Lower soil moisture threshold for transpiration decline under water deficit correlates with lower canopy conductance and higher transpiration efficiency in drought-tolerant cowpea.

As water availability is critical for reproduction, terminal drought tolerance may involve water-saving traits. Experiments were undertaken under different vapour pressure deficit (VPD) and water regimes (water stress (WS) and well watered (WW)) to test genotypic differences and trait relationships in the fraction of transpirable soil water (FTSW) at which transpiration declines, canopy conductance (proxied by transpiration rate (TR, g H2Ocm-2h-1)), canopy temperature depression (CTD, °C), transpiration efficiency (TE, gkg-1) and growth parameters, using 15 contrasting cowpea (Vigna unguiculata (L.) Walp.) genotypes. Under WW conditions at the vegetative and early podding stages, plant mass and leaf area were larger under low VPD, and was generally lower in tolerant than in sensitive genotypes. Several tolerant lines had lower TR under WW conditions and restricted TR more than sensitive lines under high VPD. Under WS conditions, transpiration declined at a lower FTSW in tolerant than in sensitive lines. Tolerant lines also maintained higher TR and CTD under severe stress. TE was higher in tolerant genotypes under WS conditions. Significant relationships were found between TR, and TE, CTD and FTSW under different water regimes. In summary, traits that condition how genotypes manage limited water resources discriminated between tolerant and sensitive lines. Arguably, a lower canopy conductance limits plant growth and plant water use, and allows tolerant lines to behave like unstressed plants until the soil is drier and to maintain a higher TR under severe stress, as lower TR at high VPD leads to higher TE.

A SNP and SSR based genetic map of asparagus bean (Vigna. unguiculata ssp. sesquipedialis) and comparison with the broader species.

Asparagus bean (Vigna. unguiculata ssp. sesquipedialis) is a distinctive subspecies of cowpea [Vigna. unguiculata (L.) Walp.] that apparently originated in East Asia and is characterized by extremely long and thin pods and an aggressive climbing growth habit. The crop is widely cultivated throughout Asia for the production of immature pods known as 'long beans' or 'asparagus beans'. While the genome of cowpea ssp. unguiculata has been characterized recently by high-density genetic mapping and partial sequencing, little is known about the genome of asparagus bean. We report here the first genetic map of asparagus bean based on SNP and SSR markers. The current map consists of 375 loci mapped onto 11 linkage groups (LGs), with 191 loci detected by SNP markers and 184 loci by SSR markers. The overall map length is 745 cM, with an average marker distance of 1.98 cM. There are four high marker-density blocks distributed on three LGs and three regions of segregation distortion (SDRs) identified on two other LGs, two of which co-locate in chromosomal regions syntenic to SDRs in soybean. Synteny between asparagus bean and the model legume Lotus. japonica was also established. This work provides the basis for mapping and functional analysis of genes/QTLs of particular interest in asparagus bean, as well as for comparative genomics study of cowpea at the subspecies level.

A multicystatin is induced by drought-stress in cowpea (Vigna unguiculata (L.) Walp.) leaves.

Cystatins are protein inhibitors of cystein proteinases belonging to the papain family. In cowpea, cystatin-like polypeptides and a cDNA have been identified from seeds and metabolic functions have been attributed to them. This paper describes VuC1, a new cystatin cDNA isolated from cowpea leaves (Vigna unguiculata (L.) Walp.). Sequence analysis revealed a multicystatin structure with two cystatin-like domains. The recombinant VUC1 protein (rVUC1) was expressed in an heterologous expression system and purified to apparent homogeneity. It appeared to be an efficient inhibitor of papain activity on a chromogenic substrate. Polyclonal antibodies against rVUC1 were obtained. Involvement of the VuC1 cDNA in the cellular response to various abiotic stresses (progressive drought-stress, dessication and application of exogenous abscissic acid) was studied, using Northern blot and Western blot analysis, in the leaf tissues of cowpea plants corresponding to two cultivars with different capacity to tolerate drought-stress. Surprisingly, these abiotic stresses induced accumulation of two VuC1-like messages both translated into VUC1-like polypeptides. Difference in the transcript accumulation patterns was observed between the two cultivars and related to their respective tolerance level. Presence of multiple cystatin-like polypeptides and their possible involvement in the control of leaf protein degradation by cysteine proteinases is discussed.