Extended: Breaking out of isotropy: does pectin methylation control cell growth and cell shape?

Plants grow in a variety of shapes and sizes, and understanding how they do so is essential for our food security and crop sustainability. Transition from isotropic to anisotropic growth, where cells grow in specific directions, underpins the formation of these varied shapes. This process depends on the mechanical properties of the cell wall, which are determined by the structure, chemistry and interactions of its constituent polysaccharides. Pectin is one of the most abundant polysaccharides in primary cell walls, and its chemical modification, particularly pectin methylation, has been proposed to play an essential role in controlling plant cell growth and shape. However, the precise role of pectin methylation in cell growth and shape is far from being understood.

The first step for pectin methylation is to incorporate S-adenosyl methionine (SAM) molecules into the Golgi lumen; therefore, all pectin methylation is underpinned by the existence of Golgi SAM transporters (GoSAMTs). The GoSAMT discovery has allowed us to study the paramount relevance of pectin methylation in plant growth and development. To investigate this further, we developed a novel inducible CRISPR-based system called "pectin methylation switch-off" (PMSO). This system allows us to deplete GoSAMT activity upon induction, effectively turning off methylated pectin production in response to a specific stimulus.

Our findings revealed that induced PMSO impeded the transition from isotropic to anisotropic growth across various tissues. Remarkably, this effect persisted even under conditions known to induce anisotropic growth, such as etiolation and exposure to far-red light. Overall, this work shows clear evidence that pectin methylation is an essential requirement for plants to grow and take shape.