Exclusion of plastid nucleoids and ribosomes from stromules in tobacco and Arabidopsis

Abstract

SUMMARYStromules are stroma-filled tubules that extend from the surface of plastids and allow the transfer of proteins as large as 550 kDa between interconnected plastids. The aim of the present study was to determine if plastid DNA or plastid ribosomes are able to enter stromules, potentially permitting the transfer of genetic information between plastids. Plastid DNA and ribosomes were marked with green fluorescent protein (GFP) fusions to LacI, the lac repressor, which binds to lacO-related sequences in plastid DNA, and to plastid ribosomal proteins Rpl1 and Rps2, respectively. Fluorescence from GFP-LacI co-localised with plastid DNA in nucleoids in all tissues of transgenic tobacco (Nicotiana tabacum L.) examined and there was no indication of its presence in stromules, not even in hypocotyl epidermal cells, which contain abundant stromules. Fluorescence from Rpl1-GFP and Rps2-GFP was also observed in a punctate pattern in chloroplasts of tobacco and Arabidopsis [Arabidopsis thaliana (L.) Heynh.], and fluorescent stromules were not detected. Rpl1-GFP was shown to assemble into ribosomes and was co-localised with plastid DNA. In contrast, in hypocotyl epidermal cells of dark-grown Arabidopsis seedlings, fluorescence from Rpl1-GFP was more evenly distributed in plastids and was observed in stromules on a total of only four plastids (<0.02% of the plastids observed). These observations indicate that plastid DNA and plastid ribosomes do not routinely move into stromules in tobacco and Arabidopsis, and suggest that transfer of genetic information by this route is likely to be a very rare event, if it occurs at all.

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

“…Although it is known that plastids do not exchange genetic information, except very rarely, in the case of protoplast fusion, 12 the new study also showed that plastid ribosomes do not routinely move into stromules in tobacco and Arabidopsis. 16 Our findings on stromules made through the use of photoconvertible mEosFP provided a better picture than was available through the use of single colored fluorescent proteins. Accordingly we have been able to suggest a minor modification to the definition of 'stromule' as 'a stroma-filled tubule extended by an independent plastid' to differentiate them from stroma filled tubules that form an isthmus between pleomorphic elongated-dividing plastids.…”

“…Whether other smaller proteins are exchanged between plastids with or without the involvement of stromules still remains an open question. However, another recent study, 16 was undertaken to determine if plastid DNA or plastid ribosomes are able to enter stromules and thus aid in transfer of genetic information between plastids. Although it is known that plastids do not exchange genetic information, except very rarely, in the case of protoplast fusion, 12 the new study also showed that plastid ribosomes do not routinely move into stromules in tobacco and Arabidopsis.…”

“…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. Observations of dynamic green fluorescent stromules strongly suggested that plastids frequently interact with each other while photo-bleaching of interconnected plastids indicated that proteins can move within the stroma filled tubules.…”

Low-Phosphate Induction of Plastidal Stromules Is Dependent on Strigolactones But Not on the Canonical Strigolactone Signaling Component MAX2

“…1). SFs were similar to those previously reported for Arabidopsis (Holzinger et al, 2008;Newell et al, 2012). Because hypocotyl epidermal cells were used to study stromules in numerous previous publications (Hanson and Sattarzadeh, 2013;Kwok and Hanson, 2004;Holzinger, Kwok et al, 2008;Newell et al, 2012), we selected these cells to evaluate the effect of different treatments involving low phosphate, sugars, or strigolactones.…”

Chromatin Tagging Systems Contribute to Live Imaging Analyses for Chromatin Dynamics

“…The chromatin tagging system is also widely used to visualize the chromatin beyond subnuclear events. The dynamics of mitochondrial and chlorophyll DNA were observed with lacO/LacI-EGFP systems in S. cerevisiae and Nicotiana tabacum L., respectively (Osman et al 2015, Newell et al 2012, Newell and Gray 2010. The chromatin tagging system revealed that the arrangement of chromatin positioning was closely related to the regulation of gene activity.…”