Plasticity in chloroplast development in the shade-adapted Selaginella erythropus in response to light

Background/Question/Methods

Bizonoplasts were first reported in a deep-shade clubmoss Selaginella erythropus. These unique giant chloroplasts are located in the dorsal epidermal cells of leaves (microphylls) and possess a dimorphic ultrastructure. This unique chloroplast type is composed of two distinct zones: an upper zone occupied by parallel layers of 2–4 stacked thylakoid membranes and a lower zone containing both unstacked stromal thylakoids and grana structures typical of chloroplasts of land plants. In contrast, other cell types in the leaves contain chloroplasts with typical structure.  After examination of 15 other Selaginella species, we found a second taxon with bizonoplasts, S. heterostachys, also a deep-shade plant. We hypothesized that the specialized structure of bizonoplasts is an adaptation to low light conditions. If so, this structure might show plasticity to the light environment. To test this idea, S. erythropus was cultivated under three different light conditions: low light (15-20 μmol/m²/s) above the leaves, low light below the leaves, and high light (280-300 μmol/m²/s) above the leaves. The newly matured ventral leaves were examined for anatomical and chloroplast ultrastructure by light and electron microscopy.

 Results/Conclusions

As anticipated, typical cup-shaped bizonoplasts with dimorphic ultrastructure were found under low light. In contrast, under high light single bizonoplasts did not occur in the dorsal epidermal cells. Instead, the dorsal epidermal cells contained one or two spherical chloroplasts without the dimorphic ultrastructure of bizonoplasts. These spherical chloroplasts contained grana structure and unstacked stromal thylakoids typical of chloroplasts of land plants. Interestingly, low light from below also led to formation of one or two spherical chloroplasts without dimorphic ultrastructure. Moreover, these chloroplasts were located near the top of the dorsal epidermal cells. Our results clearly demonstrate, for the first time, plasticity in chloroplast development in the dorsal epidermal cells of S. erythropus in response to light intensity, including changes in plastid number and ultrastructure. The presence of the upper zone in bizonoplasts is influenced by light intensity and direction. Our results suggest that the unique bizonoplasts are modified chloroplasts, with specific adaptive value in deep shade environments. It is very likely that they have the same genetic background as normal chloroplasts in mesophyll cells of S. erythropus. However, the dorsal epidermal chloroplasts possess the ability to differentiate into bizonoplasts under low light from above during development.