RESUMO
SUMMARY The hxc1 mutant was identified by screening an EMS (ethylmethane sulphonate) mutagenized population of Arabidopsis Col-0 plants for an altered hypersensitive response (HR), after spray inoculation with an HR-inducing isolate of Xanthomonas campestris pv. campestris (Xcc) (strain 147). The hxc1 mutant shows a susceptible phenotype several days after initiation of the interaction with the avirulent strain. This macroscopically observed phenotype was confirmed by measurement of in planta bacterial growth and by microscopical analysis. Interestingly, the hxc1 mutation acts very specifically. Hxc1 displays a pathophenotype identical to that observed in the wild-type with several extensively characterized avirulent and virulent bacteria, except in response to Pseudomonas syringae pv. tomato strain DC3000/avrRpm1, for which a partial loss of resistance was observed. Finally, the mutation causes an attenuation of expression of several defence markers regulated through different signalling pathways. Together, these data underline the complexity of this novel defence mutant, and support the hypothesis of a mutation affecting a key component acting during the first steps of the plant defence response leading to resistance to Xcc147 and Pseudomonas syringae pv. tomato containing the avr gene, avrRpm1.
RESUMO
The effect of elevated light treatment (25 degrees C, PPFD 360 mumol m-2 sec-1) or chilling temperatures combined with elevated light (5 degrees C, PPFD 360 mumol m-2 sec-1) on the activity of six antioxidant enzymes, guaiacol peroxidases, and glutathione peroxidase (GPx, EC 1.11.1.9) protein accumulation were studied in tobacco Nicotiana tabacum cv. Petit Havana SR1. Both treatments caused no photooxidative damage, but chilling caused a transient wilting. The light treatment increased the activities of ascorbate peroxidase (APx, EC 1.11.1.11) and guaiacol peroxidases while catalase (EC 1.11.1.6), superoxide dismutase (SOD, EC 1.15.1.1), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2) were unchanged. In contrast, chilling treatment did not increase any of the antioxidant enzyme activities, but decreased catalase and to a lesser extent DHAR activities. Glutathione peroxidase protein levels increased sporadically under light treatment and constantly under chilling. Both chilling and light stress caused induction of glutathione synthesis and accumulation of oxidised glutathione, although the predominant part of the glutathione pool remained in the reduced form. Antioxidant enzymes from the chilling treated plants were measured at both 25 degrees C and 5 degrees C. Measurements at 5 degrees C revealed a 3-fold reduction in catalase activity, compared with that measured at 25 degrees C, indicating that the overall reduction in catalase after four days of chilling was approximately 10-fold. The overall reduction in activity for the other antioxidant enzymes after four days of chilling was 2-fold for GR and APx, 1.5-fold for MDHAR, 3.5-fold for DHAR. The activity of SOD was the same at 25 and 5 degrees C. These results indicate that catalase and DHAR are most strongly affected by the chilling treatment and may be the rate-limiting factor of the antioxidant system at low temperatures.
