Effect of Mechanical Stimulation on the Morphology, Anatomy, Hormone Homeostasis, and Transcriptome of Bioenergy Sorghum Stems

Abstract

Like most crops, sorghum [Sorghum bicolor (L.) Moench] is susceptible to stem lodging, typically caused by mechanical forces associated with severe weather like high winds. However, in response to less severe mechanical stimulation, plants may exhibit alterations in growth and development through a process known as thigmomorphogenesis that enhances the plant’s ability to withstand stronger forces. Understanding mechanisms regulating thigmomorphogenesis may facilitate developing new varieties with greater lodging resistance. This study investigated the effect of mechanical stimulation on the morphology, anatomy, hormone homeostasis and transcriptome expression of sweet sorghum stems and investigated if this effect was dependent on the developmental stage of the internodes or the duration of treatment. Mechanical stimulation reduced internode lengths, which might be associated with reduced number of cells and decreased cell elongation. Mechanical stimulation increased the density and radial length of pith vascular bundles, enhanced the stem lignification level, and also boosted the rind thickness, likely contributing to increased strength and flexibility of stems. Transcriptome profiling of internode rind revealed that mechanical stimulation altered the expression of over 900 genes, including a large number of transcription factors and genes related to hormone signaling. The abundances of IAA, GA1 and ABA generally declined following mechanical stimulation, while JA was elevated. Weighted Gene Co-expression Network Analysis (WGCNA) identified three modules highly correlated with mechanical stimulation and morphological and biomechanical traits, which were enriched in pathways associated with cell wall biology, hormone signaling and general stress responses. Additionally, mechanical stimulation-triggered responses were found to be developmental stage- and dose-dependent. The hormonal responses of different stem tissues including pulvinus, white band, zone of division, and zone of maturation to mechanical stimulation were also studied. JA level was reduced exclusively in the pulvinus, while GA1 levels declined specifically in the zone of division in response to mechanical stimulation. In contrast, the levels of GA20, IAA, and ABA decreased in all tissues following mechanical stimulation. Mechanical stimulation also induced hormonal memory and habituation of sorghum stems. The findings from this study may offer opportunities to improve lodging resistance in sorghum and other crops.