Trichomes are highly differentiated cell types found on the aerial epidermis of plants. Most commonly they manifest themselves as hairs on leaves, stems and flower buds, and serve a range of functions from protection against insects to heat resistance and moisture conservation. The salt tolerant plant Mesembryanthemum crystallinum (common name Ice plant) has specialized balloon-like trichomes called epidermal bladder cells (EBC) that are thought to play a role in sodium storage and salt tolerance, however, EBC are present on plants even in the absence of salt. In order to gain an integral understanding of the development and role of these cells we have taken a systems biology approach, combining data from global quantitative transcriptomics, proteomics, metabolomics, and ionomics studies with flow cytometry and fluorescence microscopy. Our results demonstrate that these cells are highly metabolically active and not just passive stores for sodium. The rapid expansion of the cells during development is due to a huge increase in ploidy, beginning in emerging leaves, to higher than 512C in adult plants, with nuclei of greater than 60 microns in diameter measured in these cells. Salt-treatment leads to a further increase in ploidy and transcripts and proteins involved in endoploidy are shown to be upregulated by salt in the EBC suggesting environmental as well as developmental cues. It has been proposed that increased ploidy helps to mitigate stress damage, and the increase in cell size and ploidy observed in M. crystallinum under salinity may contribute to tolerance by increasing the store size for sodium sequestration. However, as significant increases in ploidy are observed in the untreated plants it is possible that higher ploidy also facilitates higher cellular metabolic activity and rapid cell expansion in this plant.
Authors would like to acknowledge financial support from an SCU seed grant and support from SCPS.