Distribution and Evolution of Forms and Types of Sieve-element Plastids in the Dicotyledons

Abstract

Sieve-element plastids may contain any combination of protein crystals (c), protein filaments (f), and starch grains (s), or none of these. All structurally distinct combinations (=forms) possible (s, cs, cfs, c, cf, fs, f, o) are recorded among the 2100 dicotyledons investigated so far with the transmission electron microscope (representing 381 of more than 460 families described). The six forms that include c and/or f define the P-type and are found in some 620 species, mainly confined to Magnolianae, Caryophyllanae, Vitanae, and Rutanae (Fabales, Linales, Rhizophorales). A detailed family-by-family analysis and quantitative form-by-form comparison suggests that form-Ss plastids give rise to all other forms. Based on a logic model connecting all forms via one-step-alterations (loss or gain of starch, protein crystals, or filaments), the distribution of P-type plastids within dicotyledons, their interconnection, and their relationships to the S-type taxa are discussed. Due to the diversity of sieve-element plastids found within one taxon, the Magnolianae (Pand S-type) are held to be a center of P-type evolution and more primitive than Caryophyllanae (P-type, only) or other superorders. The significance of sieve-element plastid data to the shaping of some of the higher dicotyledon taxa is discussed.

Differential expression and phylogenetic analysis suggest specialization of plastid-localized members of the PHT4 phosphate transporter family for photosynthetic and heterotrophic tissues

“…However, such plastids have been identified in the sieve elements and companion cells that comprise phloem. 49,50 Promoter-GUS fusions used in this study revealed that PHT4;3 and PHT4;5 were expressed in the phloem tissue of leaves and cotyledons, but not roots. Expression of both PHT4;3 and PHT4;5 in leaves was diurnal with transcripts more abundant in the light.…”

“…Little has been reported on the metabolic functions of phloem plastids, although they may be the primary sites of tryptophan biosynthesis, 51 and at least some of these plastids accumulate starch. 50 Given that amino acid and starch synthesis are ATP consuming and Pi liberating processes, we hypothesize that PHT4;3 and PHT4;5 contribute to Pi export from phloem plastids.…”

Kollektiver Kraftakt zweier Exzentriker: Phloemtransport

“…In allen Familien der Spermatophyten findet man Plastiden in den Siebelementen [3]. Ihre Ultrastruktur ist sehr unterschiedlich, aber familienspezifisch.…”

The ER Within Plasmodesmata

“…What remains at maturity is the plasma membrane, a thin layer of parietal cytoplasm composed of stacked ER or fenestrated ER (Thorsch and Esau 1981a,b;Sjolund and Shih 1983) and a few dilated mitochondria (Evert 1990). Phloem-specific plastids of two types, with proteinaceous (P-type plastids) or starch (S-type plastids) inclusions (Esau 1969;Behnke 1991a;van Bel and Knoblauch 2000;van Bel 2003), and SE phloem proteins (Eleftheriou 1990;Evert 1990;Cronshaw and Sabnis 1990;Behnke 1991b;Iqbal 1995) are also conspicuous in the SE. To prevent these organelles being dragged along in the turbulent mass flow within the SE, 7-nm-long macromolecular extensions anchor the ER, mitochondria and plastids to each other and to the plasma membrane (Ehlers et al 2000).…”