Characterization of Drought Tolerance of the Chinese Asparagus Bean Mini-Core and Transcriptional Profiling of Cultivars Representing Different Drought Tolerance Types

Asparagus bean (Vigna unguiculata  ssp. Sesquipedialis L) is a subspecies of cowpea (Vigna unguiculata L. Walp.). It is widely consumed as a vegetable in Asian countries and some regions of Africa and America. Like indigenous African cowpea, asparagus beans are highly drought tolerant and their capacities and patterns of drought tolerance vary by genotype. Through morphological and physiological assessments of the Chinese asparagus bean mini-core with the parameters trifoliate relative water content, stem greenness at early and late stages after water stress, and old-new leaf consistency, we were able to identify both Type I and Type II drought tolerant accessions as previously reported in African cowpea. Type I tolerant lines showed growth cessation in almost all plant tissues, allowing recovery of the shoot following rehydration; on the contrary Type II tolerant lines “sacrificed” old leaves to maintain higher water content and greenness in young leaves. Two lines representing each drought tolerance type were selected for comparison of global transcriptional responses under water stress using a custom microarray designed based on the HarvEST:cowpea unigene assembly P12. Many drought related pathways were found to be shared between the two lines, such as osmotic adjustment, wax accumulation, ion homeostasis and ABA signaling. There were also genes/pathways specific to or differentially regulated between the two lines, for instance, water channel regulation and ethylene signaling. A set of these differentially regulated genes has been verified by real-time RT-PCR. These results provide a basis for fine-scale pinpointing the mechanisms of drought tolerance in asparagus bean/cowpea.