Dhurrin [(S)‐p‐hydroxymandelonitrile‐β‐D‐glucopyranoside] is a cyanogenic glucoside produced by sorghum (Sorghum bicolor L. Moench) and is generally considered a natural defense compound capable of producing the toxin hydrogen cyanide (HCN) to deter animal herbivory. Recently, high levels of leaf dhurrin have been found in grain sorghum genotypes that also exhibit stay‐green during postanthesis water deficit. Post‐flowering drought tolerance (stay‐green) in sorghum is an economically important trait in sorghum production regions where drought stress postanthesis is common. Stay‐green is associated with reduced lodging, charcoal rot resistance, increased grain fill, and increased grain yield. The genetic control of stay‐green is complex, with multiple quantitative trait loci (QTL) being identified in affecting expression of stay‐grain. Here we report the discovery of a dhurrin QTL (Dhu1) on SBI01 using a recombinant inbred line (RIL) mapping population derived from BTx642/Tx7000. Leaf dhurrin was highly heritable and Dhu1 explained a large percentage of the variation of leaf dhurrin in the population. Dhu1 is aligned with genes involved in dhurrin biosynthesis (CYP79A1, CYP71E1, UGT85B1). Protein sequence variants found in CYP71E1 and UGT85B1 could be the cause of the observed differences in leaf dhurrin levels in BTx642, Tx7000, and other sorghum lines that vary in leaf dhurrin content. Dhu1 is also aligned with a previously un‐identified stay‐green QTL (Stg5) on SBI01, consistent with prior studies showing an association between high leaf dhurrin levels, this region of SBI01, and expression of the stay‐green trait.

Sorghum bicolor is a drought-resilient facultative short-day C4 grass that is grown for grain, forage, and biomass. Adaptation of sorghum for grain production in temperate regions resulted in the selection of mutations in Maturity loci ( Ma 1 –Ma 6 ) that reduced photoperiod sensitivity and resulted in earlier flowering in long days. Prior studies identified the genes associated with Ma 1 ( PRR37 ), Ma 3 ( PHYB ), Ma 5 ( PHYC ) and Ma 6 ( GHD7 ) and characterized their role in the flowering time regulatory pathway. The current study focused on understanding the function and identity of Ma 2 . Ma 2 delayed flowering in long days by selectively enhancing the expression of SbPRR37 ( Ma 1 ) and SbCO , genes that co-repress the expression of SbCN12 , a source of florigen. Genetic analysis identified epistatic interactions between Ma 2 and Ma 4 and located QTL corresponding to Ma 2 on SBI02 and Ma 4 on SBI10. Positional cloning and whole genome sequencing identified a candidate gene for Ma 2 , Sobic.002G302700, which encodes a SET and MYND (SYMD) domain lysine methyltransferase. Eight sorghum genotypes previously identified as recessive for Ma 2 contained the mutated version of Sobic.002G302700 present in 80M ( ma 2 ) and one additional putative recessive ma 2 allele was identified in diverse sorghum accessions.

Sorghum bicolor is a drought-resilient facultative short-day C4 grass that is grown for grain, forage, and biomass. Adaptation of sorghum for grain production in temperate regions resulted in the selection of mutations in Maturity loci (Ma1 – Ma6) that reduced photoperiod sensitivity and resulted in earlier flowering in long days. Prior studies identified the genes associated with Ma1 (PRR37), Ma3 (PHYB), Ma5 (PHYC) and Ma6 (GHD7) and characterized their role in the flowering time regulatory pathway. The current study focused on understanding the function and identity of Ma2. Ma2 delayed flowering in long days by selectively enhancing the expression of SbPRR37 (Ma1) and SbCO, genes that co-repress the expression of SbCN12, a source of florigen. Genetic analysis identified epistatic interactions between Ma2 and Ma4 and located QTL corresponding to Ma2 on SBI02 and Ma4 on SBI10. Positional cloning and whole genome sequencing identified a candidate gene for Ma2, Sobic.002G302700, which encodes a SET and MYND (SYMD) domain lysine methyltransferase. Nine sorghum genotypes previously identified as recessive for Ma2 contained the mutated version of Sobic.002G302700 present in 80M (ma2).

Bioenergy sorghum hybrids are being developed with enhanced drought tolerance and high levels of stem sugars. Raffinose family oligosaccharides (RFOs) contribute to plant environmental stress tolerance, sugar storage, transport, and signaling. To better understand the role of RFOs in sorghum, genes involved in myo-inositol and RFO metabolism were identified and relative transcript abundance analyzed during development. Genes involved in RFO biosynthesis (SbMIPS1, SbInsPase, SbGolS1, SbRS) were more highly expressed in leaves compared to stems and roots, with peak expression early in the morning in leaves. SbGolS, SbRS, SbAGA1 and SbAGA2 were also expressed at high levels in the leaf collar and leaf sheath. In leaf blades, genes involved in myo-inositol biosynthesis (SbMIPS1, SbInsPase) were expressed in bundle sheath cells, whereas genes involved in galactinol and raffinose synthesis (SbGolS1, SbRS) were expressed in mesophyll cells. Furthermore, SbAGA1 and SbAGA2, genes that encode neutral-alkaline alpha-galactosidases that hydrolyze raffinose, were differentially expressed in minor vein bundle sheath cells and major vein and mid-rib vascular and xylem parenchyma. This suggests that raffinose synthesized from sucrose and galactinol in mesophyll cells diffuses into vascular bundles where hydrolysis releases sucrose for long distance phloem transport. Increased expression (>20-fold) of SbAGA1 and SbAGA2 in stem storage pith parenchyma of sweet sorghum between floral initiation and grain maturity, and higher expression in sweet sorghum compared to grain sorghum, indicates these genes may play a key role in non-structural carbohydrate accumulation in stems.