Myths and Facts on cytosolic sucrose hydrolysis

Abstract

Sucrose plays an essential role in nature as part of the carbon dioxide assimilation pathway in the biosphere. Particularly, in plants occupies a unique position being essential for the allocation of carbon resources for growth and respiration, and participates in a regulatory network that coordinates metabolism and development. Its metabolism is strictly coordinated with photosynthesis-related genes by a feed-back regulation and a sugar signaling pathway. Also, sucrose accumulation has been shown to be involved in environmental stress responses and its degradation has been associated with a high demand of carbon and energy in sink tissues and anoxic environments. In the last decades, several studies on sucrose degradation in plant cells brought to light novel trails on carbon trafficking and the metabolic paths leading to regulation of gene expression and metabolic adjustment. However, recent findings point to an intricate relationship between sucrose metabolism and the whole plant physiology. This review is focused on the elusive Alkaline/Neutral-Invertase (A/N-Inv) isoforms, which have been long believed as cytosol-exclusive enzymes that hydrolyze the disaccharide for general or maintenance functions in the plant cell. Nevertheless, the story has changed in last few years when novel subcellular localizations and essential functions for a number of key physiological processes were demonstrated for A/N-Invs. From being minor proteins, scarcely mentioned in comprehensive descriptions of carbon flux in plants, these enzymes became leading actors in sucrose metabolism with significant roles in root morphogenesis, photosynthetic apparatus biosynthesis, and abiotic stress adaptation. The aim of this review article is to present a detailed and updated discussion of the most significant findings on the physiological properties of A/N-Invs. Also, we incorporate this group of invertases in the metabolic paths leading sucrose into the different circuits that govern carbon partitioning, storage and regulation of the molecular biology of the plant cell.