Proline-Rich Proteins May Regulate Free Cellular Proline Levels during Drought Stress in Tomato

Proline (Pro)-rich proteins (PRPs), initially identified as structural proteins of cell wall, have emerged as multifunctional plant proteins in recent past. Their vibrant role in plant development and environmental stress promoted us to study a SlPRP gene of tomato, which was significantly downregulated under drought stress in a microarray experiment performed in our laboratory. Promoter analysis of SlPRP revealed a number of stress-responsive protein-binding sites, confirming its expression in response to stress. Expression of SlPRP gene in different tissues of tomato, viz. root, stem, leaf and flower was studied to analyse the gene expression pattern in response to drought stress. Further, we have correlated the expression of SlPRP gene with Pro levels of the respective plant tissues under drought stress. In anticipation, it has been observed that downregulation of SlPRP gene is coupled with simultaneous increase in cellular Pro concentration in all the tissues under drought stress, except the roots. This could help preserve the available cellular proline to function as osmoprotectant during stress. The present results propose a hypothesis where PRPs may regulate free cellular proline levels during drought stress by regulating their own gene expression. Thus, it may be concluded that transcription of PRPs in plants is synchronized with the cellular Pro concentration under environmental stress in order to provide drought tolerance to plants.