The Arabidopsis thaliana NPR1 has been shown to be a key regulator of gene expression during the onset of a plant disease-resistance response known as systemic acquired resistance. The npr1 mutant plants fail to respond to systemic acquired resistance-inducing signals such as salicylic acid (SA), or express SA-induced pathogenesis-related (PR) genes. Using NPR1 as bait in a yeast two-hybrid screen, we identified a subclass of transcription factors in the basic leucine zipper protein family (AHBP-1b and TGA6) and showed that they interact specifically in yeast and in vitro with NPR1. Point mutations that abolish the NPR1 function in A. thaliana also impair the interactions between NPR1 and the transcription factors in the yeast two-hybrid assay. Furthermore, a gel mobility shift assay showed that the purified transcription factor protein, AHBP-1b, binds specifically to an SA-responsive promoter element of the A. thaliana PR-1 gene. These data suggest that NPR1 may regulate PR-1 gene expression by interacting with a subclass of basic leucine zipper protein transcription factors.Systemic acquired resistance (SAR) is a general plantresistance response that can be induced during a local infection by an avirulent pathogen. Although early studies of SAR were conducted by using tobacco mosaic virus and its Solanaceous hosts (1), SAR has been demonstrated in many plant species and shown to be effective against not only viruses but also bacterial and fungal pathogens (2, 3). A necessary signal for SAR induction is salicylic acid (SA); plants that fail to accumulate SA because of the expression of an SA-oxidizing enzyme, salicylate hydroxylase, are impaired in SAR (4). Conversely, an elevation in the endogenous level of SA or exogenous application of SA or its synthetic analogs, such as 2,6-dichloroisonicotinic acid (INA), not only results in an enhanced, broad-spectrum resistance but also stimulates concerted expression of a battery of genes known as pathogenesisrelated (PR) genes (5-12). PR genes may play direct roles in conferring resistance because their expression coincides with the onset of SAR and some of the PR genes encode enzymes with antimicrobial activities (11,12). Therefore, understanding the regulation of PR gene expression has been a focal point of research in plant disease resistance.Using PR genes as reporters, two classes of Arabidopsis thaliana mutants have been identified. One class constitutively expresses PR genes whereas the other class is impaired in the SA-or INA-induced PR gene expression (13)(14)(15)(16)(17)(18)(19)(20). Interestingly, from the second class of mutants only one genetic locus, NPR1 (also known as NIM1), has been identified. NPR1 has been shown to be a key component of the SA-regulated PR gene expression and disease resistance because npr1 mutants fail to express PR1, PR2, and PR5 and display enhanced susceptibility to infection even after treatment with SA or INA (17-20). Furthermore, transgenic plants overexpressing NPR1 display a more dramatic induction of PR genes durin...

This protocol is used to induce transgenic roots on soybean to study the function of genes required in biological processes of the root. Young seedlings with unfolded cotyledons are infected at the cotyledonary node and/or hypocotyl with Agrobacterium rhizogenes carrying the gene construct to be tested and the infection sites are kept in an environment of high humidity. When the emerged hairy roots can support the plants, the main roots are removed and the transgenic roots can be tested. Using this method, almost 100% of the infected plants form hairy roots within 1 month from the start of the experiments.

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...

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 steroid-inducible system, we clearly demonstrate that nuclear localization of NPR1 is essential for its activity in inducing PR genes.

The Glycine max nodule autoregulation receptor kinase (GmNARK) plays a central role in the systemic signal transduction pathway controlling nodulation in soybean. We used transcriptional profiling to identify potential downstream signals of this receptor kinase. These studies revealed that GmNARK-mediated signaling controls the expression of genes involved in the jasmonic acid (JA) pathway. Genes encoding the key enzymes controlling JA biosynthesis as well as JA-response genes were regulated systemically but not locally by root inoculation with Bradyrhizobium japonicum. This systemic regulation was abolished in Gmnark mutant plants, indicating that their expression was specifically controlled by signaling events associated with this receptor kinase. Foliar application of a JA biosynthesis inhibitor significantly reduced nodulation specifically in supernodulating mutant plants. These results indicate that the receptor-mediated regulation of JA signaling plays an important role in the AON signal transduction pathway. A second class of genes was identified that were controlled by GmNARK in a rhizobia-independent manner. These candidates provide insight on additional, nonsymbiotic signaling pathways that are likely regulated by GmNARK, such as those involved in root growth and defense. The discovery of downstream components of the GmNARK receptor kinase advances our understanding of the systemic control of nodule development and its association with other signaling networks.