Elimination of salt by recretion: salt glands and gland-supported bladders in recretohalophytes.

Abstract

Abstract Elimination of mineral ions and salts by salt glands and salt hairs is called recretion, which has given the name recretohalophytes to plants using this mechanism for tolerance of salinity. There are about 370 vascular plant species of recretohalophytes comprising about 15% of all halophytes and 0.15% of angiosperms. Their evolutional origin is polyphyletic. Anatomical complexity shows trends from hydathodes to salt glands. There is different complexity between simple epidermal bladders and trichome-like salt hairs concentrating salt in terminal bladder cells. Transport pathways comprise cell-to-cell apoplastic and symplastic transport in salt gland and salt hair complexes, membrane transport by ion transporters and H+-ATPases (eccrine recretion) and possibly vesicle transport (granulocrine recretion). Salt-gland cell cytology is characterized by a very dense cytoplasm enriched in mitochondria, but lacking functional chloroplasts. The plasma-membrane surface is often inflated by a labyrinth of cell wall protuberances, typical of transfer cells. Materials recreted are preferentially Na+ and Cl-. However, many other ions and also some organic solutes may be found. Isolation of salt glands embedded in the epidermis has so far not been achieved, and salt hairs with their glandular stalk cells (which are easier to isolate) have not been used for identifying recretion-specific ion transporters. Nevertheless, transcriptomics and proteomics have been applied to compare plants with differing recretory activity, including specific recretion mutants, to identify transporters putatively involved in recretion. Recretion is under genetic control. Although the number of recretohalophytes among angiosperm halophytes is relatively low (ca 15%), individual recretohalophyte species can play an important role in saline habitats. Ecophysiologically the efficiency of recretion depends on the site characteristics to which plants are adapted. Water loss with the recreted fluid is an ecophysiological challenge. Recretion does not reduce habitat salinity, because recreted salt is recycled to the environment. Recretohalophytes provide multiple services to anthropogenic management of salinity-affected ecosystems under increasingly severe environmental challenges now and in the future of global changes. There are many promising applications of recretohalophytes in saline agriculture. They serve as pasture and fodder plants and have entered human diets. Due to salt recycling they do not appear to be applicable to phytoremediation of salinity but they may clear metals from soils, and may serve as sources of bio-energy and biofuel. Recretohalophyte species can be used in the restoration ecology of deteriorated land. Recretohalophytes provide gene pools for gene engineering and synthetic biology in managing future challenges.