the future.