New technique allows gene-expression analysis
The shoot apical meristem continues to make leaves, flowers and branches throughout a plant's life, so it must contain stem cells. These reside in a complex three-dimensional structure consisting of perhaps three dozen stem cells surrounded by niche cells and millions of differentiating cells. This makes the stem cells within the meristem hard to isolate. In fact, most researchers pursuing genomic studies of plant stem cells have turned instead to roots. Now, using an Arabidopsis thaliana mutant that produces an unusually high number of accessible shoot apical meristems, G. Venugopala Reddy of the University of California, Riverside and his colleagues have found a way to study shoots and have produced a gene-expression map of the meristem that reveals the molecular signatures of these elusive stem cells.
Reporting in the Proceedings of the National Academy of Sciences, Reddy and colleagues describe making transgenic lines of the A. thaliana mutant in which the cells expressed fluorescent versions of shoot apical meristem (SAM) marker proteins: stem cells expressed a green fluorescent protein from the CLAVATA3 promoter, differentiating cells expressed a red fluorescent protein from the FILAMENTOUS FLOWER promoter and niche cells expressed a green fluorescent protein from the WUSCHEL promoter. This allowed them to sort the SAM cells into pure cultures and analyze the gene-expression patterns of each cell type. Over 2,500 genes are differentially expressed by these 3 populations.
"I think it's a really major leap to be able to record gene-expression data with this high resolution in the SAM," says Ben Scheres, a plant biologist from University of Utrecht, the Netherlands, who did not participate in the study. "This work will certainly be used by a lot of people in the community."
Already the map is providing some intriguing insight into SAM stem cells. As in animals, for example, SAM stem cells are enriched in DNA-repair and chromatin-modification genes, says Reddy. Moreover, researchers can now use this map to unravel the expression patterns and function of these genes in maintaining SAM stem cells and in developing organs. As the SAM contains additional populations of cells that were not captured by the three marker proteins used, one of the next steps is already clear: "We need to increase the resolution of the map," says Reddy.