First approach to pod dehiscence in faba bean: genetic and histological analyses

Abstract

Pod dehiscence causes important yield losses in cultivated crops and therefore has been a key trait strongly selected against in crop domestication. In spite of the growing knowledge on the genetic basis of dehiscence in different crops, no information is available so far for faba bean. Here we conduct the first comprehensive study for faba bean pod dehiscence by combining, linkage mapping, comparative genomics, QTL analysis and histological examination of mature pods. Mapping of dehiscence-related genes revealed conservation of syntenic blocks among different legumes. Three QTLs were identified in faba bean chromosomes II, IV and VI, although none of them was stable across years. Histological analysis supports the convergent phenotypic evolution previously reported in cereals and related legume species but revealed a more complex pattern in faba bean. Contrary to common bean and soybean, the faba bean dehiscence zone appears to show functional equivalence to that described in crucifers. The lignified wall fiber layer, which is absent in the paucijuga primitive line Vf27, or less lignified and vacuolated in other dehiscent lines, appears to act as the major force triggering pod dehiscence in this species. While our findings, provide new insight into the mechanisms underlying faba bean dehiscence, full understanding of the molecular bases will require further studies combining precise phenotyping with genomic analysis.

Figure 1

Schematic diagram of the genetic pathway in the dehiscence zone (DZ). Modified from Ballester and Ferrándiz 2017.

Figure 2

Frequency distribution of dehiscence traits recorded in the Vf6 x Vf27 RIL population. (a) Opened Pods (OP) in 2017/18 and 2018/19; (b) Fissured Pods (FS) in 2017/18 and 2018/19; (c) Dehiscent Pods (DP) in 2016/17, 2017/18, 2018/19 and 2019/20. The phenotypic values (mean ± SE) of the parental lines are indicated by arrows.

Figure 3

Linkage map and QTLs for dehiscence traits detected in the Vf6 x Vf27 RIL population. QTL locations are represented with bars (2-LOD interval) and boxes (1-LOD interval). Candidate gene markers are in red. Molecular markers used for map saturation by the Kompetitive Allele Specific PCR (KASP) assay are in green. FS, Fissured pods; DPG, Dehiscent pods in the greenhouse; DPF; Dehiscent pods in the field.

Figure 4

Histological study of pods in the parental faba bean lines VF6 (left) and Vf27 (right). (a) Cross-sections of the distal part ventral suture in Vf6 and Vf27 (d). Detail of the dehiscence zone in Vf6 (b) and Vf27 (e); red arrows indicate differences in the degree of lignification of the VS cell wall. Transverse section of the lateral side of the pod in Vf6 (c) and Vf27 (f). Bars, 0.1 mm (a,c,d,f); 0.05 mm (b, e). DZ, dehiscence zone; EP, epidermis; MS, mesocarp; VS, ventral sheath; VB, vascular bundle; EN, endocarp; LFL, lignified fiber layer.

Figure 5

(a) Schematic representation of a pod transverse section. The blue line indicates lignin deposition. DZ, dehiscence zone. (b) Zones in pods where histological cuts were performed.