Prof. Jiang Liwen, professor in the School of Life Sciences of CUHK, broke new ground recently by proving that plants secrete proteins in unconventional ways that had previously been ignored by the plant research community. In the process, he also discovered a novel organelle responsible for this kind of protein secretion.

What Was Known

Conventionally in plants, protein secretion or exocytosis is achieved via a secretory pathway involving several organelles in plant cells. This method of secretion depends on the proteins having a signal peptide (a.k.a. leader peptide), which is a string of amino acids that acts like a GPS navigation system, guiding the protein into the first organelle of this pathway—the endoplasmic reticulum, before it travels from organelle to organelle along the rest of the way. An organelle is essentially a plant’s ‘organ’, a membrane-bound compartment within a cell with a specific function. There can be over a thousand organelles in a plant cell. This method of protein secretion is known as conventional constitutive secretion and it takes place within the cell, at the endoplasmic reticulum.

Proteins thus secreted include cell membrane proteins and cell wall modifying enzymes, which are used as building blocks by the plant.

Anomaly Discovered

Professor Jiang and his team had studied conventional protein secretion in plant cells. But while analysing secreted proteins in culture media, they noticed a growing number of proteins without a signal peptide on the exterior of cells. In other words, cars without a GPS system are seen on the edge of the woods. They were fascinated: How did they get there? Did they take the classic secretory pathway? Which organelle is responsible for this unconventional type of secretion?

Reacting to Pathogen Attack

Based on data obtained from various laboratory experiments, Professor Jiang hypothesized that a novel organelle (with a novel secretory pathway) is responsible for unconventional secretion in plants, and it is related to the plant’s defence mechanism.

Like human beings, plants come into contact with pathogens through air, soil and water. When a plant is challenged by infectious agents such as fungi, bacteria, viruses or insects, it can do one of two things: commit partial suicide by killing off the infected part or release antimicrobial agents to threaten or kill the attacker. Professor Jiang studies the latter process which scientists in the past had assumed belonged to the realm of conventional secretion. His recent discoveries have proven otherwise.

The research team adopted multiple methods to determine whether the proteins found on the exterior of plants had arrived there by the classic route. For example, they dyed different parts of the cell with fluorescent tag and observed the spatial overlap between two or more study targets to see if they were located in the same organelle. They also applied an inhibitor of secretory pathways to the cell, and found that proteins without a signal peptide were insensitive to the inhibitor. Together their results indicated that the proteins had certainly travelled a different path.

Discovery of EXPO

The team also found a novel double-membrane organelle which appears to mediate unconventional protein secretion in plants. They named it EXPO (exocyst-positive organelle). The exocyst is a type of protein complex involved in protein secretion in yeast and animal cells. One of EXPO’s possible functions in plants is to release internal content that warns attackers or defends the plant against them.

With the new discoveries, the cellular scenario of what happens when a plant comes under attack seems to shimmer into view. Once a pathogen has penetrated the cell wall into the cell membrane, receptors there tell the plant to ramp up its EXPO production. Each EXPO carries within it cargo proteins of antimicrobial agents which are released into the plant’s exterior when EXPO fuses with the cell membrane and cell wall.

Way Ahead

Unconventional protein secretion occurs in all plants but so far it has been underestimated due to the lack of molecular markers—proteins that can be used to define the organelle involved in the unconventional secretory pathway. Professor Jiang’s discoveries therefore are groundbreaking. However, he believes this is just the beginning of understanding unconventional protein secretion in plants: ‘It will be of great interest to learn more about EXPO’s cargo and to study the dynamics of its response to pathogen attack. We hypothesize that EXPO may play roles in cell wall biosynthesis and defence in plants. Understanding EXPO’s functions and its pathway will be useful for the fields of biofuel and plant biotechnology.’

Professor Jiang may have confirmed that cars without GPS have taken a new route to the edge of the woods, but light remains to be shed on the intricate mechanisms of this unusual pathway.