VAMP724 and VAMP726 are involved in autophagosome formation in Arabidopsis thaliana

A novel workflow for unbiased 3D quantification of autophagosomes in Arabidopsis thaliana roots

Macroautophagy is often quantified by live imaging of autophagosomes labeled with fluorescently tagged ATG8 protein (FP-ATG8) in Arabidopsis thaliana. The labeled particles are then counted in single focal planes. This approach may lead to inaccurate results as the actual 3D distribution of autophagosomes is not taken into account and appropriate sampling in the Z-direction is not performed. To overcome this issue, we developed a workflow consisting of immunolabeling of autophagosomes with an anti-ATG8 antibody followed by stereological image analysis using the optical disector and the Cavalieri principle. Our protocol specifically recognized autophagosomes in epidermal cells of Arabidopsis root. Since the anti-ATG8 antibody recognizes multiple AtATG8 isoforms, we were able to detect a higher number of immunolabeled autophagosomes than with the FP-AtATG8e marker, that most probably does not recognize all autophagosomes in a cell. The number of autophagosomes per tissue volume positively correlated with the intensity of autophagy induction. Compared with the quantification of autophagosomes in maximum intensity projections, stereological methods were able to detect the autophagosomes present in a given volume with higher accuracy. Our novel workflow provides a powerful toolkit for unbiased and reproducible quantification of autophagosomes and offers a convenient alternative to the standard of live imaging with FP-ATG8 markers.

How coat proteins shape autophagy in plant cells

Autophagy is a membrane trafficking pathway through which eukaryotic cells target their own cytoplasmic constituents for degradation in the lytic compartment. Proper biogenesis of autophagic organelles requires a conserved set of autophagy-related (ATG) proteins and their interacting factors, such as signalling phospholipid phosphatidylinositol 3-phosphate (PI3P) and coat complex II (COPII). The COPII machinery, which was originally identified as a membrane coat involved in the formation of vesicles budding from the endoplasmic reticulum, contributes to the initiation of autophagic membrane formation in yeast, metazoan, and plant cells; however, the exact mechanisms remain elusive. Recent studies using the plant model species Arabidopsis thaliana have revealed that plant-specific PI3P effectors are involved in autophagy. The PI3P effector FYVE2 interacts with the conserved PI3P effector ATG18 and with COPII components, indicating an additional role for the COPII machinery in the later stages of autophagosome biogenesis. In this Update, we examined recent research on plant autophagosome biogenesis and proposed working models on the functions of the COPII machinery in autophagy, including its potential roles in stabilizing membrane curvature and sealing the phagophore.

A condensates-to-VPS41-associated phagic vacuoles conversion pathway controls autophagy degradation in plants

Autophagy is a universal degradation system in eukaryotic cells. In plants, although autophagosome biogenesis has been extensively studied, the mechanism of how autophagosomes are transported to the vacuole for degradation remains largely unexplored. In this study, we demonstrated that upon autophagy induction, Arabidopsis homotypic fusion and protein sorting (HOPS) subunit VPS41 converts first from condensates to puncta, then to ring-like structures, termed VPS41-associated phagic vacuoles (VAPVs), which enclose autophagy-related gene (ATG)8s for vacuolar degradation. This process is initiated by ADP ribosylation factor (ARF)-like GTPases ARLA1s and occurs concurrently with autophagy progression through coupling with the synaptic-soluble N-ethylmaleimide-sensitive factor attachment protein rmleceptor (SNARE) proteins. Unlike in other eukaryotes, autophagy degradation in Arabidopsis is largely independent of the RAB7 pathway. By contrast, dysfunction in the condensates-to-VAPVs conversion process impairs autophagosome structure and disrupts their vacuolar transport, leading to a significant reduction in autophagic flux and plant survival rate. Our findings suggest that the conversion pathway might be an integral part of the autophagy program unique to plants.

Autophagosome biogenesis and organelle homeostasis in plant cells

Autophagy is one of the major highly inducible degradation processes in response to plant developmental and environmental signals. In response to different stimuli, cellular materials, including proteins and organelles, can be sequestered into a double membrane autophagosome structure either selectively or nonselectively. The formation of an autophagosome as well as its delivery into the vacuole involves complex and dynamic membrane processes. The identification and characterization of the conserved autophagy-related (ATG) proteins and their related regulators have greatly advanced our understanding of the molecular mechanism underlying autophagosome biogenesis and function in plant cells. Autophagosome biogenesis is tightly regulated by the coordination of multiple ATG and non-ATG proteins and by selective cargo recruitment. This review updates our current knowledge of autophagosome biogenesis, with special emphasis on the core molecular machinery that drives autophagosome formation and autophagosome-organelle interactions under abiotic stress conditions.

Vacuolar degradation of plant organelles

Plants continuously remodel and degrade their organelles due to damage from their metabolic activities and environmental stressors, as well as an integral part of their cell differentiation programs. Whereas certain organelles use local hydrolytic enzymes for limited remodeling, most of the pathways that control the partial or complete dismantling of organelles rely on vacuolar degradation. Specifically, selective autophagic pathways play a crucial role in recognizing and sorting plant organelle cargo for vacuolar clearance, especially under cellular stress conditions induced by factors like heat, drought, and damaging light. In these short reviews, we discuss the mechanisms that control the vacuolar degradation of chloroplasts, mitochondria, endoplasmic reticulum, Golgi, and peroxisomes, with an emphasis on autophagy, recently discovered selective autophagy receptors for plant organelles, and crosstalk with other catabolic pathways.

