Polymerization-mediated SRFR1 condensation in upper lateral root cap cells regulates root growth.

Primary root growth, regulated by internal hormone signals, adapts to external factors such as water availability, soil compactness, and microbial interactions. An essential step in root growth consists of cell divisions in the meristem, with the outermost root cap layer thought to provide protection. However, recent studies reveal that lateral root cap (LRC) cells control meristem size and lateral root initiation. In this study, we identified an upper LRC-specific protein condensation mechanism involving SUPPRESSOR of rps4-RLD1 (SRFR1) that governs root growth and show that growth conditions and hormone treatment dynamically modulate condensate accumulation. SRFR1 condensate formation is driven by its plant-associated N-terminal tetratricopeptide repeat (PANT) polymerization domain and fine-tuned by the adjacent intrinsically disordered region 1 (IDR1). Mutational and biophysical analyses show that IDR1's zwitterionic nature is essential for its regulatory role, acting as a chaperone to promote PANT polymerization at low temperatures while preventing aggregation at high temperatures. This enables SRFR1 condensate formation across a wide temperature range. Notably, the zwitterionic IDR1 can be functionally substituted by zwitterionic dehydrins. Shifting IDR1 toward a negative state impairs, whereas a positive shift enhances SRFR1 condensation and further improves root growth. The association of zwitterionic IDRs with polymerization domains is common, suggesting that this mechanism broadly prevents irreversible aggregation and promotes physiological polymerization under varying temperatures.