Cells, Genes & Molecules

Rice chloroplasts become more spherical under salt stress

Though the cause isn’t clear, the chloroplasts may be reacting to carbon dioxide or light levels.

Soil salinity is a significant environmental factor limiting the health and productivity of crop plants. One of the key metabolic effects of salt stress is to limit photosynthesis. Past research using two dimensional images of salt-stressed chloroplasts has suggested that the organelles swell in response to the salt. However, the need to slice specimens into ultrathin sections for transmission electron microscopy has prevented three dimensional visualization.

In a recent study published in Annals of Botany, author Takao Oi and colleagues examined the leaves of rice plants that had been treated with saline solution. The authors used focussed ion beam scanning electron microscopy (FIB-SEM) and image processing software to reconstruct the 3-D ultrastructure of the mesophyll cells and their organelles.

In control cells, chloroplasts were lens-shaped and arranged closely along the cell wall. In the salt-treated cells, however, the chloroplasts became more oval-shaped, but with no overall change in volume, though the surface area to volume ratio decreased. The positioning of the treated chloroplasts was affected as well. Rather than being spread out sheet-like and in close contact with one another along the cell wall like the controls, they became more separated.

Top views in parallel with transverse (xy) sections showing the 3-D models of chloroplasts in mesophyll cells. (Left) control, (Right) NaCl-treated. Full details in Oi et al. 2020

While the reason behind these structural changes is not clear, the authors speculate that they may be in response to lowered carbon dioxide concentration brought on by stomatal closure due to osmotic stress. Alternatively, the change may represent an acclimation to salinity as the cell attempts to reduce light intensity that can cause photodamage during times of salt stress. Finally, the change may simply be a result of damage caused by the salt treatment. “The physiological significance of this morphological change remains unclear,” the authors write, “and, therefore, the relationship between chloroplast ultrastructure and their functionality under salinity stress should be further investigated in the future.”

This method of viewing cellular components in 3-D could prove useful in avoiding similar errors in our understanding of such structures in the future. “Previous views of swelling in chloroplasts were misinterpretations based on 2-D observations,” the authors note, “suggesting that the use of 3-D analysis would be useful for future studies to prevent similar misinterpretations.”

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