Abstract
In sugarcane, invertase enzymes play a key role in sucrose accumulation and are also involved in futile reactions where sucrose is continuously degraded during the pre- and post-harvest period, thereby reducing sugar yield and recovery. Invertase inhibitor (INVINH) proteins play a key role in post-translation regulation of plant invertases through which sucrose hydrolysis is controlled. INVINH proteins are small (18 kDa) members of the pectin methylesterase inhibitor superfamily and they are moderately conserved across plants. In the present study, we identified two INVINH genes from sugarcane, ShINH1 and ShINH2. In silico characterization of the encoded proteins revealed 43% sequence identity at the amino acid level, confirming the non-allelic nature of the proteins. The presence of putative signal peptide and subcellular targeting sequences revealed that ShINH1 and ShINH2 likely have apoplasmic and vacuolar localization, respectively. Experimental visualization of ShINH1–GFP revealed that ShINHI is indeed exported to the apoplast. Differential tissue-specific and developmental expression of ShINH1 between leaf, stalk, flower and root suggest that it plays a role in controlling source-sink metabolic regulation during sucrose accumulation in sugarcane. ShINH1 is expressed at relatively high levels in leaves and stalk compared to flowers and roots, and expression decreases significantly toward internodal maturity during stalk development. ShINH1 is expressed at variable levels in flowers with no specific association to floral maturity. Production of recombinant ShINH1 enabled experimental validation of protein function under in vitro conditions. Recombinant ShINH1 potently inhibited acid invertase (IC50 22.5 nM), making it a candidate for controlling pre- and post-harvest deterioration of sucrose in sugarcane. Our results indicate that ShINH1 and ShINH2 are likely to play a regulatory role in sucrose accumulation and contribute to the improvement of sugar yield and recovery in sugarcane.
Highlights
Sugarcane (Saccharum spp.) is the major source of sucrose worldwide
Both maize (NM001157609) and tobacco (Y12805 & Y12806) Invertase inhibitor (INVINH) genes were used as query sequences, only the maize sequence aligned to sugarcane scaffolds, with three regions of homology identified (Figure S2)
We identified two INVINH gene homologs (ShINH1 and ShINH2) in sugarcane and, for the first time, experimentally demonstrated that sugarcane INVINH proteins are potent invertase inhibitors
Summary
Sugarcane (Saccharum spp.) is the major source of sucrose worldwide. Annually, 1,800 million tons of cane produced in an area of 27 Mha contributes ∼80% of world sugar production (FAOSTAT, 2014). Sucrose metabolism is a complex process in sugarcane involving transfers between a number of compartments for synthesis, transport, and accumulation. The major source tissue for sucrose synthesis is mature leaves and sucrose is subsequently transported to sinks including growing points and the storage tissues of mature internodes where it may be further metabolized or accumulated. During the postharvest period, activation of endogenous invertase enzymes leads to deterioration of sucrose in the cut cane and low sugar recovery (Solomon, 2009). The rise in invertase activity and consequent loss of sucrose become increasingly problematic as the time gap between harvesting and milling increases. Suppression of invertase activity to stabilize sucrose content at cane maturity and after harvest has been investigated as a possible solution (Solomon, 2009)
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