Nuclear Localization of NPR1 Is Required for Activation of PR Gene Expression

Abstract

Systemic acquired resistance (SAR) is a broad-spectrum resistance in plants that involves the upregulation of a battery of pathogenesis-related ( PR ) genes. NPR1 is a key regulator in the signal transduction pathway that leads to SAR. Mutations in NPR1 result in a failure to induce PR genes in systemic tissues and a heightened susceptibility to pathogen infection, whereas overexpression of the NPR1 protein leads to increased induction of the PR genes and enhanced disease resistance. We analyzed the subcellular localization of NPR1 to gain insight into the mechanism by which this protein regulates SAR. An NPR1-green fluorescent protein fusion protein, which functions the same as the endogenous NPR1 protein, was shown to accumulate in the nucleus in response to activators of SAR. To control the nuclear transport of NPR1, we made a fusion of NPR1 with the glucocorticoid receptor hormone binding domain. Using this steroidinducible system, we clearly demonstrate that nuclear localization of NPR1 is essential for its activity in inducing PR genes. INTRODUCTIONPlants, like animals, are capable of mounting an immune response after a primary pathogen infection. One such response is known as systemic acquired resistance (SAR). SAR, which is often triggered by a local infection, can provide long-term resistance throughout the plant to subsequent infections by a broad range of pathogens (Ross, 1961;Kuc, 1982;Ryals et al., 1996). The activation of SAR correlates with the expression of the pathogenesis-related ( PR ) genes. Even though the functions of most PR gene products are unknown, some of these proteins have been shown to confer various degrees of pathogen resistance (Schlumbaum et al., 1986;Mauch et al., 1988; Broglie et al., 1991; Woloshuk et al., 1991;Terras et al., 1992 Terras et al., , 1995 Alexander et al., 1993;Liu et al., 1994;Ponstein et al., 1994; Zhu et al., 1994).Activation of PR gene expression and the establishment of SAR require the signal molecule salicylic acid (SA). Concentrations of SA have been shown to increase in both infected and uninfected tissues after pathogen infection (Malamy et al., 1990;Métraux et al., 1990Métraux et al., , 1991Rasmussen et al., 1991). The exogenous application of SA or its synthetic analogs, such as 2,6-dichloroisonicotinic acid (INA) and benzo(1,2,3)thiadiazole-7-carbothioic acid S -methyl ester, results in expression of the PR genes and activation of SAR (White, 1979;Ward et al., 1991; Görlach et al., 1996;Lawton et al., 1996). The essential role of SA in SAR has been demonstrated in transgenic tobacco and Arabidopsis plants that express the bacterial salicylate hydroxylase ( nahG ) gene. In these plants, SA is converted to the inactive compound catechol, and the induction of PR gene expression and SAR is inhibited (Gaffney et al., 1993; Delaney et al., 1994;Lawton et al., 1995).Transduction of the SA signal requires the function of NPR1, a protein first identified in Arabidopsis through a mutant screen (Cao et al., 1994). The npr1 (nonexpressor of PR genes) mutant f...

Vitis vinifera VvNPR1.1 is the functional ortholog of AtNPR1 and its overexpression in grapevine triggers constitutive activation of PR genes and enhanced resistance to powdery mildew

“…Following treatment with SA or an analog, the NPR1 oligomer is reduced to monomers that are relocalized to the nucleus, resulting in the activation of PR gene expression (Kinkema et al 2000;Mou et al 2003). Several conserved cystein residues play an important role in redox-regulated NPR1 localization in Arabidopsis.…”

“…In non-induced Arabidopsis plants, NPR1 is present in the cytoplasm as an oligomer maintained through redoxsensitive intermolecular disulfide bonds (Kinkema et al 2000;Mou et al 2003). Following treatment with SA or an analog, the NPR1 oligomer is reduced to monomers that are relocalized to the nucleus, resulting in the activation of PR gene expression (Kinkema et al 2000;Mou et al 2003).…”

“…Upon SAR induction, the disulfide bonds are reduced, and NPR1 is converted to a monomeric form. Monomeric NPR1 is translocated to the nucleus, where it interacts with transcription factors from the TGA family to regulate the expression of defense genes (Kinkema et al 2000;Mou et al 2003).…”

Expression of callose synthase genes and its connection with Npr1 signaling pathway during pathogen infection

“…Our results suggest that the high induction of CalS1 and CalS12 is Npr1 dependent whereas the weak induction of CalS1, 5, 9, 10 and 12 genes is Npr1indepen-dent. The Arabidopsis NPR1 plays an important role in inducible plant disease resistance (Johnson et al 2003;Kinkema et al 2000;Li et al 1999;Ryals et al 1996). The expression of Npr1 is also induced by SA and pathogen (Cao et al 1997).…”

Light conditions influence specific defence responses in incompatible plant?pathogen interactions: uncoupling systemic resistance from salicylic acid and PR-1 accumulation

“…Induced systemic resistance (ISR), triggered by certain non-pathogenic, root-colonizing rhizobacteria, is SA independent and not associated with the expression of PR genes (Pieterse et al 1996). Both SAR and ISR have in common that they depend on NPR1, a protein mediating SA-induced PR gene expression and exhibiting distinct signalling functions in the cytosol and in the nucleus (Kinkema et al 2000;Spoel et al 2003). Interestingly, NPR1 function is regulated by changes in cellular redox potential to induce PR gene expression and SAR, and this might be mediated by SA accumulation (Mou et al 2003).…”