Plants possess complex internal systems for growth, development, and environmental response. MLO (Mildew Locus O) proteins are among the many components contributing to these processes. Found across the plant kingdom, they play various roles in plant biology. Understanding their function offers insights into how plants interact with their environment.
What Are MLO Proteins?
MLO proteins are a family found mainly in plants, involved in various cellular activities. They are characterized by a distinct structure, featuring seven transmembrane domains, spanning the cellular membrane multiple times. This arrangement embeds them within the plasma membrane, with an extracellular N-terminus and an intracellular C-terminus. MLO proteins also possess a calmodulin-binding domain (CaMBD) in their C-terminal tails, which is involved in their regulation.
How MLO Proteins Influence Cellular Processes
MLO proteins modulate various cellular processes in plants. They function as calcium channels, playing a role in calcium signaling, a fundamental communication system within plant cells. Calcium acts as a second messenger, and its concentration changes in the cytoplasm are important for relaying environmental stimuli and coordinating cellular responses, including those against pathogens. MLO proteins are also involved in membrane trafficking, the movement of cellular components within and out of the cell. For example, the MLO protein NORTIA (MLO7) undergoes signal-mediated polar trafficking to accumulate at specific sites and promote pollen tube bursting, which is important for plant reproduction.
MLO and Plant Disease Resistance
MLO proteins are known for their association with plant disease, particularly their role in susceptibility to powdery mildew. Powdery mildew is a widespread fungal disease that can severely reduce crop yields. Under normal conditions, MLO proteins act as susceptibility factors, meaning they facilitate the infection process for powdery mildew fungi.
However, specific mutations in MLO genes, known as “mlo” mutations, can lead to enhanced and broad-spectrum resistance against this pathogen. This “mlo resistance” is recessive and durable, meaning it remains effective against a wide range of powdery mildew isolates over time. For instance, the inactivation of the barley MLO protein prevents fungal penetration by promoting mechanisms like callose deposition and cell wall strengthening.
Agricultural Impact and Future Outlook
Understanding MLO proteins has significantly impacted agriculture, particularly in developing more resilient crop varieties. Natural mlo mutations in barley, for example, have been successfully incorporated into breeding programs, leading to widespread powdery mildew resistance in European barley agriculture for nearly four decades. Mlo-based resistance has also been found or generated in various other plant species, including wheat, tomato, pea, melon, cucumber, apple, and grape, through methods like induced mutagenesis, gene silencing, or gene knockout. Future research will explore the molecular functions of MLO proteins and their interactions with other cellular components. This understanding could lead to broader applications beyond powdery mildew, such as enhancing plant resilience to other stresses or improving various plant traits.