The characterisation of low temperature tolerance in legumes.

Legumes underpin the global food network, providing the majority of the world’s dietary protein. However, global productivity of grain legumes is limited by environmental stresses, particularly chilling and freezing. For example, chilling temperatures (0-15 ºC) limit the production of soybean (Glycine max; a tropical legume), while extreme freezing temperatures (<0 ºC) limit use of temperate legumes such as faba bean (Vicia faba L). The following studies were performed to gain new insights into chilling and freezing tolerance in these two important legumes. Transgenic soybean lines, overexpressing the rice cysteine protease inhibitor, oryzacystatin I (OCI), showed decreased chillinginduced inhibition of photosynthesis compared to wild type (Wt). These lines also showed an increased abundance of transcripts encoding the strigolactone (SL) biosynthesis enzymes: carotenoid cleavage dioxygenases 7 and 8 (CCD7, CCD8). Pea (rms3, rms4, rms5) and Arabidopsis thaliana (max2-1, max3-9, max4-1) mutants, deficient in SL synthesis and signalling, showed enhanced sensitivity to dark-chilling. Differences in chilling and freezing sensitivity were also identified in 5 faba bean cultivars. Transcriptome profiling comparisons were performed on the most chilling sensitive (Wizard) and most chilling tolerant (Hiverna) cultivars to identify specific differences in gene expression, underpinning stress tolerance. Moreover, genome sequencing of the Wizard cultivar enabled the assembly and annotation of the mitochondrial and chloroplast genomes. Single nucleotide polymorphisms (SNP) were found in several organelle genes, when comparing read sequences to published references. Furthermore, based on published SNP orientated linkage maps, contiguous read sequences could be mapped to chromosomal loci, leading to the identification of 8 putative nuclear gene sequences and an increase in sequence length data at 147 loci. Together with these new genomic resources, the discovery that cysteine proteases, phytocystatins and SL are important in legume low temperature tolerance will enable the development of stress tolerance markers, for use in faba bean selective breeding programs.