Plastid stromules: video microscopy of their outgrowth, retraction, tensioning, anchoring, branching, bridging, and tip-shedding

Abstract

Stromules are stroma-containing tubules which can grow from the surface of plastids, most commonly leucoplasts and chromoplasts, but also chloroplasts in some tissues. Their functions are obscure. Stills from video rate movies are presented here. They illustrate interaction of stromules with cytoskeletal strands and the anchoring of stromules to unidentified components at the cell surface. Anchoring leads to stretching and relaxation of stromules when forces arising from cytoplasmic streaming act on the attached, freely suspended plastid bodies. Data on stromule growth, retraction, and regrowth rates are provided. Formation and movement of stromular branches and bridges between plastids are described. The shedding of a tip region into the streaming cytoplasm is recorded in frame-by-frame detail, in accord with early observations.

Mobilization of Rubisco and Stroma-Localized Fluorescent Proteins of Chloroplasts to the Vacuole by anATGGene-Dependent Autophagic Process

“…In cinephotomicrographic and video microscopic studies, chloroplast protuberances have been observed to segment into vesicular structures (Wildman et al, 1962;Gunning, 2005). The release of bodies approximately 1.5 mm in diameter from the ends of stromules has been observed (Gunning, 2005). In transgenic plants expressing stroma-targeted GFP, GFP bodies of similar size, which are presumably separated from plastids or stromules, have also been observed by LSCM (Arimura et al, 2001;Waters et al, 2004).…”

“…In cinephotomicrographic and video microscopic studies, chloroplast protuberances have been observed to segment into vesicular structures (Wildman et al, 1962;Gunning, 2005). The release of bodies approximately 1.5 mm in diameter from the ends of stromules has been observed (Gunning, 2005).…”

New insights on stromules: Stroma filled tubules extended by independent plastids

“…[1][2][3][4][5] Time-lapse movies using differential interference contrast microscopy suggested that the sporadically extending and retracting tubules could result in plastid interactions. 6 However, it was the discovery of the green fluorescent protein (GFP) and its subsequent targeting to the plastid stroma that clearly highlighted fluorescent tubules and resulted in the coining of a simple descriptive term 'stromule' by Kohler and Hanson. 4,5 Further, GFP photo-bleaching showed that there is a flow and exchange of macromolecules between interconnected plastids.…”

“…4,7,8 Since their re-discovery numerous conditions leading to stromule induction have been reported in original publications and discussed in several reviews. [5][6][7][8][9][10][11][12][13][14] The authoritative review by Hanson and Sattarzadeh 12 specifically appraised the status of stromules and provided a balanced discussion on the different possible roles that stromules might play within the cell. Most importantly this review clearly states that "most plastids are not connected at any one time, although over the course of a day, it is possible that many plastids within a cell establish transient contacts through stromules".…”

“…12 Nevertheless, an oversimplification of the original observations on stromules has resulted in leading text books presenting stromules as conduits involved in macromolecule exchange between plastids. 15 Further many plant biologists have probably not critically assessed the experimental design reported in original publications [3][4][5][6][7][8] and appear to think of stromules as tubular interconnections between plastids. 9,16 The recognition of stromules as sporadically extended stroma filled tubules from all kinds of plastids constitutes one of the major insights that resulted from the direct application of green fluorescent protein aided imaging of living plant cells.…”