Endopolyploidy in the mesophyll and bundle sheath cells of C4 Poaceae

Most C4 plants share photosynthetic reactions between mesophyll and bundle sheath cells. Bundle sheath cells in these C4 plants are enlarged and have higher organelle content, and these traits are considered crucial enablers of the evolution of C4 photosynthesis. Endopolyploidy, the occurrence of varying ploidy levels in the somatic tissues of an organism, has been implicated in the enlargement of cells. Endopolyploidy may therefore contribute to the enlargement of bundle sheath cells, but it remains elusive as to whether the nuclei of bundle sheath cells are endopolyploid in C4 Poaceae.

We assessed endopolyploidy in the leaf cells of 18 species belonging to the two largest C4 Poaceae subfamilies (Panicoideae and Chloridoideae) using flow cytometry and fluorescence microscopy. Additionally, we examined whether genetic relatedness affected endopolyploidy among species using phylogenetic signals.

In C4 Poaceae, the endoreduplication index of the nuclei in the bundle sheath cells was substantially higher than that in the mesophyll cells. Significant variations were noted in the endoreduplication index among the species in the mesophyll and bundle sheath cells. Furthermore, the variation in the endoreduplication index exhibited various patterns between the mesophyll and bundle sheath cells. Pagel's λ, a kind of phylogenetic signal, indicates that factors other than phylogeny markedly influence the variation in endoreduplication index of these cell types among the species.

This study uncovered various tendencies concerning endopolyploidy in the cell types of C4 Poaceae. Bundle sheath cells tended to have endopolyploid nuclei, whereas mesophyll cells did not. However, the extent of endopolyploidy differs among the species, and such variation is derived from factors other than phylogeny, which vary depending on the cell type.