Sugar accumulation enhancement in sorghum stem is associated with reduced reproductive sink strength and increased phloem unloading activity

Sweet sorghum has emerged as a promising source of bioenergy mainly due to its high biomass and high soluble sugar yield in stems. Studies have shown that loss-of-function Dry locus alleles have been selected during sweet sorghum domestication, and decapitation can further boost sugar accumulation in sweet sorghum, indicating that the potential for improving sugar yields is yet to be fully realized. To maximize sugar accumulation, it is essential to gain a better understanding of the mechanism underlying the massive accumulation of soluble sugars in sweet sorghum stems in addition to the Dry locus. We performed a transcriptomic analysis upon decapitation of near-isogenic lines for mutant (d, juicy stems, and green leaf midrib) and functional (D, dry stems and white leaf midrib) alleles at the Dry locus. Our analysis revealed that decapitation suppressed photosynthesis in leaves, but accelerated starch metabolic processes in stems. SbbHLH093 negatively correlates with sugar levels supported by genotypes (DD vs. dd), treatments (control vs. decapitation), and developmental stage post anthesis (3d vs.10d). D locus gene SbNAC074A and other programmed cell death-related genes were downregulated by decapitation, while sugar transporter-encoding gene SbSWEET1A was induced. Both SbSWEET1A and Invertase 5 were detected in phloem companion cells by RNA in situ assay. Loss of the SbbHLH093 homolog, AtbHLH093, in Arabidopsis led to a sugar accumulation increase. This study provides new insights into sugar accumulation enhancement in bioenergy crops, which can be potentially achieved by reducing reproductive sink strength and enhancing phloem unloading. 64 Samples from the stem and a leaf from two genotypes (D and d), a control and decapitation treatment, and two lengths of treatment before collecting (3d and 10d).

甜高粱（Sweet sorghum）已成为极具应用前景的生物能源作物，这主要归功于其茎秆较高的生物量与可溶性糖产量。研究发现，功能丧失型Dry位点（Dry locus）等位基因在甜高粱驯化过程中受到选择；而摘心处理可进一步促进甜高粱的糖分积累，这表明提升糖分产量的潜力尚未得到充分发掘。为最大化糖分积累，除明确Dry位点的调控作用外，我们还需更深入地解析甜高粱茎秆中可溶性糖大规模积累的分子机制。我们针对Dry位点携带突变型（d，茎秆多汁、叶中脉呈绿色）与功能型（D，茎秆干燥、叶中脉呈白色）等位基因的近等基因系（near-isogenic lines），开展了摘心处理后的转录组分析（transcriptomic analysis）。分析结果显示，摘心会抑制叶片的光合作用，但会加速茎秆中的淀粉代谢进程。SbbHLH093的表达水平与糖分含量呈负相关，这一相关性在不同基因型（DD vs. dd）、处理组（对照组 vs. 摘心组）以及花后发育阶段（3d vs.10d）中均得到验证。Dry位点基因SbNAC074A以及其他程序性细胞死亡（programmed cell death）相关基因的表达会被摘心下调，而编码糖转运蛋白（sugar transporter）的基因SbSWEET1A则会被诱导表达。通过RNA原位杂交（RNA in situ assay）实验，我们在韧皮部伴胞（phloem companion cells）中检测到了SbSWEET1A与转化酶5（Invertase 5）的表达。拟南芥（Arabidopsis）中SbbHLH093的同源基因AtbHLH093功能丧失后，会导致植株糖分积累量上升。本研究为生物能源作物的糖分积累提升提供了新的思路，该策略可通过降低生殖库强度、增强韧皮部卸载来实现。本研究共采集64份样本，涵盖两种基因型（D与d）的茎秆与叶片样本、对照组与摘心处理组样本，以及两个处理后采集时间点（花后3天与10天）的样本。