Title : Alkaline/Neutral invertases: Involvement of cytosolic or organellar isoforms in abiotic stress responses in wheat and poplar
S ucrose is a central metabolite in plant life. In recent years, novel functions have been uncovered for alkaline/neutral invertases (A/N-Invs), enzymes that irreversibly hydrolyze sucrose into glucose and fructose. A/N-Invs play important roles in growth, development, and stress response. They occur as different isoenzymes located either in the cytosol or inside organelles (e.g., mitochondria, chloroplasts). In wheat, A/N-Invs were shown to play an important role associated with a more efficient cytosolic sucrose hydrolysis during environmental stresses (e.g., water deficit and low temperatures). The availability of complete sequenced genomes allowed us to identify and characterize the members of the A/N-Inv family in wheat (Triticum aestivum) and poplar (Populus sp.), and then investigate the expression of different isoforms in plants subjected to different stress conditions. In wheat, four of the seven A/N-Inv genes (TaNIN) were predicted as encoding cytosolic isoforms, which turned out to be phylogenetic and structurally similar to their rice homologs. RT-PCR analysis revealed differential gene expression in leaves and roots of two contrasting wheat varieties (winter Baguette 31 and spring BioINTA 1005) subjected to low (chilling) temperature or water deficit. Remarkably, only the winter cultivar, with higher fructan content, exhibited up-regulation of two cytosolic isoforms in leaves, and of other two different forms in roots. Conversely, the response in leaves was similar for both cultivars exposed to water deficit, with the enhanced expression of another isoform. The contrasting expression between cultivars under low temperature led us to hypothesize that certain cytosolic isoforms could be involved in the intricate sugar-signaling pathway that senses chilling and resulting in low temperature tolerance. On the other hand, in poplar, we retrieved 12 A/N-invertase homologous sequences from the recent sequenced Populus tremula genome (Ptr-NINVs). Six of the 12 putative PtrNINV deduced proteins were predicted as cytosolic isoforms and grouped in the ß-clade together with characterized cytosolic proteins in other plant species. The 6 remaining Ptr-NINV sequences that grouped in the α-clade were characterized as putative mitochondrial and chloroplastic isoforms. To characterize the organellar location, we identified the transit peptide sequences, made PtrNINV::GFP constructs and carried out transient expression in tobacco leaf cells. Preliminary experiments to identify isoforms related to stress tolerance indicated that one of the mitochondrial isoform may be involved in cold and salt stress response. Taken together, our results show that either cytosolic or organellar A/NInvs may be important in the response to abiotic stress.