Cold Tolerance in Juvenile Sorghum Is Associated with Root Traits and Is Influenced By Accumulation of Soluble Sugars.

Cold tolerance at seedling /juvenile stage is an important and desirable trait in sorghum cultivars grown in northern latitudes.  To address the need for this favorable trait, analysis of the important metabolites during cold stress treatment using a sorghum diversity panel was conducted and results showed association of tolerance to cold stress with anthocyanin transcription factors. In silico analysis of co-expression networks of sorghum myb transcription factors of interest revealed that genes for sugar transport pathways were co-expressed and in the sorghum cold response gene network. To elucidate the role of soluble sugars (monosaccharides) in the thermal tolerance of sorghum seedling, we analyzed sugar content of shoots and roots in four genotypes under controlled conditions at 28°C and 15°C at seven days after germination in roots and shoots. There were no significant differences in the sugar content of shoots, but fructose levels significantly varied in roots under cold stress treatment, distinctly showing higher levels of root fructose content in cold tolerant germplasm. These results suggested the possible role of fructose to provide protective role in sorghum roots under cold stress conditions. To complement the metabolome studies with genomic information global transcriptome analyses of cold treated roots using RNASeq was performed to identify the possible genes associated with differential fructose accumulation. We extended the study to include twenty sorghum germplasm with varying tolerance to cold stress under field and controlled conditions. The extended results showed that root fructose content positively correlates with the field germination (0.67) and vigor (0.55). This study suggests the significant role of root soluble sugars in providing better seedling establishment under early season cold stress.  This also indicates that root fructose could be a candidate biomarker for use in screening sorghum germplasm for cold tolerance levels at early season planting.