Abstract

Stem internodes of bioenergy sorghum inbred R.07020 are longer at high plant density (shade) than at low plant density (control). Initially, the youngest newly‐formed subapical stem internodes of shade‐treated and control plants are comparable in length. However, full‐length internodes of shade‐treated plants are three times longer than the internodes of the control plants. To identify the early molecular events associated with internode elongation in response to shade, we analyzed the transcriptome of the newly‐formed internodes of shade‐treated and control plants sampled between 4 and 6 hr after the start of the light period (14 hr light/10 hr dark). Sorghum genes homologous to the Arabidopsis shade marker genes ATHB2 and PIL1 were not differentially expressed. The results indicate that shade signals promote internode elongation indirectly because sorghum internodes are not illuminated and grow while enclosed with leaf sheaths. Sorghum genes homologous to the Arabidopsis morning‐phased circadian clock genes LHY, RVE, and LNK were downregulated and evening‐phased genes such as TOC1, PRR5, and GI were upregulated in young internodes in response to shade. We hypothesize that a change in the function or patterns of expression of the circadian clock genes is the earliest molecular event associated with internode elongation in response to shade in bioenergy sorghum. Increased expression of CycD1, which promotes cell division, and decreased expression of cell wall‐loosening and MBF1‐like genes, which promote cell expansion, suggest that shade signals promote internode elongation in bioenergy sorghum in part through increasing cell number by delaying transition from cell division to cell expansion.

Highlights

  • Bioenergy sorghum accumulates ~80% of its biomass in its stem (McKinley, Olson, et al, 2018)

  • | 3 shade signals in adult plants, we investigated the shoot elongation response of bioenergy sorghum inbred line R.07020 to high plant density treatments beginning at 60 days after planting

  • Our study revealed that internode elongation in R.07020 during the vegetative phase is a response to high plant density, a typical growth response of plants anticipating shading by their neighbors known as the shade avoidance response (Casal, 2013; Martinez-Garcia et al, 2010; Smith & Whitelam, 1997)

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Summary

| INTRODUCTION

Bioenergy sorghum accumulates ~80% of its biomass in its stem (McKinley, Olson, et al, 2018). Since the length of the youngest newly-formed subapical internode in the shade-treated plants was comparable to the corresponding internode in the control plants (Figure 3b), the identification of differentially expressed genes (DEGs) and associated developmental changes that distinguish these internodes could potentially identify key early regulators promoting internode elongation in response to shade signals. Differential expression of SbLHY and SbTOC1 early during the light period suggests that these genes play a role in regulating internode elongation in response to shade in the bioenergy sorghum plants. The BT2 gene mediates response to hormones, sugar, and nutrients (Mandadi et al, 2009) Another evening expressed sorghum gene, Sobic.001G138000.1, homologous to the Arabidopsis THIAMIN C SYNTHASE (THIC) gene that encodes a protein involved in thiamin biosynthesis was upregulated in the newly-formed internode of shade-treated sorghum plants (Figure 7f). Shading downregulated the expression of morning genes and upregulated the expression of evening genes around 4 hr after the start of the 14-hr-long light period

| DISCUSSION
Findings
| CONCLUSION
| MATERIALS AND METHODS
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