Table 2.

A non-comprehensive list of developmental conditions and exogenous treatments known to affect the formation of organelle extensions

Treatment or condition	Phenotype	Postulated mechanism	Reference
Related to tissue/organ-specific development/cautious interpretation
Fresh callus and cell suspensions of Tobacco	Developmental condition: Chloroplast body present. Generally elongated plastids and stromules	High sucrose and phytohormone media. Possible effect on internal membranes in chloroplasts	Köhler and Hanson (2000)
In normal, mature leaves mesophyll chloroplasts	Developmental condition: Low stromule abundance	Normal phenomenon compared to epidermal chloroplasts. Plastids are fully differentiated.	Waters et al. (2004)
Normal non-photosynthetic cells of epidermis, petals, roots	Developmental condition: High stromule abundance. Stromules may be confused with leucoplasts.	Immature plastids, etioplasts, and non-green plastids are usually elongated.	Holzinger et al. (2007); Köhler et al. (1997); Köhler and Hanson (2000)
Ripe fruit (tomato)	Developmental condition: Increased stromule abundance	Mostly leucoplasts and chromoplasts. Might fall under tubular organelles.	Waters et al. (2004)
Temperature fluctuation	Undetectable stromules at 10°C, many stromules at 40°C	Higher temperatures are associated with more fluid membranes.	Holzinger et al. (2007)
Anoxia for more than 4 h	Elongated and fused mitochondria (Tobacco cell culture)	Mitochondrial fusion as a stress response in order to optimize recombination of mitochondrial DNA.	Van Gestel and Verbelen (2002)
Hypoxia resulting from been kept underwater ca. 45 min (Arabidopsis and tobacco tissue)	Elongation of mitochondria followed by their expansion into giant mitochondria	Mitochondrial expansion as a way of dealing with low O2. Could also involve compromised osmoregulation involving increased permeability of mitochondrial envelope membranes	Jaipargas et al. (2015)
Vacuum infiltration of 40 mM sucrose or glucose solution into leaves	Increased stromule abundance	Unclear mechanism. Speculated as an osmotic response.	Schattat and Klösgen (2011) Barton et al. (2018)
500 mM mannitol solution (Wheat)	Increased stromules and some filamentous chloroplasts in leaves	Osmotic stress triggers stromule formation.	Gray et al. (2012)
≥ 200 mM potassium chloride (tobacco)	Over 80% of chloroplasts with extended stromules	Salt stress triggers stromule formation.	Gray et al. (2012)
100 µM abscisic acid (tobacco)	Over 80% of plastids with extended stromules	Mimics salt stress and dark growing conditions, dependent on cytosolic protein synthesis.	Gray et al. (2012)
6% polyethylene glycol (tobacco)	Over 80% of plastids emitting stromules	Stimulated drought conditions at the plastid trigger stromules.	Gray et al. (2012)
Leaf exposed at room temperature for 4 h (tobacco)	Greater stromule abundance	Desiccation triggers stromule formation.	Gray et al. (2012)
Antimycin A (50 and 10 µM)	Extreme, reversible plastid elongation in root cortex	Treatment blocks the mitochondrial bc1 complex in the electron transport chain, triggering the alternative oxidase pathway. Could involve elongated leucoplast.	Itoh et al. (2010)
0.08–0.8 M H2O2	Significant increase in peroxules	Hydroxyl stress increases subcellular demand for peroxisomal catalase. Peroxules appear before the formation of completely tubular peroxisomes that undergo fission.	Sinclair et al. (2009)
15–90 s UV-A treatment	Elongated peroxisomes	Produces subcellular H2O2, increases subcellular demand for peroxisomal catalase. Leads to peroxisomal fission.	Sinclair et al. (2009)
Cells undergoing arbuscule formation during mycorrhizae colonization (tobacco)	Increased localization of chloroplasts to the nucleus and the formation of stromules	Cell invasion triggers ROS accumulation and alters cellular sugar status, peroxisomes may respond.	Fester and Hause (2005); Fester et al. (2001)
100 µM s-triazine	Results in elongated mitochondria	Produces superoxide by blocking the chloroplast electron transport chain at photosystem II.	Scott and Logan (2008); Bliek (2009)
Abutilon mosaic virus infected cells	Increased stromule formation	A combination of ROS-induced responses that affect chloroplast envelope	Krenz et al. (2012)
Viral/bacterial infection of N. benthamiana leaves	Induced stromules that aggregated around the nucleus; Replicated upon salicylic acid and H2O2 treatment	Implicate pro-defence molecules and innate immunity signaling pathways	Caplan et al. (2015)
100 µM methyl viologen	Triggers mitochondrial elongation	Produces superoxide radicals that can trigger apoptosis.	Scott and Logan (2008); Bliek (2009)
1-Aminocyclopropane-1-carboxylic acid (ACC)	Increased stromule abundance	Mimics pathogen attack conditions when the cell produces ethylene (ACC is the first precursor in the committed ethylene synthesis pathway).	Gray et al. (2012)
1 µM silver nitrate	Decreases stromule abundance	Inhibits ethylene activity. May also link to ROS management.	Gray et al. (2012)
Mechanical wounding. Agrobacterium infiltration	Increased stromule abundance in cells near wound site	Altered ROS and sugar status in cell	Schattat et al. (2012b); Mathur (unpublished data)
Treatment with photosynthetic electron transport chain (pETC) inhibitors	Increased stromule frequency in chloroplasts	Implicate internal light-sensitive redox signaling pathways	Brunkard et al. (2015)
Isolated chloroplasts	Independent chloroplasts can form stromules	Internal redox signals suggested	Brunkard et al. (2015)
Senescent cotyledon and leaf tissue	Increased stromules and peroxules	Altered permeability of organelle membranes and major changes in ROS perception	Mathur (unpublished data)
Exposure to high light conditions	Increased peroxules and breakdown of mitochondria	Typical response to increased ROS levels in cell	Jaipargas et al. (2016); Mathur et al. (2018)