Introduction

In all eukaryotic cells, the transport of proteins and lipids from the Endoplasmic Reticulum (ER) to the Golgi apparatus is a fundamental process for cell function and survival. While this pathway is well-understood in animal and yeast cells to occur via small transport vesicles, the mechanism in higher plants is a subject of considerable scientific debate. Unlike the stationary Golgi in animal cells, plant Golgi stacks are highly mobile and move along the ER network. This review brings together four leading cell biologists to present their divergent and often opposing views on whether this transport occurs via classical vesicles, direct tubular connections, or an alternative mechanism.

Research Objective

This article aimed to critically evaluate the existing evidence and competing models for ER-to-Golgi transport in plant cells by:

• Presenting the molecular evidence for the “secretory unit” model, where ER Exit Sites (ERES) and Golgi stacks move together.
• Summarizing the ultrastructural and live-cell imaging data that supports the existence of direct tubular connections between the ER and Golgi.
• Defending the classical vesicle-mediated transport model and addressing the challenges in visualizing these structures in plants.
• Introducing novel insights from super-resolution microscopy in yeast that suggest a hybrid “hug-and-kiss” model, which may offer a resolution to the controversy.

Key Findings

The review highlights a deep disagreement within the scientific community, with each expert presenting compelling but conflicting evidence:

• The Secretory Unit Model (Brandizzi): Live-cell imaging shows that COPII coat proteins (essential for transport) are found in punctate structures (ERES) that are physically associated with and move along with the motile Golgi stacks. This “secretory unit” model suggests a tightly coupled system, but does not resolve the nature of the transport carrier itself.
• The Case for Tubes (Hawes): Evidence from historical electron microscopy studies, particularly using osmium impregnation techniques, suggests the presence of direct membrane-bounded tubules connecting the ER and Golgi. The rarity of COPII vesicle sightings in plant electron micrographs is presented as strong evidence against a vesicle-only model.
• The Case for Vesicles (Robinson): This viewpoint argues that vesicles are physiologically necessary to maintain the distinct biochemical identities and pH gradients of the ER and Golgi. The lack of convincing images of gap-spanning tubules is emphasized, and the difficulty in visualizing vesicles is attributed to their transient nature, rapid transport, and the extremely narrow (~500 nm) interface between the two organelles.
• An Alternative from Yeast (Nakano): Using super-resolution live imaging microscopy (SCLIM) in yeast, a “hug-and-kiss” mechanism was observed. In this model, the cis-Golgi makes brief contact with ERES to directly capture cargo without the full release of free vesicles into the cytoplasm. This could represent a more efficient transport mechanism that explains the lack of free vesicles seen in micrographs.

Methodology

This publication is a critical review and synthesis of expert opinions, not a primary research study. It analyzes data from a wide range of methodologies used in the field:

• Organisms Studied: The discussion centers on higher plants such as Arabidopsis thaliana and tobacco (Nicotiana tabacum), with comparative insights from yeast (Saccharomyces cerevisiae) and mammalian cells.
• Experimental Approach: The authors critically review findings from decades of research, contrasting results from different techniques.
• Key Techniques Discussed: The debate hinges on interpretations of data from live-cell fluorescence microscopy (confocal, photobleaching), electron microscopy (including high-pressure freezing and tomography), immunolabeling of key proteins (like COPII and COPI), and the use of genetic mutants.

Importance for Space Missions

Understanding the fundamental mechanisms of plant cell biology is crucial for the success of bioregenerative life support systems on long-duration space missions.

• Plant Health and Growth: The ER-Golgi pathway is central to synthesizing and secreting proteins and polysaccharides needed for cell wall construction, plant growth, and stress responses. An incomplete understanding of this pathway limits our ability to predict how plants will function in the unique environment of space.
• Optimizing Life Support: Efficient plant growth is required for reliable food production, oxygen generation, and water recycling. Any disruption to fundamental cellular processes like protein transport could compromise plant health and the stability of the entire life support system.
• Countermeasure Development: By identifying the precise mechanisms of cellular transport, scientists can better diagnose and potentially mitigate issues with plant growth in microgravity and high-radiation environments.

Knowledge Gaps & Future Research

The core of this review is the identification of a major knowledge gap. The primary unanswered question is the definitive nature of the transport vector between the ER and Golgi in plants. Future research must focus on:

• Applying advanced imaging technologies like super-resolution live imaging (SCLIM) and correlative light and electron microscopy (CLEM) to plant cells to directly visualize the transport event in real-time at high resolution.
• Elucidating the molecular machinery that tethers the Golgi to the ER and facilitates their co-movement as a “secretory unit.”
• Determining if and how transport is regulated during the “stop-and-go” phases of Golgi movement.
• Investigating whether the “hug-and-kiss” model observed in yeast is also applicable to plant cells.

Results

The primary result of this review is the clear articulation that the mechanism of ER-to-Golgi transport in plants remains a significant and unresolved scientific controversy. The traditional vesicle-based model, while functionally logical, faces challenges from the lack of direct visual evidence in plants. Alternative models involving direct tubular connections or transient contact-based transfer present compelling counterarguments. Resolving this fundamental question is essential for a complete model of plant cell biology and is a prerequisite for accurately predicting and ensuring plant health and productivity in the challenging environments of future space exploration missions.