Invertase inhibitors in potato: towards a biochemical and molecular understanding of cold-induced sweetening

Abstract

Invertase inhibitors classified as cell wall/apoplastic and vacuolar belonging to the pectin methylesterase family, play a major role in cold-induced sweetening (CIS) process of potato tubers. The CIS process is controlled at the post-translational level via an interaction between invertase (cell wall/apoplastic and vacuolar) by their compartment-specific inhibitors (cell wall/apoplastic and vacuolar). Invertase inhibitors have been cloned, sequenced and functionally characterized from potato cultivars differing in their CIS ability. The secondary structure of the invertase inhibitors consisted of seven alpha-helices and four conserved cysteine residues. The well-conserved three amino acids i.e. Pro-Lys-Phe are known to interact with invertase. Location of the genes encoding cell wall/apoplastic and vacuolar invertase inhibitors on potato chromosome number twelve in a tandem orientation without any intervening genes suggest their divergence into the cell wall and vacuole forms following the event of gene duplication. Under cold storage conditions, the vacuolar invertase inhibitor gene showed developmentally regulated alternative splicing and produce hybrid mRNAs which were the result of mRNA splicing of an upstream region of vacuolar invertase inhibitor gene to a downstream region of the apoplastic invertase inhibitor gene. Transgenic potato tubers overexpressing invertase inhibitors resulted in decreased invertase activity, low reducing sugars and improved processing quality making invertase inhibitors highly potential candidate genes for overcoming CIS. Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) gene-editing technology offers transgene-free breeding for developing CIS resistant potato cultivars. Moreover, the post-transcriptional regulation of invertase inhibitors during cold storage can be warranted. This review summarizes progress and current knowledge on biochemical and molecular approaches used for the understanding of invertase inhibitors with special reference to key findings in potato.

KEYWORDS:

• Cell wall
• cold-induced sweetening
• invertase inhibitor
• potato
• reducing sugars
• vacuolar

Acknowledgments

I would like to thank Prof. Dr. Sharon Regan, Biology Department, Queen’s University, Canada for providing guidance and the infrastructural facilities. A very special thanks to Prof. Stephen Lougheed, Department of Biology & School of Environmental Studies, Baillie Family Chair in Conservation Biology, Director, Queen's University Biological Station, Kingston, Canada for providing the phylogenetic tree. I would like to extend my thanks to Dr. David Brummell, Science Team Leader, Fresh Food Metabolism, Plant & Food Research, Palmerston North 4442, New Zealand for providing the guidance and granting the permissionto modify Fig. 2 in this article.

 

Phylogenetic tree constructed based on the cell wall/apoplastic invertase inhibitor and the vacuolar invertase inhibitor from potato (Table 2; GenBank database (https://www.ncbi.nlm.nih.gov/). A tree has been kindly constructed by Prof. Stephen Lougheed, Department of Biology & School of Environmental Studies, Queen's University Biological Station, Canada. We used ProtTest version 3.4.2 (Darriba et al. 2011) and the Bayesian Information Criterion (BIC) option to select the appropriate model of evolution for Bayesian analysis. Using the suggested VT model (Muller and Vingron 2000) we inferred evolutionary relationships among amino acid sequences with MrBayes v3.2.6 × 64 (Huelsenbeck and Ronquist 2001, Ronquist et al. 2012) with two simultaneous runs of 2 million generations, each with random starting trees with one cold and three incrementally heated Markov chains. The analysis was run until the standard deviation of split frequencies < 0.01, sampling every 1000 generations. We visually assessed that our runs had achieved stationarity using the program Tracer version 1.7.1 (Rambaut et al., 2018). For each analysis, we constructed a 50% majority rule consensus tree (discarding the first 25% of trees as burnin).Group A consists of the vacuolar invertase inhibitors while Group B consists of the cell wall/apoplastic invertase inhibitors. Stkunitztype was set as an outgroup.