Plastid Envelope-Localized Proteins Exhibit a Stochastic Spatiotemporal Relationship to Stromules

Abstract

Plastids in the viridiplantae sporadically form thin tubules called stromules that increase the interactive surface between the plastid and the surrounding cytoplasm. Several recent publications that report observations of certain proteins localizing to the extensions have then used the observations to suggest stromule-specific functions. The mechanisms by which specific localizations on these transient and sporadically formed extensions might occur remain unclear. Previous studies have yet to address the spatiotemporal relationship between a particular protein localization pattern and its distribution on an extended stromules and/or the plastid body. Here, we have used discrete protein patches found in several transgenic plants as fiducial markers to investigate this relationship. While we consider the inner plastid envelope-membrane localized protein patches of the GLUCOSE 6-PHOSPHATE/PHOSPHATE TRANSLOCATOR1 and the TRIOSE-PHOSPHATE/ PHOSPHATE TRANSLOCATOR 1 as artifacts of fluorescent fusion protein over-expression, stromule formation is not compromised in the respective stable transgenic lines that maintain normal growth and development. Our analysis of chloroplasts in the transgenic lines in the Arabidopsis Columbia background, and in the arc6 mutant, under stromule-inducing conditions shows that the possibility of finding a particular protein-enriched domain on an extended stromule or on a region of the main plastid body is stochastic. Our observations provide insights on the behavior of chloroplasts, the relationship between stromules and the plastid-body and strongly challenge claims of stromule-specific functions based solely upon protein localization to plastid extensions.One sentence summaryObservations of the spatiotemporal relationship between plastid envelope localized fluorescent protein fusions of two sugar-phosphate transporters and stromules suggest a stochastic rather than specific localization pattern that questions the idea of independent functions for stromules.

Membrane contacts with the endoplasmic reticulum modulate plastid morphology and behaviour

“…Patches of varying sizes, similar to those observed for BnCLIP1-eGFP were observed for both transporters. Similar to that concluded for the transporters ( Delfosse et al., 2018 ) BnCLIP1-eGFP localization could occur on any part of a stromule and was considered stochastic Figure 1C ).…”

“…However, clear differences in behaviour became apparent between the punctae in control plants expressing GPT1mEosFP ( Figure 1F ) and TPT1-mEosFP ( Figure 1G ) and BnCLIP1-eGFP punctae ( Figure 1E ) in extended stromules ( Figure 1E versus Figure 1G ). While BnCLIP1-eGFP punctae maintained their small size ( Figure 1E ; Supplementary Movie 2 ) the punctae in TPT1-mEosFP ( Figure 1G ) and the second control plants of GPT1-mEosFP ( Figure 1F ); Delfosse et al., 2018 ) extended considerably to cover large regions of a stromule (arrowheads in Figure 1G ).…”

Chloroplast Auxin Efflux Mediated by ABCB28 and ABCB29 Fine-Tunes Salt and Drought Stress Responses in Arabidopsis

“…High expression levels in transient expression assays of OE and IE proteins can lead to alterations in the shape of the respective membrane and might be considered artifacts of fusion protein overexpression [ 38 , 39 ]. Similarly, localized protein patches were observed in several stable transgenic plants overexpressing OE and IE fluorescence protein fusions [ 40 , 41 , 42 , 43 ] without altering growth and development [ 41 , 44 ]. Moreover, the effect was proposed to be a product of the GFP construct [ 44 ].…”

“…Therefore, the formation of punctate protrusions of ABCB28-GFP signals in the ABCB28 pro :ABCB28:GFP line might suggests that ABCB28 is highly expressed in the leaves. Alternatively, ABCB28 might actually localize as small domains to increase its outreach and interactivity with other cytosolic and organelle-resident proteins, as was suggested for proteins known to be essential for plastid functions [ 40 ]. In contrast, the fluorescence of the stromal marker PTCK-CFP [ 42 ] overlapped with the autofluorescence of chlorophyll (Chl) ( Figure 1 b and Figure S2b ).…”

A review on prospective production of biofuel from microalgae

“…Few algae that are of research interest are Chlamydomonas reinhardtii (21% lipids, 48% sugars), Spirulina platensis (8% lipids, 60% sugars) and Chlorella sp. (19% lipids, 56% sugars) [ 35 , 36 ]. Today nearly 10 countries are keen in biofuel production from the algae biomass.…”