Co-regulation of heat-shock protein 90 (HSP90) and transglutaminase (TGase) in Brachypodium distachyon under abiotic stress

Drought and salinity are two most encountered and severe abiotic stress conditions that cause reduction in crop yield and productivity. Plants continuously encounter stress conditions due to their sessile structure and basic response pathways of plants include heat-shock proteins (HSPs). Main function of HSPs is defined as performing intracellular chaperone role by facilitating protein folding. HSPs can be found in different molecular weights as HSP60, HSP70, HSP90, HSP100 and small heat shock proteins (sHSPs). HSP90 constitutes 1 to 6 percent of the total protein in the cell. HSP90 expression levels are influenced by free or bound polyamines (PAs). Transglutaminase (TGase) is another protein that interacts with PAs and work by means of catalyzation of protein cross-linking and post-translational protein modification as well as free PA incorporation. Both HSP90 and TGase are found to be involved in stress response and their expression levels are affected by stress conditions. In the current study, a monocot model plant Brachypodium distachyon (Bd21 line) was exposed to drought (12 days water withholding) and salinity (14 days 320 mM NaCl treatment) stress at their vegetative stage in order to determine whether HSP90 and TGase are co-regulated under abiotic stress. Under drought stress, both HSP90 and TGase transcript levels were increased by approximately 2-fold and 1.11-fold respectively whereas HSP90 protein levels were upregulated by 2-fold. However, TGase protein levels were slightly downregulated. Upon salinity treatment, TGase transcript levels were downregulated by almost 6-folds and also HSP90 transcripts were in a downregulation trend whereas HSP90 and TGase protein levels displayed downregulation by 80 and 82 percent, respectively. Under salinity stress, transcript levels changes of mitochondrial and chloroplastic HSP90 and TGase are parallel together with close downregulation of protein expression levels. Hence, there might be a possible interaction between HSP90 and TGase in Brachypodium distachyon under salinity stress. This study highlights the importance of molecular mechanisms behind drought and salinity stress.