Abstract
Phenylalanine ammonia-lyase (PAL) has a crucial role in secondary phenylpropanoid metabolism and is one of the most extensively studied enzymes with respect to plant responses to biotic and abiotic stress. Here, we identified the pepper (Capsicum annuum) PAL (CaPAL1) gene, which was induced in pepper leaves by avirulent Xanthomonas campestris pv. vesicatoria (Xcv) infection. CaPAL1-silenced pepper plants exhibited increased susceptibility to virulent and avirulent Xcv infection. Reactive oxygen species (ROS), hypersensitive cell death, expression of the salicylic acid (SA)-dependent marker gene CaPR1, SA accumulation, and induction of PAL activity were significantly compromised in the CaPAL1-silenced pepper plants during Xcv infection. Overexpression (OX) of CaPAL1 in Arabidopsis conferred increased resistance to Pseudomonas syringae pv. tomato (Pst) and Hyaloperonospora arabidopsidis infection. CaPAL1-OX leaves exhibited restricted Pst growth, increased ROS burst and cell death, and induction of PR1 expression and SA accumulation. The increase in PAL activity in healthy and Pst-infected leaves was higher in CaPAL1-OX plants than in wild-type Arabidopsis. Taken together, these results suggest that CaPAL1 acts as a positive regulator of SA-dependent defence signalling to combat microbial pathogens via its enzymatic activity in the phenylpropanoid pathway.
Highlights
Plants have evolved multiple defence signalling pathways to cope with adverse environmental conditions and pathogen attack (Jones and Dangl, 2006)
The strict conservation of a Phenylalanine ammonia-lyase (PAL) domain suggests that CaPAL1 may act as a PAL enzyme in nitrogen metabolism, phenylpropanoid biosynthesis, and alkaloid biosynthesis in pepper plants (Fritz et al, 2006)
PALs are encoded by a multi-gene family in plant species such as Arabidopsis (Wanner et al, 1995; Cochrane et al, 2004), Solanum lycopersicum (Guo and Wang, 2009), Bambusa oldhamii (Hsieh et al, 2010a, b), and Phyllostachys edulis (Gao et al, 2012)
Summary
Plants have evolved multiple defence signalling pathways to cope with adverse environmental conditions and pathogen attack (Jones and Dangl, 2006). The evolutionary emergence of the phenylpropanoid pathway in plants is an important adaptation that enables plant defence against abiotic and biotic stresses (Ferrer et al, 2008). Phenylpropanoids have important functions in several different pathways: in plant defence against pathogens and predators, in protection from UV irradiation, in signal transduction and communication with other organisms, and as regulatory molecules (Hahlbrock and Scheel, 1989; Dixon and Paiva, 1995; Ferrer et al, 2008; Vogt, 2010).
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