Overexpression of Sucrose Phosphate Synthase Enhanced Sucrose Content and Biomass Production in Transgenic Sugarcane

Abstract

Sucrose phosphate synthase (SPS) is a key enzyme in sucrose synthesis, which controls sucrose content in plants. This study was designed to examine the efficacy of the overexpression of SoSPS1 gene on sucrose accumulation and carbon partitioning in transgenic sugarcane. The overexpression of SoSPS1 gene increased SPS activity and sucrose content in transgenic sugarcane leaves. More importantly, the overexpression enhanced soluble acid invertase (SAI) activity concomitant with the increase of glucose and fructose levels in the leaves, whereas sucrose synthase activity exhibited almost no change. In the stalk, a similar correlation was observed, but a higher correlation was noted between SPS activity and sugar content. These results suggest that SPS overexpression has both direct and indirect effects on sugar concentration and SAI activity in sugarcane. In addition, SPS overexpression resulted in a significant increase in plant height and stalk number in some transgenic lines compared to those in non-transgenic control. Taken together, these results strongly suggest that enhancing SPS activity is a useful strategy for improving sugarcane yield.

Keywords: biomass; soluble acid invertase; sucrose; sucrose phosphate synthase; transgenic sugarcane.

Figure 1

Expression of sucrose phosphate synthase (SPS), phosphoenolpyruvate carboxylase (PEPC), and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the leaf of non-transgenic (NT) and transgenic sugarcane lines (SP1, SP3, SP9). (A) Transcript levels of SoSPS1 and Actin (reference control) in the sugarcane lines as determined by RT-PCR. Cycle numbers in PCR were 25 and 20 min for SoSPS1 and Actin, respectively. (C) Protein levels of SPS, PEPC, and Rubisco-large subunit (LSU) detected by immuno-blotting. (B,D) Intensities of the amplified cDNA and protein bands analyzed by ImageJ free software (https://imagej.nih.gov/). The results are expressed as relative values of the control NT (=1.0). Fully expanded two-month-cultivated sugarcane leaves were harvested at daytime and divided into two parts for RNA and protein extraction. One microgram of total RNA was reverse-transcribed to first strand cDNA and used for PCR. Then, 30, 10, and 5 µg of total soluble proteins were subjected for immunoblot analysis for SPS, PEPC, and Rubisco-LSU proteins, respectively.

Figure 2

Activities of SPS (A), soluble acid invertase (SAI) (B), and sucrose synthase (SuSy) (C) in leaves of NT and transgenic sugarcane lines (SP1, SP3, SP9). Total soluble protein was extracted from fully expanded sugarcane leaves as described in the legend of Figure 1. The activities of enzymes were measured as described in Section 4. Values are means ± SD for three independent plants. Asterisks denote statistically significant differences (t-test: p < 0.05).