Expression Of Pathogenesis-related Protein Genes Research Articles

Cloning of a Pathogenesis-Related Protein-1 Gene from Nicotians glutinosa L. and Its Salicylic Acid-Independent Induction by Copper and β-Aminobutyric Acid

Summary Induction of pathogenesis-related (PR) protein gene expressions are one of the unique features of disease resistant reactions of plants against pathogen infection. To better understand the molecular mechanism of plants by monitoring expression of PR protein gene, we isolated a PR-1 cDNA from TMV- induced Nicotiana glutinosa cDNA library using PCR-based method. Southern blot hybridization showed that there may present 2 copies of PR-1 gene in diploid plant N. glutinosa and possibly 4 copies in amphidiploid plant N. tabacum . Nucleotide sequence analysis showed that the PR-1 clone is 682 bp in length which containing one open reading frame of 168 amino acids. Deduced amino acid sequence of this cDNA has 88 % identity with PR-1 protein from N. tabacum cv. Samsun NN. Northern blot hybridization revealed that PR-1 mRNA of N. glutinosa is induced by TMV infection in TMV resistant but not in susceptible tobacco cultivar. However, salicylic acid (SA) treatment induces this gene both in TMV resistant and susceptible tobacco cultivars. Some chemicals were tested as possible inducers, resulting in identification of copper and β-aminobutyric acid(β-ABA) as potent inducers of tobacco PR gene expression. Temporal expression studies of PR-1 and glucanase (PR-2) genes in tobacco revealed that induction of PR gene expressions in tobacco by copper and (β-ABA treatment are not dependent on accumulation of endogenous SA following treatment of those chemicals. This result suggests that there may exist an SA-independent signal transduction pathway leading to induce expression of PR-genes in tobacco.

Two inducers of plant defense responses, 2,6-dichloroisonicotinec acid and salicylic acid, inhibit catalase activity in tobacco.

2,6-Dichloroisonicotinic acid (INA) and salicylic acid (SA) are potent inducers of plant defense responses including the synthesis of pathogenesis-related (PR) proteins and the development of enhanced disease resistance. A soluble SA-binding protein has been purified from tobacco with an affinity and specificity of binding that suggest it is a SA receptor. Recently, this protein has been shown to be a catalase whose enzymatic activity is inhibited by SA binding. We have proposed that the resulting increase in intracellular levels of reactive oxygen species plays a role in the induction of defense responses such as PR protein gene expression. Here we report that INA, like SA, binds the SA-binding protein/catalase and inhibits its enzymatic activity. In fact, the dose-response curves for inhibition of catalase by these two compounds are similar. Furthermore, the ability of both INA analogues and SA derivatives to bind and inhibit tobacco catalase correlates with their biological activity to induce PR-1 gene expression and enhance resistance to tobacco mosaic virus. Comparison of the structures of INA, SA, and their analogues reveals several common features that appear to be important for biological activity. Thus, these results not only suggest that INA and SA share the same mechanism of action that involves binding and inhibition of catalase but also further indicate an important role for reactive oxygen species in the induction of certain plant defense responses. This is supported by the demonstration that INA-mediated PR-1 gene activation is suppressed by antioxidants.