Post

Time for a thread!🧵 How different is the molecular organization of thylakoids in “higher” plants🌱? To find out, we teamed up with @profmattjohnson.bsky.social to dive into spinach chloroplasts with #CryoET ❄️🔬. Curious? ..Read on! #TeamTomo #PlantScience 🧪 🧶🧬 🌾 elifesciences.org/articles/105... 1/🧵

Unlike in algae and other phototrophs, plant thylakoids form specialized grana stacks🥞 interconnected by single thylakoids called stromal lamellae. The 3D membrane architecture we observed is consistent with the “helical staircase model” proposed from classical TEM (💚🔬 Andrew Staehelin, RIP). 2/🧵

We made an interesting observation about plastoglobules, lipid-protein storage droplets that share one leaflet with thylakoid membranes. We saw that plastoglobules often sat within little holes in the thylakoids 🟠🕳️, perhaps increasing the area of the contact site, facilitating exchange 🔄 3/🧵

Our data enabled us to quantify native thylakoid architecture in the grana. Getting precise numbers is important for modeling photosynthetic regulation. Here’s a comparison to similar measurements made by classical TEM. We’d like to apply #CryoET to the dark-to-light💡 transition in the future. 4/🧵

Contrary to previous models, our native views showed that grana are not perfect cylinders, but have wavy undulating edges 🌊🏄‍♀️. Here we slice through a few grana🥞 from the top– Do you see all the little protein densities in the membranes?🔍 Those are photosynthetic complexes. How to analyze them? 5/🧵

The talented @lorenzlamm.bsky.social (joint PhD w/ @tingyingpeng.bsky.social) developed #MemBrain 🤖🧠software to automatically segment the thylakoid membranes and localize all the photosystem II (PSII) complexes embedded within them. Let’s get analyzing!👨‍💻🤓 Try MemBrain: bsky.app/profile/lore... 6/🧵

For decades, the idea has persisted that grana margins have a distinct protein composition and provide a zone where PSI & PSII can intermix. However, our "membranograms" revealed strict lateral heterogeneity between PSI & PSII at the margins, just like we previously observed in green algae 🟦🟨 7/🧵

We also observed that PSII concentration was uniform throughout each granum, irrespective of distance from the margin. Thus, we propose a simple two-domain model of thylakoid organization in green algae and plants. And yes, we speculate about domain #PhaseSeparation. I await your hate mail 📬🤗 8/🧵

There have been many studies on PSII arrays, including #CryoET showing that they can stack thylakoid membranes together. The arrays also form in thylakoids from a variety of species, even diatoms. So it isn’t just a “plant thing”🌿. The catch is—all these studies looked at isolated membranes 🤔 10/🧵

We never saw PSII arrays within intact chloroplasts. However, we spotted them in chloroplasts that broke open and exposed their thylakoids to buffer. This is a known in vitro phenomenon called “thylakoid restacking”. Thus, interactions between arrays in two membranes seem to drive restacking!🤐 11/🧵

So, PSII arrays seem to be an artifact of isolating thylakoid membranes. And yet… We still wonder if these beautiful (and likely more static) structures play some physiological role during stress or damage. @wojwie.bsky.social & I wrote a mini-review on this: academic.oup.com/plcell/artic... 12/🧵