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A research team led by Professor Zhuang Xiaohong from the School of Life Sciences at The Chinese University of Hong Kong (CUHK) has discovered a plant-unique membrane-less compartment via biomolecular condensation in plant cells for autophagosome biogenesis to facilitate degradation of heat-induced protein aggregates, providing a novel insight into the mechanism of plant high-temperature stress resilience. The research findings have recently been published in the top multidisciplinary scientific journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).

Biology textbooks tell us that cellular compartments, also known as organelles, are separated by membrane boundaries, so that each can have different contents and perform specific functions. Autophagy, derived from the Greek words “auto” (“self”) and “phagy” (“eating”), is a self-eating process that breaks down and reuses old cell contents, akin to the process of packing waste into a bin bag – in this case, a structure called an autophagosome. As opposed to other membrane-bound organelles, an autophagosome is a unique double-membraned organelle which is formed de novo. The autophagosome is not inherited from the mother cell, like mitochondria or chloroplasts. Instead, they are highly induced to meet cellular needs or, under stress conditions, to scavenge unwanted or damaged cellular contents for recycling. The creation of an autophagosome starts from the assembly of a membrane sac known as a phagophore, which expands into a sheet to sequester the cargo and finally seals into a complete, doughnut-shaped, double-membraned vesicle. The mechanism of phagophore initiation in plant cells is poorly understood.

Autophagy requires a set of autophagy-related proteins (ATGs) to execute specific functions at different steps during autophagosome formation. ATG8 is a key player, which is linked to the autophagosome membrane through a series of reactions and used as a reporter of autophagosome structures. In addition, ATG8 recognises the cargo directly or indirectly through selective autophagy receptors. A research team led by Professor Zhuang Xiaohong from the School of Life Sciences at CUHK applied proximity-dependent labelling methods for proteomic profiling of ATG8 interactome in the model plant Arabidopsis and uncovered a novel ATG8-interacting protein family, named ERC (Figure 2). In particular, ERC proteins form large, membrane-less droplets together with ATG8 proteins when blocking ATG8 from the membrane.

Recently, increasing studies have observed that certain cellular molecules might undergo a phase transition to form membrane-less droplets known as biomolecular condensates, akin to oil droplets in water, which are capable of organising cellular functions without membrane boundaries. With a combination of temporal and spatial analysis, the research team discovered that ERC-mediated biomolecular condensate represents a plant-unique type of biomolecular condensate, displaying a complex spatial architecture in close association with the membranes (Figure 2). Subsequent investigation of the ERC proximitome revealed that ERC also interacts with NBR1, a selective autophagy receptor in plants and animals. NBR1 is well known for targeting ubiquitinated substrates for degradation through autophagy to aid plant heat tolerance. Further functional analysis showed that ERC proteins are essential for NBR1 turnover and plant growth under heat stress conditions.

“We are very excited to observe that plant-unique biomolecular condensates are involved at the very beginning of autophagosome formation, before the membrane boundary is set,” remarked Professor Zhuang Xiaohong. “Biomolecular condensation is emerging as a crucial cellular assembly process for plant stress responses. We will be further exploring the underlying mechanism for the assembly of these biomolecular condensates, which might illuminate how to create synthetic condensates that can improve plant resilience.”

PhD students Chung Ka-kit, Law Kai-ching and Zhao Ziwei contributed equally to this project as the co-first authors of the research paper.

The full research article can be accessed here: https://doi.org/10.1073/pnas.2425689122