Thaumatin was isolated as a sweet-tasting protein. Arabidopsis has over 20 Thaumatin-Like Protein (TLP)/Osmoti-Like Protein (OLP) genes that belong to the PR5 family. Although biotic stress-related functions of TLPs have been reported from transgenic lines expressing TLPs, it is nonetheless necessary to investigate genetic phenotypes produced by defects in the TLP genes. In this report, four TLP genes were selected based on sequence similarities (Thau1/2/3/4), and the corresponding mutant thau1/2/3/4 was examined for biotic and abiotic stress responses. The thau1/2/3/4 mutant showed increased susceptibility to the Pseudomonas syringae pv. tomato DC3000 infection, and reduced sensitivity to the ABA and drought stress treatments. Each of the four thaumatin genes showed different gene expression patterns for ABA treatment. Moreover, ABA-inductions of Thau1/2/3/4 were largely dependent on the intact ABA signaling pathway mediated by PYR/PYL receptors. Among the many ABA-responsive genes affected by the defects of Thau1/2/3/4, reduced expression of P5CS1 with decreased accumulation phenotype of prolines indicates that compromised proline synthesis may be associated with the stress phenotypes of thau1/2/3/4. Our data suggest that Thau1/2/3/4 have a function in both biotic stress and abiotic stress signal transduction through the regulation of proline synthesis.
