DST INSPIRE Faculty Fellow University of Kashmir, Srinagar, India Srinagar, Jammu and Kashmir, India
Body of Abstract: The calcium oxalate (CaOx) crystals, produced within idioblasts, are distributed in nearly all taxonomic echelons from small algae to giant gymnosperms. This biomineralization process involves a genetically controlled coordination between calcium uptake, oxalate production, and occasionally environmental stress rather than being a random event. Nonetheless, despite their prevalence and ascribed functions, the factors responsible for their synthesis and accumulation are less known. In this context, the present study aimed to understand the CaOx biology of five congeneric species of Rheum from the Indian Himalayas including R. australe, R. webbianum, R. moorcroftianum, R. spiciforme, and R. tibeticum. They were found to harbor 24 different crystal morphologies with druse and conglomerate types being the most prevalent highlighting their species delimitation potential. The crystals tend to accumulate around the vascular bundles more towards the xylem with spatial variability in crystal number within idioblasts. SEM-EDX, XRD, and ICP-OES analysis showed that crystals contain sixteen common and heavy metals across species wherein metal impurities were associated with the weddellite form. Indeed, the higher quantity of crystals in the plant was found to depend on the soluble oxalate as well as the calcium and heavy metal contents within the soil. Moreover, the evaluation of stress indicators in rhubarb species deciphers consonance of crystal accumulation with environmental cues. Interestingly, ascorbate and dehydro-ascorbate (precursors of oxalate) levels paralleled with crystal content in the root in contrast to the shoot. Further, the transcript abundance of nine genes of calcium regulation, oxalate/ascorbate synthesis, and stress response positively correlate with crystal content, whereas other (5) genes of oxalate oxidation/decarboxylation showed a negative correlation. These findings on CaOx crystal morphology, their distribution, accumulation vis-à-vis environmental factors, and different synthetic aspects in rhubarb may act as a prelude for new insights toward understanding their biosynthesis and subsequent fate.