Aggregative movement of mesophyll chloroplasts occurs in a wide variety of C4 plant species

Abstract

• Redistributions of chloroplasts in 36 C 4 monocots and eudicots were examined. • Mesophyll chloroplasts in 35 C 4 plants aggregated towards the bundle sheath. • This chloroplast movement would be a general physiological response in C 4 plants. • Mesophyll chloroplasts in monocots aggregated more strongly than in eudicots. • Degrees of movement in NADP-ME were lower than in PCK species. C 4 plants have two types of photosynthetic cells, mesophyll (M) and bundle sheath (BS) cells with different subcellular structures. In C 4 plants, maize and finger millet, the chloroplasts of M cells are normally distributed toward the periphery but redistribute toward the BS cell side in response to environmental stresses. This directional movement of M chloroplasts is called aggregative movement, of which physiological significance remains unknown. Furthermore, few C 4 plant species have been examined for aggregative movement, and it has not been established whether this is a common phenomenon in C 4 plants. Hence, this study aims to investigate whether the aggregative movement is widely observed and diverse in C 4 plants. Thirty-six monocotyledonous and dicotyledonous C 4 plant species (Poaceae, Amaranthaceae, Asteraceae, and Portulaceae) including all three C 4 subtypes were examined. Leaf segments were infiltrated with the abscisic acid solution and irradiated with blue light to induce aggregative movement of M chloroplasts. Aggregative movement of M chloroplasts was observed in 35 C 4 species across 11 evolutionary lineages. The degree of aggregative movement was significantly greater in monocots than that in eudicots; phospho enol pyruvate carboxykinase subtype C 4 plants showed significantly greater aggregative movement than NADP-ME type C 4 plants. The aggregative movements would be broadly shared stress responses in C 4 plants. The differences in the degree of aggregative movement could be related to morphological and physiological differences between monocots and eudicots, and among C 4 subtypes.