Suppression of polyphenol oxidases increases stress tolerance in tomato

Abstract

Polyphenol oxidases (PPOs) have been proposed to function in the Mehler reaction, photoreduction of molecular oxygen by PSI [Physiol. Plant.72 (1988) 659]. Under drought stress where photosynthesis is reduced, the Mehler reaction may provide a nondestructive sink for absorbed light energy not used in photochemistry [Plant Physiol. 112 (1996) 265]. If so, water-stressed plants with suppressed PPO are expected to exhibit photooxidative damage and plants with elevated PPO may show increased stress tolerance. However, in experiments examining drought response of tomato plants with different levels of PPO expression, we report more favorable water relations and decreased photoinhibition in transgenic plants with suppressed PPO (SP plants) compared to nontransformed controls (NT plants) and transgenic plants overexpressing PPO (OP plants). In addition, chlorophyll content is higher in water-stressed SP plants compared to NT and OP plants indicating lower photooxidative stress in SP plants. These results suggest that PPO may have a role in the development of plant water stress and potential for photoinhibition and photooxidative damage that may be unrelated to any effects on the Mehler reaction. As a preliminary attempt to understand the role PPO may play in potentiating stress-induced damage, we examined the expression of PPOs during these experiments. Among the seven-member PPO gene family, we found that in response to water stress, two members, PPO B and D, are transcriptionally upregulated in abscission zones of leaf petioles. In addition, PPO B is induced in leaf veins throughout foliar development and in stem vascular tissues, cortical and pith cells. Similarly, tissue printing of stem tissues following drought stress showed increased immunoreactive PPO in vascular tissues, cortical and pith cells of NT and OP plants. Foliar PPO activity of NT plants also increased throughout development in response to water stress. The physiological role(s) of drought-induced PPO activity remains to be determined.