Gigantic chloroplasts, including bizonoplasts, are common in shade‐adapted species of the ancient vascular plant family Selaginellaceae

Abstract

et al. 2020. Gigantic chloroplasts, including bizonoplasts, are common in shade-adapted species of the ancient vascular plant family Selaginellaceae.PREMISE: Unique among vascular plants, some species of Selaginella have single giant chloroplasts in their epidermal or upper mesophyll cells (monoplastidy, M), varying in structure between species. Structural variants include several forms of bizonoplast with unique dimorphic ultrastructure. Better understanding of these structural variants, their prevalence, environmental correlates and phylogenetic association, has the potential to shed new light on chloroplast biology unavailable from any other plant group. METHODS:The chloroplast ultrastructure of 76 Selaginella species was studied with various microscopic techniques. Environmental data for selected species and subgeneric relationships were compared against chloroplast traits. RESULTS:We delineated five chloroplast categories: ME (monoplastidy in a dorsal epidermal cell), MM (monoplastidy in a mesophyll cell), OL (oligoplastidy), Mu (multiplastidy, present in the most basal species), and RC (reduced or vestigial chloroplasts). Of 44 ME species, 11 have bizonoplasts, cup-shaped (concave upper zone) or bilobed (basal hinge, a new discovery), with upper zones of parallel thylakoid membranes varying subtly between species. Monoplastidy, found in 49 species, is strongly shade associated. Bizonoplasts are only known in deep-shade species (<2.1% full sunlight) of subgenus Stachygynandrum but in both the Old and New Worlds. CONCLUSIONS:Multiplastidic chloroplasts are most likely basal, implying that monoplastidy and bizonoplasts are derived traits, with monoplastidy evolving at least twice, potentially as an adaptation to low light. Although there is insufficient information to understand the adaptive significance of the numerous structural variants, they are unmatched in the vascular plants, suggesting unusual evolutionary flexibility in this ancient plant genus.Plants carry out photosynthesis over a huge range of environmental conditions. Although the key organelle, the chloroplast, might be expected to vary adaptively in size, number, and structure over this range, chloroplast traits are generally highly conserved in land plants. Exceptions to this rule have the potential to be especially instructive. The monogeneric seedless vascular plant family 564 • American Journal of Botany

Light-Dependent Morphological Changes Can Tune Light Absorption in Iridescent Plant Chloroplasts: A Numerical Study Using Biologically Realistic Data

“…Their photonic properties were recently characterized for the first time (32). A schematic of their structure is represented in Figure 4 a and it behaves as a one-dimensional photonic crystal which reflects strongly in the blue, around 450 nm (21,22,32,33).…”

The hornworts: morphology, evolution and development

“…Unique amongst land plants with the exception of a small number of lycophyte species, all hornworts have one (or just a few) chloroplast(s) per cell (Fig. 7) (Vaughn et al ., 1992; de Vries & Gould, 2018; Liu et al ., 2020). All other land plants have several chloroplasts per vegetative cell.…”

“…Sporophytes bear stomata, which may be homologous to those of tracheophytes (Renzaglia et al ., 2017; Harris et al ., 2020). (2) Chloroplast: it is the only extant land plant lineage, together with some lycophytes (Liu et al ., 2020), that has a single chloroplast (or just a few) per cell. The chloroplast of several hornwort species may contain a pyrenoid, a structure also found in many streptophyte algae and other algal lineages but not in other land plants (Li et al ., 2017; Meyer et al ., 2017).…”

A simpleAgrobacterium-mediated stable transformation technique for the hornwort modelAnthoceros agrestis

“…Hornworts display a unique combination of features (Frangedakis et al, 2020) such as a sporophyte, that is produced by an indeterminate basal meristem, and bears stomata similar to mosses and vascular plants (Renzaglia et al, 2017). In addition, it is the only extant land plant lineage (together with a few Selaginella species (Liu et al, 2020)), that has a single (or just a few) algal-like chloroplast(s) per cell. The chloroplasts resemble those of algae in that they may contain pyrenoids, a carbon-concentrating structure that is shared with many algal lineages (Villarreal and Renner, 2012; Li et al, 2017).…”