Alexander N. Deryabin,and Тamara I. Trunova
Laboratory of Cold Resistance, Timiryazev Institute of Plant Physiology, Moscow, Russian Federation
In higher plant apoplastic acid invertase (β-D-fructofuranoside fructohydrolase, E.C. 188.8.131.52) is the key enzyme of carbohydrate metabolism, which is involved in important physiological processes such as phloem unloading, control of cell differentiation and sucrose level in cell free space, and transport of sucrose across the plasmalemma. In this study, we investigated into a functional role of the yeast-derived acid invertase in the transformed potato plant (Solanum tuberosum L., cv. Désirée), which expressed the SUC2 gene of Saccharomyces cerevisiae under the transcriptional control of the patatin class I tuber-specific B33-promoter. Potato line with the SUC2 gene expression offer unique opportunities for research since the integrated SUC2 gene encodes the invertase of yeast with an N-end-connected potato proteinase II inhibitor signal peptide, which provides apoplastic localization of foreign invertase.
It was shown the SUC2 gene encoding the extracellular invertase of the yeast is actively expressed in the genome of the transformed potato plants. The soluble form of the invertase protein occurs in the apoplast and is weakly adsorbed on the cell-wall of the transformed plants. The constitutive SUC2 gene expression resulted in elevated activity of the apoplastic invertase in transformants compared to that in the WT-plants ones promoted more active accumulation of metabolically active sugars: glucose and sucrose in the leaves; fructose in the apoplast, and especially, glucose in the roots. The increased total sugar contents in the transformed potato plant led to some changes in their morphometric parameters (decreased offshoot length, less developed root system, and larger water content in tissues). It is indicative of a regulatory function of apoplastic invertase. It was reported that potato plants with the in-built SUC2 gene could be effectively used to examine the role that apoplastic invertase played in adaptation to hypotermia. Prolonged cold exposure contributed to the increased activity of apoplastic invertase and contents sugar in the transformed plants compared to that in the WT-plants. The transformed potato plant possess a higher tolerance to hypotermia and less lipid peroxidation activity, apparently due to the changes in sugar ratio produced by the yeast invertase. This fact allows us to consider apoplastic invertase as a stress enzyme of carbohydrate metabolism, playing a significant regulatory role in the modulation of metabolic signaling during the formation of increased plant resistance to hypothermia.
The authors are grateful to Dr. L. Willmitzer (Max Planck Institute of Molecular Plant Physiology, Germany) and Dr. G.A. Romanov (Timiryazev Institute of Plant Physiology, Russia) for the potato plants provided for the research.
Keywords: Saccharomyces cerevisiae, Solanum tuberosum, apoplast, invertase, SUC2 gene, sugars, transformed plant.