β-aminobutyric acid (BABA): towards understanding its biosynthesis, localization and hormonal regulation in "Arabidopsis thaliana"

Abstract

Priming is defined as a physiological state induced by a priming stimulus that allows a plant to deploy a more rapid and more robust defense response to stresses compared with a non-primed plant. β-aminobutyric acid (BABA) has emerged as one of the best stimuli to study priming. Plants can synthesize BABA and accumulate it after being exposed to both biotic or abiotic stress. The plant immune system regulates BABA accumulation during pathogen infection. BABA concentrations vary depending on organ type and with the developmental stage. Flowers, senescent leaves and seeds are the sites of major accumulation. Interestingly, the early senescence and constitutive priming mutant cpr5-2 shows higher basal and induced BABA concentrations compared to its wild type. Besides cpr5-2, no other mutants related to BABA have been characterized up to now. Therefore, we performed RNA-seq analysis to identify common genes expressed during various BABA-inducing biotic and abiotic stresses in Arabidopsis Col-0 and cpr5-2. The analysis revealed ten genes up-regulated in common and one down-regulated gene. Nevertheless, T-DNA insertional lines of the up-regulated genes did not show a wild-type BABA concentration after salt stress application, keeping unsolved the search for genes involved in BABA metabolism in plants. Furthermore, we are looking at the relation between BABA and plant hormones. Exposition of Arabidopsis to different plant hormones revealed that only ABA led to an increase of the BABA concentration. The bioactive form of ABA, (+)-ABA, showed a more robust BABA induction phenotype. Mutants of the three SnRK2 genes, key regulators of ABA signaling, showed BABA induction after (+)-ABA application, suggesting a compensation effect. Finally, to get further information on BABA localization in plants, BABA has been modified chemically, adding an alkyne tag. Successively, a copper-catalyzed azide-alkyne cycloaddition (CuAAC) between tagged BABA and an azide Alexa fluor® was performed, generating fluorescence tagged BABA molecules. These molecules were then visualized on confocal microscopy, showing a cell wall localization of BABA in Arabidopsis roots and globular subcellular structures. Leaves showed no labeling. However, the results are too early to define precisely the exact localization of BABA in plants.