Salicylic Acid restricts cell elongation and induces changes of vacuolar morphology and pH

The phytohormone salicylic acid (SA) is a key factor to balance plant defence as well as growth and development. While its role in plant defence has been investigated for decades, regulation of plant growth and development has only come into focus recently. It has been demonstrated that SA application inhibits growth independently of the established Non-expressor of Pathogenesis Related (NPR) receptors. However, the underlying mechanism of this growth inhibition on the cellular level remains largely elusive. Here we show that SA restricts cell elongation and induces changes of vacuolar morphology and pH. Rapidly upon SA application we observe homotypic vacuole fusion and a significant increase in vacuolar pH. These changes seem to be independent of the phytohormone auxin which has been reported to crosstalk with SA. By increasing vacuolar pH, SA directly impacts basic cellular functions such as vesicle trafficking or nutrient storage, leading eventually to cell size restriction and limited growth. Our results demonstrate an NPR-independent mechanism to attenuate growth, potentially allowing free resources to be relocated to withstand environmental stresses.