Cold-Triggered Induction of ROS- and Raffinose Metabolism in Freezing-Sensitive Taproot Tissue of Sugar Beet.

Isabel Keller,H Ekkehard Neuhaus,C Martins Rodrigues,Frank Ludewig,Dominik Kischka,Uwe Sonnewald,Wolfgang Zierer,Benjamin Pommerrenig,Wolfgang Koch,Karin Fiedler-Wiechers,Christina Müdsam,Karsten Harms,Olaf Czarnecki

doi:10.3389/fpls.2021.715767

Abstract

Sugar beet (Beta vulgaris subsp. vulgaris) is the exclusive source of sugar in the form of sucrose in temperate climate zones. Sugar beet is grown there as an annual crop from spring to autumn because of the damaging effect of freezing temperatures to taproot tissue. A collection of hybrid and non-hybrid sugar beet cultivars was tested for winter survival rates and freezing tolerance. Three genotypes with either low or high winter survival rates were selected for detailed study of their response to frost. These genotypes differed in the severity of frost injury in a defined inner region in the upper part of the taproot, the so-called pith. We aimed to elucidate genotype- and tissue-dependent molecular processes during freezing and combined analyses of sugar beet anatomy and physiology with transcriptomic and metabolite profiles of leaf and taproot tissues at low temperatures. Freezing temperatures induced strong downregulation of photosynthesis in leaves, generation of reactive oxygen species (ROS), and ROS-related gene expression in taproots. Simultaneously, expression of genes involved in raffinose metabolism, as well as concentrations of raffinose and its intermediates, increased markedly in both leaf and taproot tissue at low temperatures. The accumulation of raffinose in the pith tissue correlated with freezing tolerance of the three genotypes. We discuss a protective role for raffinose and its precursors against freezing damage of sugar beet taproot tissue.

Highlights

In temperate climate zones (Europe and North America), sugar beet (Beta vulgaris subsp. vulgaris) is the exclusive source of sugar for the food industry and a source for bio-energy generation
To determine whether the low temperature exposure in general was associated with formation of reactive oxygen species (ROS), we performed staining of taproot slices with diaminobenzidine (DAB) and nitroblue tetrazolium (NBT) to reveal accumulation of H2O2 and superoxide, respectively
Our observations showed that frost-induced tissue damage and emergence of ROS are clustered in the upper part of the sugar beet taproot, mainly in the pith tissue

Summary

Introduction

In temperate climate zones (Europe and North America), sugar beet (Beta vulgaris subsp. vulgaris) is the exclusive source of sugar (sucrose) for the food industry and a source for bio-energy generation. Successful vernalization of sugar beet plants can only occur at low, but above-zero temperatures This is because, despite the high accumulation of sugars, which are known to stabilize membranes and protect against freezing-induced damages (Anchordoguy et al, 1987; Pommerrenig et al, 2018), sugar beet is sensitive to subzero temperatures (Barbier et al, 1982; Loel and Hoffmann, 2015). This sensitivity limits cultivation of the crop to regions with a vegetative period from spring to late autumn. Ice formation and thawing leads to cell rupture and leakage of root sap, which attracts microorganisms and leads to rot of the taproot (Barbier et al, 1982)

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