How the Softest Part of the Plant Cell Wall Makes It Strong: Pectin-Cellulose Entanglement Prevents Embolism Spread in the Plant Vascular System

Water transport in plants is blocked by the entry and spread of air embolisms in the vascular system. Pit membranes are cell walls found in the vasculature that help prevent embolism spread, but the relationship between their structure and ability to block air remains controversial. The current paradigm is that the spread of air across pit membranes is determined by defects in a rigid cellulose network, with the other main component of the wall -- pectin hydrogel -- being too soft to have a measurable impact. In contrast, we hypothesize that the pectin network plays a critical role in preventing embolism spread by locally stabilizing the dynamic cellulose network via polymer entanglements and calcium crosslinks. Using red maple (Acer rubrum) as our model system, we found that 1) removing pectin network increases the permeability to air, supporting the idea that the pectin network locally restrict the deformation of the cellulose network; and 2) calcium removal in the pectin network has a small effect on embolism spread, which suggests that pectin-cellulose entanglements may be more important than calcium crosslinks. Our study is relevant to predicting plant responses to drought in a warming climate.