The Effects of Salicylic Acid and Tobacco Mosaic Virus Infection on the Alternative Oxidase of Tobacco

Abstract

Salicylic acid (SA) is a signal in systemic acquired resistance and an inducer of the alternative oxidase protein in tobacco (Nicofiana tabacum cv Xanthi nc) cell suspensions and during thermogenesis i n aroid spadices. The effects of SA on the levels of alternative oxidase protein and the pathogenesis-related l a mRNA (a marker for systemic acquired resistance), and on the partitioning of electrons between the Cyt and alternative pathways were investigated in tobacco. Leaves were treated with 1.0 mM SA and mitochondria isolated at times between 1 h and 3 d after treatment. Alternative oxidase protein increased 2.5-fold within 5 h, reached a maximum (9-fold) after 12 h, and remained at twice the level of control plants after 3 d. Measurements of isotope fractionation of '*O by intact leaf tissue gave a value of 23% at all times, identical to that of control plants, indicating a constant 27 to 30% of electron-flow partitioning to the alternative oxidase independent of treatment with SA. Transgenic NahC tobacco p!ants that express bacterial salicylate hydroxylase and possess very low levels of SA gave a fractionation of 23% and showed control levels of alternative oxidase protein, suggesting that steady-state alternative oxidase accumulates i n an SA-independent manner. lnfection of plants with tobacco mosaic virus resulted in an increase in alternative oxidase protein in both infected and systemic leaves, but no increase was observed in comparably infected NahC plants. Total respiration rate and partitioning of electrons to the alternative pathway in virus-infected plants was comparable to that in uninfected controls.

Cyanide Restores N Gene–Mediated Resistance to Tobacco Mosaic Virus in Transgenic Tobacco Expressing Salicylic Acid Hydroxylase

“…In contrast, Aox transcript levels were not increased in either mock-inoculated plants or TMVinoculated plants of the susceptible cultivar Xanthi nn. Furthermore, when exogenous SA is fed to Xanthi nn tobacco, there is increased resistance to TMV (White et al, 1983), and we have shown that this resistance is accompanied by Aox gene induction (Figure 1; Chivasa et al, 1997).Our findings are consistent with the results of Lennon et al (1997), who showed that AOX protein accumulation was increased in directly inoculated and noninoculated leaves of Xanthi NN tobacco infected with TMV but was not increased in the corresponding leaves of TMV-infected SH-L transgenic tobacco plants. Our results, together with those of Lennon et al (1997), provide mounting evidence in support of a role for AOX in resistance to viruses.…”

“…Our findings are consistent with the results of Lennon et al (1997), who showed that AOX protein accumulation was increased in directly inoculated and noninoculated leaves of Xanthi NN tobacco infected with TMV but was not increased in the corresponding leaves of TMV-infected SH-L transgenic tobacco plants. Our results, together with those of Lennon et al (1997), provide mounting evidence in support of a role for AOX in resistance to viruses.…”

“…The work has also lent further support to the hypothesis that AOX is involved in the induction of resistance (Lennon et al, 1997), specifically in the induction of resistance to viruses (Chivasa et al, 1997). We have found that a synthetic SAR-inducing chemical, INA, and two metabolic poisons, AA and KCN, also induce resistance to viruses via the SHAM-sensitive signaling pathway.…”

“…Our results, together with those of Lennon et al (1997), provide mounting evidence in support of a role for AOX in resistance to viruses. However, the only way to definitively test the role of AOX in resistance to viruses is to use transgenic plants containing modified levels of AOX (Vanlerberghe et al, 1994) when they are made generally available.…”

“…Thus, our results suggest that the AOX may play a role in the signal transduction pathway leading to SAR against viruses (Chivasa et al, 1997). Independently, work by another group has also implicated AOX in the induction of resistance (Lennon et al, 1997). Here, we report further studies on the SHAM-sensitive signal transduction pathway that leads to resistance to viruses.…”