Energy Sorghum: A Genetic Model and Commercial Design Platform for C4 Energy Grasses

Energy sorghum is emerging as an excellent genetic reference species for C4 energy grass crops due to its tractable genetics, good genome platform, diverse germplasm, and its commercial use as an annual hybrid energy crop.  Energy sorghum hybrids are four meters tall and accumulate ~20-40 dry metric tons of biomass per hectare, depending primarily on availability of water (Olson et al., 2012).  Increased biomass accumulation by energy sorghum compared to grain sorghum is due to longer duration of growth, higher radiation interception, and increased radiation use efficiency. Energy sorghum is grown using the same N-input as grain sorghum but the energy crop accumulates more than twice the biomass resulting in high nitrogen use efficiency (NUE; biomass/plant N).  High NUE was correlated with a high stem:leaf biomass ratio, stems with low N-content, and efficient N-remobilization from leaves and stems lower in the canopy to growing tissues at the top of the canopy.  NIR analysis of diverse energy sorghum germplasm revealed a wide range of biomass composition, in addition to sweet sorghum’s cane-like ability to accumulate sucrose. The genetic basis of composition variation is being analyzed and used to optimize energy sorghum composition for enhanced conversion to biofuels. Energy sorghum hybrids flower late in the season due to allele complementation that results in increased photoperiod sensitivity of the hybrids relative to parental inbred lines used in hybrid construction. Photoperiod sensitivity in sorghum is regulated by six major loci, termed Ma₁-Ma₆, that delay flowering in long days.  The central genes/proteins in this pathway include the red light photoreceptor phyB (Ma₃), SbPRR37 (Ma₁), a gene encoding a pseudoresponse regulator protein (Murphy et al., 2011) and Ma₆, a repressor that acts together with PRR37 to inhibit flowering in long days.  Expression of SbPRR37 and Ma₆ in leaves is regulated by the circadian clock and light signaling through phyB.  In long days, light and the clock induce peaks of SbPRR37 and Ma₆ expression in the morning and evening.  In short days, plants flower because the clock-gated evening peak of potential SbPRR37 expression occurs in darkness eliminating expression of SbPRR37 and Ma₆ during this time of day and reducing their repressing action on genes that activate flowering (i.e., Ehd1, FT, ZCN8).