Chloroplast starch granules reflect solar radiation and protect plants from photo-oxidative stress under hypothermia

Starch is one of the primary products of photosynthesis, which accumulates in chloroplasts as discrete starch granules during the day and degrades throughout the night. It was found that Nicotiana tabacum L. showed a response to hypothermia by hyperaccumulation of starch in its leaves, whereas in Secale cereale L. cold acclimation was accompanied by a decrease of leaf starch. Transmission electron microscopy showed that the size of starch granules in tobacco chloroplasts during cold acclimation reached almost half of the chloroplast area, while in rye chloroplasts starch granules completely disappeared. The formation of large starch granules in tobacco chloroplasts resulted in a significant increase in absorbance of both isolated chloroplasts and leaves and also greatly increased plant resistance to photo-oxidative stress induced by high light and methylviologen. Starch granules of minimal size (less than 1% of chloroplast area) in rye had no effect on leaf and chloroplast absorbance and the plant resistance to photo-oxidative stress. It is suggested that large-sized starch granules can effectively reflect light and create a shading effect for both individual chloroplasts and the whole leaf. Reducing the intensity of chloroplast membrane illumination decreases the generation of reactive oxygen species during hypothermia and allows plants to avoid photo-oxidative damage. These results provide new insights into the function of leaf starch, and opens new future directions for fundamental research with potential applications to breeding of chilling-tolerant agronomically valuable varieties.