Dynamic organelle changes and autophagic processes in lily pollen germination

Pollen germination is a crucial process in the life cycle of flowering plants, signifying the transition of quiescent pollen grains into active growth. This study delves into the dynamic changes within organelles and the pivotal role of autophagy during lily pollen germination. Initially, mature pollen grains harbor undifferentiated organelles, including amyloplasts, mitochondria, and the Golgi apparatus. However, germination unveils remarkable transformations, such as the redifferentiation of amyloplasts accompanied by starch granule accumulation. We investigate the self-sustained nature of amylogenesis during germination, shedding light on its association with osmotic pressure. Employing BODIPY 493/503 staining, we tracked lipid body distribution throughout pollen germination, both with or without autophagy inhibitors (3-MA, NEM). Typically, lipid bodies undergo polarized movement from pollen grains into elongating pollen tubes, a process crucial for directional growth. Inhibiting autophagy disrupted this essential lipid body redistribution, underscoring the interaction between autophagy and lipid body dynamics. Notably, the presence of tubular endoplasmic reticulum (ER)-like structures associated with developing amyloplasts and lipid bodies implies their participation in autophagy. Starch granules, lipid bodies, and membrane remnants observed within vacuoles further reinforce the involvement of autophagic processes. Among the autophagy inhibitors, particularly BFA, significantly impede germination and growth, thereby affecting Golgi morphology. Immunogold labeling substantiates the pivotal role of the ER in forming autophagosome-like compartments and protein localization. Our proposed speculative model of pollen germination encompasses proplastid differentiation and autophagosome formation. This study advances our understanding of organelle dynamics and autophagy during pollen germination, providing valuable insights into the realm of plant reproductive physiology.

An Arabidopsis Rab18 GTPase promotes autophagy by tethering ATG18a to the ER in response to nutrient starvation

The expansion of autophagosomes requires a controlled association with the endoplasmic reticulum (ER). However, the mechanisms governing this process are not well defined. In plants, ATG18a plays a key role in autophagosome formation in response to stress, yet the factors regulating the process are unknown. This study finds that ATG18a acts as a downstream effector of RABC1, a member of the poorly characterized Rab18/RabC GTPase subclass in plants. Active RABC1 interacts with ATG18a on the ER, particularly under nutrient starvation. In rabc1 mutants, autophagy is compromised, especially under nutrient deprivation, affecting the ER association and expansion of ATG18a-positive autophagosomes. Furthermore, both dominant-negative and constitutively active RABC1 forms inhibit autophagy. The dominant inactive RABC1 impedes the ER association of ATG18a, whereas the constitutively active RABC1 delays ATG18a detachment from the ER. Collectively, RABC1 regulates the ER association and the subsequent detachment of ATG18a-positive autophagosomes during nutrient starvation.

VAMP726 and VAMP725 regulate vesicle secretion and pollen tube growth in Arabidopsis

KEY MESSAGE

VAMP726/VAMP725 and SYP131 can form a part of a SNARE complex to mediate vesicle secretion at the pollen tube apex. Secretory vesicle fusion with the plasma membrane of the pollen tube tip is a key step in pollen tube growth. Membrane fusion was mediated by SNAREs. However, little is known about the composition and function of the SNARE complex during pollen tube tip growth. In this study, we constructed a double mutant vamp725 vamp726 via CRISPR‒Cas9. Fluorescence labeling combined with microscopic observation, luciferase complementation imaging, co-immunoprecipitation and GST pull-down were applied in the study. We show that double mutation of the R-SNAREs VAMP726 and VAMP725 significantly inhibits pollen tube growth in Arabidopsis and slows vesicle exocytosis at the apex of the pollen tube. GFP-VAMP726 and VAMP725-GFP localize mainly to secretory vesicles and the plasma membrane at the apex of the pollen tube. In addition, fluorescence recovery after photobleaching (FRAP) experiments showed that mCherry-VAMP726 colocalizes with Qa-SNARE SYP131 in the central region of the pollen tube apical plasma membrane. Furthermore, we found that VAMP726 and VAMP725 can interact with the SYP131. Based on these results, we suggest that VAMP726/VAMP725 and SYP131 can form a part of a SNARE complex to mediate vesicle secretion at the pollen tube apex, and vesicle secretion may mainly occur at the central region of the pollen tube apical plasma membrane.

VAMP726 from maize and Arabidopsis confers pollen resistance to heat and UV radiation by influencing lignin content of sporopollenin

Sporopollenin in the pollen cell wall protects male gametophytes from stresses. Phenylpropanoid derivatives, including guaiacyl (G) lignin units, are known to be structural components of sporopollenin, but the exact composition of sporopollenin remains to be fully resolved. We analyzed the phenylpropanoid derivatives in sporopollenin from maize and Arabidopsis by thioacidolysis coupled with nuclear magnetic resonance (NMR) and gas chromatography-mass spectrometry (GC-MS). The NMR and GC-MS results confirmed the presence of p-hydroxyphenyl (H), G, and syringyl (S) lignin units in sporopollenin from maize and Arabidopsis. Strikingly, H units account for the majority of lignin monomers in sporopollenin from these species. We next performed a genome-wide association study to explore the genetic basis of maize sporopollenin composition and identified a vesicle-associated membrane protein (ZmVAMP726) that is strongly associated with lignin monomer composition of maize sporopollenin. Genetic manipulation of VAMP726 affected not only lignin monomer composition in sporopollenin but also pollen resistance to heat and UV radiation in maize and Arabidopsis, indicating that VAMP726 is functionally conserved in monocot and dicot plants. Our work provides new insight into the lignin monomers that serve as structural components of sporopollenin and characterizes VAMP726, which affects sporopollenin composition and stress resistance in pollen.