Plant lipid transfer proteins (LTPs) are small, cysteine-rich proteins secreted into the extracellular space. They belong to the pathogenesis-related proteins (PR-14) family and are believed to be involved in several physiological processes including plant disease resistance, although their precise biological function is still unknown. Here, we show that a recombinant tobacco LTP1 is able to load fatty acids and jasmonic acid. This LTP1 binds to specific plasma membrane sites, previously characterized as elicitin receptors, and is shown to be involved in the activation of plant defense. The biological properties of this LTP1 were compared with those of LTP1-linolenic and LTP1-jasmonic acid complexes. The binding curve of the LTP1-linolenic acid complex to purified tobacco plasma membranes is comparable to the curve obtained with LTP1. In contrast, the LTP1-jasmonic acid complex shows a strongly increased interaction with the plasma membrane receptors. Treatment of tobacco plants with LTP1-jasmonic acid resulted in an enhancement of resistance toward Phytophthora parasitica. These effects were absent upon treatment with LTP1 or jasmonic acid alone. This work presents the first evidence for a biological activity of a LTP1 and points out the crucial role of protein-specific lipophilic ligand interaction in the modulation of the protein activity.
SummaryCuticle plays a major role as a protective barrier in plants. Despite its physiological importance, the mode of formation of this complex structure remains poorly understood. In particular, none of the putative enzymes involved in the biosynthesis of the cutin, the matrix of cuticle, have been cloned. We have shown previously that peroxygenase is able to catalyze in vitro the epoxidation step required for the biosynthesis of C18 cutin monomers. In the present work, we have con®rmed in planta that this oxidase is indeed a key enzyme involved in the formation of cutin. Thus, in maize leaves, the speci®c inactivation of peroxygenase by organophosphorothioates resulted in a dramatic decrease of cuticular epoxide content, as visualized by a speci®c histochemical technique that was accompanied by a reduced thickness of the cuticle. A strict correlation could also be established between the extent of inhibition of the peroxygenase and the modi®cation of the cuticle triggered by a family of structurally related inhibitors. Importantly, these effects were restricted to plants that contain a cutin originating from C18 monomers. The altered cuticle of maize, treated with the peroxygenase inhibitor, was characterized by an increased permeability to pesticides. In addition, such plants became largely susceptible to infection by fungi, implying that the cuticle represents a crucial target for the modulation of the response in plant±pathogen interactions.
Elicitation of defense reactions in tobacco by cryptogein, triggered a production of active oxygen species (AOS) via the NADPH oxidase, NtrbohD, and an accumulation of b1din, a defense induced b-type subunit of 20S proteasome. The proteasome inhibitor, MG132, stimulated this AOS production. Tobacco cells transformed with sense constructs of b1din showed an inhibition of the AOS production following elicitin treatment, whereas the antisense transformed cells showed a strongly enhanced AOS production. In cells transformed with sense construct of b1din, the NtrbohD transcripts failed to be induced by cryptogein as observed in control and antisense transformed cells. Conversely, in tobacco cells transformed with antisense constructs for NtrbohD, b1din transcripts remained at a low level after elicitation. These results constitute the first demonstration of proteasome comprising b1din acting as a negative regulator of NtrbohD and contributes to the regulation of AOS generation during plant defense reactions.
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