Bulletin No. 1, 2013

Unconventional Protein Secretion in Plants 41 organelle along the rest of the way. An organelle is essentially a plant’s ‘organ’. This method of protein secretion takes place within the cell, at the endoplasmic reticulum. Proteins thus secreted include ones that are used as building blocks by the plant. Anomaly Discovered Professor Jiang had studied conventional secretion in plant cells, but while analysing secreted proteins in culture media, he noticed a growing number of proteins without a signal peptide on the exterior of cells. Cars without a GPS system are seen on the edge of the woods. 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 or bacteria, it can do one of two things: commit partial suicide by killing off the infected part or it can release antimicrobial agents to threaten or kill the attacker. Professor Jiang studies the latter process. And while scientists in the past assumed it 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, such as by dyeing different parts of the cell with fluorescent tag and observing the spatial overlap between two or more study targets to see if they were located in the same organelle. The results indicated the proteins have certainly taken a different path. Discovery of EXPO In the process, 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 the trafficking of certain organelles. One of EXPO’s possible functions 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 production of EXPO. Each EXPO carries within it a cargo of antimicrobial agents which are released when EXPO fuses with the cell membrane and cell wall, into the plant’s exterior. Way Ahead and Application Professor Jiang’s discoveries are groundbreaking but he believes that 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 potential functions in cell wall biosynthesis and defence has implications on the development of biofuel crop plants for second-generation bioethanol technology which presumably can supply more fuel more sustainably, affordably and with less harmful impact on the environment. Such knowledge may also contribute to the application of plant biotechnology for defence against pathogens. Professor Jiang may have confirmed that cars without GPS have taken a new route to the edge of the woods, but questions about the intricate mechanisms of this pathway remain to be illuminated by future studies. EXPO outside the plasma membrane just before releasing its protein cargo; tiny black dots indicate presence of proteins in EXPO