Narrowing Down a Major QTL Region Conferring Pod Fiber Contents in Yardlong Bean (Vigna unguiculata), a Vegetable Cowpea

Yardlong bean (Vigna unguiculata (L.) Walp. ssp. sesquipedalis), a subgroup of cowpea, is an important vegetable legume crop of Asia where its young pods are consumed in both fresh and cooked forms. Pod fiber contents (cellulose, hemicellulose and lignin) correlates with pod tenderness (softness/hardness) and pod shattering. In a previous study using populations derived from crosses between yardlong bean and wild cowpea (V. unguiculata ssp. unguiculata var. spontanea), three major quantitative trait loci (QTLs), qCel7.1, qHem7.1 and qLig7.1, controlling these fibers were identified on linkage group 7 (cowpea chromosome 5) and are co-located with QTLs for pod tenderness and pod shattering. The objective of this study was to identify candidate gene(s) controlling the pod fiber contents. Fine mapping for qCel7.1, qHem7.1 and qLig7.1 was conducted using F(2) and F(2:3) populations of 309 and 334 individuals, respectively, from the same cross combination. New DNA markers were developed from cowpea reference genome sequence and used for fine mapping. A QTL analysis showed that in most cases, each pod fiber content was controlled by one major and one minor QTLs on the LG7. The major QTLs for cellulose, hemicellulose and lignin in pod were always mapped to the same regions or close to each other. In addition, a major QTL for pod shattering was also located in the region. Although there were several annotated genes relating to pod fiber contents in the region, two genes including Vigun05g266600 (VuBGLU12) encoding a beta glucosidase and Vigun05g273500 (VuMYB26b) encoding a transcription factor MYB26 were identified as candidate genes for the pod fiber contents and pod shattering. Function(s) of these genes in relation to pod wall fiber biosynthesis and pod shattering was discussed.