Potentiation of pathogen-specific defense mechanisms in <i>Arabidopsis</i> by β-aminobutyric acid

The nonprotein amino acids γ-aminobutyric acid (GABA) and β-aminobutyric acid (BABA) have known biological effects in animals and plants. Their mode of action has been the object of thorough research in animals but remains unclear in plants. Our objective was to study the mode of action of BABA in the protection of <i>Arabidopis</i> plants against virulent pathogens. BABA protected <i>Arabidopsis</i> against the oomycete pathogen <i>Peronospora parasitica</i> through activation of natural defense mechanisms of the plant such as callose deposition, the hypersensitive response, and the formation of trailing necroses. BABA was still fully protective against <i>P. parasitica</i> in transgenic plants or mutants impaired in the salicylic acid, jasmonic acid, and ethylene signaling pathways. Treatment with BABA did not induce the accumulation of mRNA of the systemic acquired resistance (SAR)-associated <i>PR-1</i> and the ethylene- and jasmonic acid-dependent <i>PDF1.2</i> genes. However, BABA potentiated the accumulation of <i>PR-1</i> mRNA after attack by virulent pathogenic bacteria. As a result, BABA-treated <i>Arabidopsis</i> plants were less diseased compared with the untreated control. In the case of bacteria, BABA protected mutants insensitive to jasmonic acid and ethylene but was not active in plants impaired in the SAR transduction pathway. Thus, BABA protects <i>Arabidopsis</i> against different virulent pathogens by potentiating pathogen-specific plant resistance mechanisms. In addition, we provide evidence that BABA-mediated papilla formation after <i>P. parasitica</i> infection is independent of the SAR signaling pathway.