The phytohormone salicylic acid (SA) is well known for its induction of pathogenesis-related proteins and systemic acquired resistance; SA also has specific effects on plant growth and development. Here we analyzed the effect of SA on Arabidopsis (Arabidopsis thaliana) root development. We show that exogenous SA treatment at low (below 50 µM) and high (greater than 50 µM) concentrations affect root meristem development in two different PR1-independent ways. Low-concentration SA promoted adventitious roots and altered architecture of the root apical meristem, whereas high-concentration SA inhibited all growth processes in the root. All exposures to exogenous SA led to changes in auxin synthesis and transport. A wide range of SA treatment concentrations activated auxin synthesis, but the effect of SA on auxin transport was dose dependent. Mathematical modeling of auxin synthesis and transport predicted auxin accumulation or depletion in the root tip following low- or high-concentration SA treatments, respectively. SA-induced auxin accumulation led to the formation of more layers of columella initials, an additional cortical cell layer (middle cortex), and extra files of epidermis, cortex, and endodermis cells. Suppression of SHORT ROOT and activation of CYCLIN D6;1 mediated the changes in radial architecture of the root. We propose that low-concentration SA plays an important role in shaping root meristem structure and root system architecture.
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