Induction of vacuolar invertase inhibitor mRNA in potato tubers contributes to cold-induced sweetening resistance and includes spliced hybrid mRNA variants

Abstract

Cold storage of tubers of potato (Solanum tuberosum L.) compromises tuber quality in many cultivars by the accumulation of hexose sugars in a process called cold-induced sweetening. This is caused by the breakdown of starch to sucrose, which is cleaved to glucose and fructose by vacuolar acid invertase. During processing of affected tubers, the high temperatures involved in baking and frying cause the Maillard reaction between reducing sugars and free amino acids, resulting in the accumulation of acrylamide. cDNA clones with deduced proteins homologous to known invertase inhibitors were isolated and the two most abundant forms, termed INH1 and INH2, were shown to possess apoplastic and vacuolar localization, respectively. The INH2 gene showed developmentally regulated alternative splicing, so, in addition to the INH2α transcript encoding the full-length protein, two hybrid mRNAs (INH2β*A and INH2β*B) that encoded deduced vacuolar invertase inhibitors with divergent C-termini were detected, the result of mRNA splicing of an upstream region of INH2 to a downstream region of INH1. Hybrid RNAs are common in animals, where they may add to the diversity of the proteome, but are rarely described in plants. During cold storage, INH2α and the hybrid INH2β mRNAs accumulated to higher abundance in cultivars resistant to cold-induced sweetening than in susceptible cultivars. Increased amounts of invertase inhibitor may contribute to the suppression of acid invertase activity and prevent cleavage of sucrose. Evidence for increased RNA splicing activity was detected in several resistant lines, a mechanism that in some circumstances may generate a range of proteins with additional functional capacity to aid adaptability.

Allele diversity for the apoplastic invertase inhibitor gene from potato

“…The Plant & Food Research cultivars 'Summer Delight' and 'Karaka' are non-processing cultivars and are susceptible to CIS (Brummell et al 2011).…”

“…The analysis revealed four sequences, StInvInh1, StInvInh2A, StInvInh2B and StInvInh3, with expression studies suggesting an interaction between StInv1 and StInvInh2 that would account for the response to cold-induced sweetening in tubers. Brummell et al (2011) characterised both apoplastic (inh1) and vacuolar invertase inhibitor (inh2) genes from potato genotypes displaying diVerent responses to cold-induced sweetening. Their results show that during cold storage the transcripts of invertase inhibitors are more abundant in cultivars resistant to cold-induced sweetening than in susceptible cultivars.…”

“…During cation-exchange chromatography they detected a putative apoplastic invertase inhibitor in fractions with strong apoplastic invertase immunosignal, but low or absent apoplastic invertase activity. Brummell et al (2011) produced a construct that fused the N-terminal part of the INH-1 protein, including the putative signal peptide to GFP (green Xuorescence protein), and detected the presence of GFP in cytoplasm as well as in the cell wall of epidermal cells of both onion bulbs and lisianthus petals. Similar StInhInv1-GFP gene fusions conWrmed membrane (and nuclear) accumulation of the translational fusion proteins in onion epidermal and tobacco BY-2 cells (Liu et al 2010).…”

Alternative Splicing at the Intersection of Biological Timing, Development, and Stress Responses

“…For example, low temperature storage of potato (Solanum tuberosum) causes sweetening due to the conversion of starch to Suc and, subsequently, Glc and Fru by vacuolar acid invertase, which is detrimental to processing. Splicing of a mini-exon that forms part of the active site of an invertase gene was modified in cold storage (Bournay et al, 1996), and lines showing resistance to cold-induced sweetening have higher expression of two splice variants (INH2a and INH2b) of the invertase inhibitor gene (INH2) (Brummell et al, 2011). Cold also regulates AS of the INDETERMINATE DOMAIN14 (IDD14) transcription factor that activates the expression of the QUA-QUINE STARCH starch-degrading enzyme, leading to inhibition of starch accumulation (Seo et al, 2011b).…”

Modulation of gene expression in cold-induced sweetening resistant potato species Solanum berthaultii exposed to low temperature

“…6a). InvInh proteins were suggested to play a critical role in posttranslational modification of invertase activity through protein-protein interactions (Rausch and Greiner 2004), and hence they may have great influence on CIS resistance Brummell et al 2011). In the present study, expression of C20-4-C15 was sharply induced by Fig.…”