Bizonoplast, a unique chloroplast in the epidermal cells of microphylls in the shade plant Selaginella erythropus (Selaginellaceae)

Abstract

Study of the unique leaf anatomy and chloroplast structure in shade-adapted plants will aid our understanding of how plants use light efficiently in low light environments. Unusual chloroplasts in terms of size and thylakoid membrane stacking have been described previously in several deep-shade plants. In this study, a single giant cup-shaped chloroplast, termed a bizonoplast, was found in the abaxial epidermal cells of the dorsal microphylls and the adaxial epidermal cells of the ventral microphylls in the deep-shade spike moss Selaginella erythropus. Bizonoplasts are dimorphic in ultrastructure: the upper zone is occupied by numerous layers of 2-4 stacked thylakoid membranes while the lower zone contains both unstacked stromal thylakoids and thylakoid lamellae stacked in normal grana structure oriented in different directions. In contrast, other cell types in the microphylls contain chloroplasts with typical structure. This unique chloroplast has not been reported from any other species. The enlargement of epidermal cells into funnel-shaped, photosynthetic cells coupled with specific localization of a large bizonoplast in the lower part of the cells and differential modification in ultrastructure within the chloroplast may allow the plant to better adapt to low light. Further experiments are required to determine whether this shade-adapted organism derives any evolutionary or ecophysiological fitness from these unique chloroplasts.

A variation on chloroplast development: The bizonoplast and photosynthetic efficiency in the deep‐shade plant Selaginella erythropus

“…The bizonoplast of S. erythropus is a monoplastid chloroplast (up to 30 µm long) found in the dorsal epidermal cells ( Sheue et al, 2007 ;Reshak and Sheue, 2012 ). The bizonoplast differs from all previously reported chloroplasts (monoplastid or not) by its distinctive dimorphic ultrastructure.…”

“…Normal chloroplasts and bizonoplasts are both present in individual microphylls of S. erythropus ( Sheue et al, 2007 ;Reshak and Sheue, 2012 ). While biozonoplasts are in the dorsal epidermal cells, normal chloroplasts are found in the mesophyll and ventral epidermal cells.…”

“…However, in these cases, the entire plastid has the lamellar structure of the upper zone of the bizonoplast. The lower zone of the bizonoplast contains both unstacked stromal thylakoids and thylakoids stacked in normal granal structures oriented in different directions ( Sheue et al, 2007 ).…”

“…The bizonoplast of S. erythropus from tropical South and Central America is a unique giant chloroplast, with a two-zoned ultrastructure ( Sheue et al, 2007 ).…”

Review: Recent status of Selaginella (Selaginellaceae) research in Nusantara

“…Selaginella is generally abundant in forest floor; its dimorphism leaf is an adaptation to low light intensity (Hebant and Lee 1984). Some species can modify chloroplast to boost exploitation of sunshine by forming bizonoplast, such as S. erythropus (Sheue et al 2007) or non-pigment optical structure of schemochromic tydallblue iridescent, such as S. willdenowii and S. uncinata (Fox and Wells, 1971;Hebant and Lee 1984;Lee 2001;Thomas 2010), and S. singalanensis (Setyawan 2009), which loose in open place (Hebant and Lee 1984). S. willdenowii also adapt to grow under shade, with ratio of chlorophyll a:b equal to 2,2 (Nasrulhaq and Duckett 1991).…”