Products of plant resistance ( R ) genes Pto and Cf contain distinct domains, and have different cellular localization. It is intriguing to compare the development mechanisms of resistance conferred by the two R genes. In the present report, two hypersensitive response (HR) initiation systems were employed to study the time_course expression induced by Avr / Cf interaction of the genes encoding Pti4, Pti5 and Pti6 which interact directly with Pto: (1) Seeds of tomato (Lycopersicon esculentum Mill.) containing complementary gene pairs Avr 4/ Cf _4 and Avr 9/ Cf _9 were obtained through crossing. Their seedlings developed HR under room temperature. (2) Avr / Cf seedlings grew normally at 33 ℃. When the temperature was shifted down to 25 ℃, HR occurred within hours in the seedlings. Results of both experiments showed that expression of Pti4, Pti5 and Pti6 was induced upon development of hypersensitive necrosis in Avr / Cf seedlings. However, the expression levels and patterns of these Pti s differed. This finding indicated that these Pti s function complementarily, and might be involved in regulation of both Pto and Cf _conferred resistance.
