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ABSTRACT In metazoans, autophagosomes fuse with late endosomes (LEs)/multivesicular bodies (MVBs) to form a hybrid organelle known as an amphisome. Subsequently upon fusion with lysosomes the contents of amphisomes are degraded. While the formation of metazoan amphisomes has been well established, it has remained an open question whether amphisomes form and deliver their cargo to the central vacuole for degradation in plant cells. In this mini review, we provide an update on recent discoveries in the field of plant autophagy that demonstrate the formation of amphisome-like organelles that are generated through several distinct autophagosome/MVB fusion pathways. 

Inducible protein degradation systems are an important but untapped resource for the study of protein function in plant cells. Unlike mutagenesis or transcriptional control, regulated degradation of proteins of interest allows the study of the biological mechanisms of highly dynamic cellular processes involving essential proteins. While systems for targeted protein degradation are available for research and therapeutics in animals, there are currently limited options in plant biology. Targeted protein degradation systems rely on target ubiquitination by E3 ubiquitin ligases. Systems that are available or being developed in plants can be distinguished primarily by the type of E3 ubiquitin ligase involved, including those that utilize Cullin-RING ligases, bacterial novel E3 ligases, and N-end rule pathway E3 ligases, or they can be controlled by proteolysis targeting chimeras. Target protein ubiquitination leads to degradation by the proteasome or targeting to the vacuole, with both pathways being ubiquitous and important for the endogenous control of protein abundance in plants. Targeted proteolysis approaches for plants will likely be an important tool for basic research and to yield novel traits for crop biotechnology. 

Abstract Ligand-induced endocytosis of the immune receptor FLAGELLIN SENSING2 (FLS2) is critical for maintaining its proper abundance in the plasma membrane (PM) to initiate and subsequently down regulate cellular immune responses to bacterial flagellin or flg22-peptide. The molecular components governing PM abundance of FLS2, however, remain mostly unknown. Here, we identified Arabidopsis (Arabidopsis thaliana) DYNAMIN-RELATED PROTEIN1A (DRP1A), a member of a plant-specific family of large dynamin GTPases, as a critical contributor to ligand-induced endocytosis of FLS2 and its physiological roles in flg22-signaling and immunity against Pseudomonas syringae pv. tomato DC3000 bacteria in leaves. Notably, drp1a single mutants displayed similar flg22-defects as those previously reported for mutants in another dynamin-related protein, DRP2B, that was previously shown to colocalize with DRP1A. Our study also uncovered synergistic roles of DRP1A and DRP2B in plant growth and development as drp1a drp2b double mutants exhibited severely stunted roots and cotyledons, as well as defective cell shape, cytokinesis, and seedling lethality. Furthermore, drp1a drp2b double mutants hyperaccumulated FLS2 in the PM prior to flg22-treatment and exhibited a block in ligand-induced endocytosis of FLS2, indicating combinatorial roles for DRP1A and DRP1B in governing PM abundance of FLS2. However, the increased steady-state PM accumulation of FLS2 in drp1a drp2b double mutants did not result in increased flg22 responses. We propose that DRP1A and DRP2B are important for the regulation of PM-associated levels of FLS2 necessary to attain signaling competency to initiate distinct flg22 responses, potentially through modulating the lipid environment in defined PM domains.