Salicylic Acid Induction–Deficient Mutants of Arabidopsis Express PR-2 and PR-5 and Accumulate High Levels of Camalexin after Pathogen Inoculation

Abstract

In Arabidopsis, systemic acquired resistance against pathogens has been associated with the accumulation of salicylic acid (SA) and the expression of the pathogenesis-related proteins PR-1, PR-2, and PR-5. We report here the isolation of two nonallelic mutants impaired in the pathway leading to SA biosynthesis. These SA induction-deficient ( sid ) mutants do not accumulate SA after pathogen inoculation and are more susceptible to both virulent and avirulent forms of Pseudomonas syringae and Peronospora parasitica. However, sid mutants are not as susceptible to these pathogens as are transgenic plants expressing the nahG gene encoding an SA hydroxylase that degrades SA to catechol. In contrast to NahG plants, only the expression of PR-1 is strongly reduced in sid mutants, whereas PR-2 and PR-5 are still expressed after pathogen attack. Furthermore, the accumulation of the phytoalexin camalexin is normal. These results indicate that SA-independent compensation pathways that do not operate in NahG plants are active in sid mutants. One of the mutants is allelic to eds5 (for enhanced disease susceptibility), whereas the other mutant has not been described previously. INTRODUCTIONInfection with necrotizing pathogens often leads to the subsequent systemic induction of resistance. This is referred to as systemic acquired resistance (SAR), which is characterized by a long-lasting defense response against a broad spectrum of pathogens (Kuc, 1982;Ryals et al., 1994;Sticher et al., 1997). After the initial observation that treatment of plants with salicylic acid (SA) leads to resistance (White, 1979), several lines of evidence have shown that SA is associated with SAR. The expression of a number of pathogenesis-related (PR) proteins is highly correlated with acquired resistance Uknes et al., 1992). The synthesis of PR proteins can be induced by treatment with SA and also by the application of functional analogs of SA, such as 2,6-dichloroisonicotinic acid (INA;Métraux et al., 1991) or benzo(1,2,3)thiadiazole-7-carbothioic acid S -methyl ester (BTH; Görlach et al., 1996).The accumulation of SA in plant tissues correlates with the presence of PR proteins and resistance (Malamy et al., 1990;Métraux et al., 1990;Yalpani et al., 1991). In addition, transgenic tobacco (Gaffney et al., 1993) and Arabidopsis plants that express the Pseudomonas putida nahG gene, encoding an SA hydroxylase, which degrades SA to catechol, are hypersusceptible to infection with virulent pathogens and cannot develop SAR. Experiments with NahG plants also showed that SA is involved in gene-for-gene defense responses . These findings provide strong evidence for the involvement of SA in SAR.SA also has been proposed to be the systemic signal for SAR (Malamy et al., 1990;Métraux et al., 1990). However, although SA synthesized in infected leaves can be shown to be transported to uninfected leaves in tobacco and cucumber (Shulaev et al., 1995;Mölders et al., 1996), grafting experiments with tobacco plants expressing NahG, cholera toxin, or reduced levels of ...

Constitutive disease resistance requires EDS1 in the Arabidopsis mutants cpr1 and cpr6 and is partially EDS1‐dependent in cpr5

“…EDS1-independent signaling may also contribute to the maintenance of constitutive PR2 and PR5 expression in cpr6 eds1 plants. Uncoupling PR1 expression from regulation of PR2 and PR5 mRNAs has been observed in other SA-signaling mutants (Nawrath and Me  traux, 1999;Rogers and Ausubel, 1997;Zhou et al, 1998). Alternatively, residual resistance in cpr6 eds1 may be a consequence of the extremely high PDF1.2 expression observed ( Figure 3a).…”

“…Applications of SA also amplify EDS1 mRNA accumulation, pointing to the existence of an SA-generated positive feedback loop in the defense circuit (Falk et al, 1999). Other Arabidopsis mutations affecting defense signaling upstream of SA accumulation include pad4 (Glazebrook et al, 1997;Zhou et al, 1998), eds5/sid1 and sid2 (Nawrath and Me  traux, 1999;Rogers and Ausubel, 1997). Like EDS1, the wild-type PAD4 gene encodes a lipase-like protein whose expression is ampli®ed by applications of SA (Jirage et al, 1999).…”

Constitutive salicylic acid‐dependent signaling in cpr1 and cpr6 mutants requires PAD4

“…The latter explanation is more likely, as cpr5 eds5 double mutants have SA levels and P. syringae resistance indistinguishable from eds5 plants (Clarke et al, 2000). The eds5 mutation blocks SA accumulation and causes enhanced susceptibility to P. syringae (Nawrath and Metraux, 1999;Rogers and Ausubel, 1997), so these observations suggest that resistance to P. syringae in cpr5 is entirely SA-dependent. Resistance to P. parasitica was also observed in cpr5 pad4 plants.…”

“…NPR1 is a soluble protein containing ankyrin repeats Ryals et al, 1997). It interacts with TGA family transcription factors in yeast two-hybrid assays, suggesting that NPR1 might activate defense gene expression by altering the activity of transcription factors (Despre  s et al, 2000;Niggeweg et al, 2000;Zhang et al, 1999;Zhou et al, 2000). The effects of npr1 mutations can be suppressed by the sni1 mutation.…”

Dissecting the β-Aminobutyric Acid–Induced Priming Phenomenon in Arabidopsis

“…The signaling pathway controlling SAR requires endogenous accumulation of the stress hormone salicylic acid (SA; Gaffney et al, 1993;Nawrath and Mé traux, 1999) and an intact defense regulatory protein No PR-1/No Immunity 1/Salicylic Acid Insensitive 1 (NPR1/NIM1/SAI1) (Cao et al, 1994;Delaney et al, 1995;Shah et al, 1997). The expression of SAR, triggered by either pathogen infection or treatment with SA or its functional analogs 2,6-dichloroisonicotinic acid or benzothiadiazole (BTH), is tightly associated with the transcriptional activation of genes encoding pathogenesisrelated proteins (PRs; Van Loon, 1997).…”