Differential Positioning of C₄ Mesophyll and Bundle Sheath Chloroplasts: Aggregative Movement of C₄ Mesophyll Chloroplasts in Response to Environmental Stresses

摘要

In C₄ plants, mesophyll (M) chloroplasts are randomly distributed along the cell walls, while bundle sheath (BS) chloroplasts are typically located in either a centripetal or centrifugal position. We investigated whether these intracellular positions are affected by environmental stresses. When mature leaves of finger millet (Eleusine coracana) were exposed to extremely high intensity light, most M chloroplasts aggregatively re-distributed to the BS side, whereas the intracellular arrangement of BS chloroplasts was unaffected. Compared with the homologous light-avoidance movement of M chloroplasts in C₃ plants, it requires extremely high light (3,000–4,000 μmol m−2 s−1) and responds more slowly (distinctive movement observed in 1 h). The high light-induced movement of M chloroplasts was also observed in maize (Zea mays), another C₄ species, but with a distinct pattern of redistribution along the sides of anticlinal walls, analogous to C₃ plants. The aggregative movement of M chloroplasts occurred at normal light intensities (250–500 μmol m−2 s−1) in response to environmental stresses, such as drought, salinity and hyperosmosis. Moreover, the re-arrangement of M chloroplasts was observed in field-grown C₄ plants when exposed to mid-day sunlight, but also under midsummer drought conditions. The migration of M chloroplasts was controlled by actin filaments and also induced in a light-dependent fashion upon incubation with ABA, which may be the physiological signal transducer. Together these results suggest that M and BS cells of C₄ plants have different mechanisms controlling intracellular chloroplast positioning, and that the aggregative movement of C₄ M chloroplasts is thought to be a protective response under environmental stress conditions.