Systemic acquired tolerance to virulent bacterial pathogens in tomato

Abstract

Recent studies on the interactions between plants and pathogenic microorganisms indicate that the processes of disease symptom development and pathogen growth can be uncoupled. Thus, in many instances, the symptoms associated with disease represent an active host response to the presence of a pathogen. These host responses are frequently mediated by phytohormones. For example, ethylene and salicylic acid (SA) mediate symptom development but do not influence bacterial growth in the interaction between tomato (Lycopersicon esculentum) and virulent Xanthomonas campestris pv vesicatoria (Xcv). It is not apparent why extensive tissue death is integral to a defense response if it does not have the effect of limiting pathogen proliferation. One possible function for this hormone-mediated response is to induce a systemic defense response. We therefore assessed the systemic responses of tomato to Xcv. SA- and ethylene-deficient transgenic lines were used to investigate the roles of these phytohormones in systemic signaling. Virulent and avirulent Xcv did induce a systemic response as evidenced by expression of defense-associated pathogenesis-related genes in an ethylene- and SA-dependent manner. This systemic response reduced cell death but not bacterial growth during subsequent challenge with virulent Xcv. This systemic acquired tolerance (SAT) consists of reduced tissue damage in response to secondary challenge with a virulent pathogen with no effect upon pathogen growth. SAT was associated with a rapid ethylene and pathogenesis-related gene induction upon challenge. SAT was also induced by infection with Pseudomonas syringae pv tomato. These data show that SAT resembles systemic acquired resistance without inhibition of pathogen growth.

Figure 1.

Symptom development of Xcv-infected tomato. Disease symptoms at 16 dpi of wild-type tomato plants (A) inoculated with avirulent and virulent Xcv and (B) following challenge with virulent Xcv. These pictures are representative of at least two independent biological experiments.

Figure 2.

Cell death of tomato 16 d after challenge with virulent Xcv. Cell death was measured in the form of percent ion leakage in plants treated with a mock challenge or challenged with virulent Xcv. The plants were exposed to a mock inoculation or inoculation with virulent (vir) or avirulent (avr) Xcv prior to treatment. These data are representative of two independent biological experiments. Bars equal se, n = 30.

Figure 3.

Bacterial growth during inoculation and challenge of tomato with Xcv. A, The growth of virulent (vir) and avirulent (avr) Xcv was measured during primary infections. B, A systemic challenge with virulent Xcv was then performed on these plants as well as mock-inoculated controls and the bacterial growth was measured. These data are representative of two independent biological experiments. Bars equal se, n = 5.

Figure 4.

Local ethylene and SA accumulation following inoculation and challenge of tomato with Xcv. Tomato plants were mock inoculated or inoculated with virulent (vir) or avirulent (avr) Xcv and (A) ethylene and (B) SA accumulation following inoculation were measured. Fourteen days later, a challenge with virulent Xcv was performed on uninfected leaves and (C) ethylene and (D) SA accumulation following challenge was measured. These data are representative of two independent biological experiments. Bars equal se, n = 4.

Figure 5.

Local and systemic PR gene induction in ethylene- and SA-deficient tomato lines in response to inoculation with virulent Xcv. Ethylene-deficient ACD, SA-deficient NahG, and their isogenic parents were inoculated with virulent Xcv. PR1a and PR1b expression levels were determined by real-time RT-PCR in local and systemic tissues. These data are representative of two independent biological experiments. Bars equal se, n = 4.

Figure 6.

Local and systemic PR gene expression during inoculations with virulent or avirulent Xcv. Wild-type (UC82B) plants were mock inoculated or inoculated with virulent (vir) or avirulent (avr) Xcv and the local and systemic expression of PR1a and PR1b was determined by real-time RT-PCR. These data are representative of two independent biological experiments. Bars equal se, n = 4.

Figure 7.

Induction of PR genes during challenge with virulent Xcv in the presence or absence of SAT. Wild-type plants were mock inoculated or inoculated with virulent (vir) or avirulent (avr) Xcv. Fourteen days later, a challenge was performed with virulent Xcv. The expression of PR1a and PR1b was determined with real-time RT-PCR following challenge. These data are representative of two independent biological experiments. Bars equal se, n = 4.

Figure 8.

SAT induction by Xcv and Pst. Wild-type plants were mock inoculated, treated with the SAR inducer Actigard, or inoculated with virulent Xcv or Pst. Fourteen days later, a challenge was performed with virulent Xcv or Pst. Populations of (A) Xcv and (B) Pst were determined at 5 d after challenge. Bars equal se, n = 5. Percent ion leakage was determined 16 d after challenge in plants challenged with (C) Xcv or (D) Pst. Bars equal se, n = 30. Each experiment was repeated at least twice.