The application of exogenous PopW increases the tolerance of Solanum lycopersicum L. to drought stress through multiple mechanisms

Abstract

Tomato is a major cultivated vegetable species of great economic importance throughout the world, but its fruit yield is severely impaired by drought stress. PopW, a harpin protein from Ralstonia solanacearum ZJ3721, plays vital roles in various plant defence responses and growth. In this study, we observed that the foliar application of PopW increased tomato drought tolerance. Our results showed that compared with water-treated plants, PopW-treated plants presented a significantly higher recovery rate and leaf relative water content under drought-stress conditions. PopW decreased the malondialdehyde content and relative electrical conductivity by 40.2% and 21%, respectively. Drought disrupts redox homeostasis through the excessive accumulation of reactive oxygen species (ROS). PopW-treated plants displayed an obvious reduction in ROS accumulation due to enhanced activities of the antioxidant enzyme catalase, superoxide dismutase and peroxidase. Moreover, PopW promoted early stomatal closure, thereby minimizing the water loss rate of plants under drought stress. Further investigation revealed that endogenous abscisic acid (ABA) levels and the transcript levels of drought-responsive genes involved in ABA signal transduction pathways increased in response to PopW. These results confirm that PopW increases drought tolerance through multiple mechanisms involving an enhanced water-retention capacity, balanced redox homeostasis, increased osmotic adjustment, reduced membrane damage and decreased stomatal aperture, suggesting that the application of exogenous PopW may be a potential method to enhance tomato drought tolerance.

Keywords: Drought tolerance; Osmotic adjustment; PopW; Redox homeostasis; Solanum lycopersicum L; Stomatal closure.

Fig. 1

Effect of PopW on the water retention ability and recovery of tomato under drought stress. Plants were exposed to drought stress after treatment with PopW or water (as a control) for 5 days. a The effect of dehydration of the tomato plants for 20 days and re-watering for 24 h; b Plant survival rate (%) after 20 days of dehydration and re-watering for 24 h; c Relative water content of the leaves from well-hydrated plants and plants that were dehydrated for 8 days. Values are the means ± SDs (n = 24). The experiments were repeated three times with similar results. Asterisks indicate a significant difference compared to the control (**P < 0.01)

Fig. 2

PopW induced physiological indexes under drought-stress conditions. Drought stress was applied to plants via the withholding of water after treatment with PopW or water (as a control) for 5 days. The MDA content (a), relative electrical conductivity (b), root recovery intension (c) and total chlorophyll content (chlorophyll a + b) (d) were measured in well-hydrated plants and plants that were dehydrated for 8 days. Values are the means ± SDs (n = 24). The experiments were repeated three times with similar results. Asterisks (* and **) indicate a significant difference compared to the control (*P < 0.05 and **P < 0.01)

Fig. 3

Effect of PopW on ROS levels and CAT, SOD and POD activities. Changes in free ROS content of plant leaves (a, b), CAT activity (c), SOD activity (d), and POD activity (e) were detected at the indicated days after treatment with PopW or water under water-deficit conditions. Dpt (days post-treatment with PopW or water), daDS (days after drought stress). Values are the means ± SDs (n = 24). The experiments were repeated three times with similar results

Fig. 4

PopW induced free proline generation in tomato leaves under drought- stress conditions. Changes in free proline content of plant leaves were detected at the designated time intervals after treatment with PopW or water under drought-stress conditions. Dpt (days post-treatment with PopW or water), daDS (days after drought stress). Values are the means ± SDs (n = 24). The experiments were repeated three times with similar results. Asterisks indicate a significant difference compared to the control (**P < 0.01)

Fig. 5

Effect of PopW on stomatal aperture and the water loss rate in tomato under dehydration-stress conditions. a Image of stomata monitored using a fluorescence microscope, scale bar = 20 μm. b Stomatal apertures were measured using ImageJ software. c Change in the water loss rate of plants was detected under drought-stress conditions at 0, 0.5, 2, 4, 8, 12 and 24 h in vitro. Dpt (days post-treatment with PopW or water), daDS (days after drought stress). Values are the means ± SDs (n ≥ 100 stomata). The experiments were repeated three times with similar results. Asterisks indicate a significant difference compared to the control (**P < 0.01)

Fig. 6

PopW triggered ABA accumulation and increased the transcript level of drought-responsive genes in tomato. a Changes in ABA content were measured at the designated time intervals. Values are the means ± SDs (n = 24). b, c, d Drought-responsive gene expression induced by PopW or water in tomato leaves under well-watered conditions and drought-stress conditions were analysed at the indicated time points through qRT-PCR: (b) SlAREB1, (c) SlRD22, and (d) SlRD29B. The SlUBI3 gene was used as a constitutively expressed internal control. Hpt (hours post-treatment with PopW or water), Dpt (days post-treatment with PopW or water), daDS (days after drought stress). Values are the means ± SDs (n = 6). The experiments were repeated three times with similar results. Asterisks indicate a significant difference compared to the control (**P < 0.01)

Fig. 7

Model by which PopW pre-treatment confers drought tolerance to the tomato. In this model, the application of exogenous PopW increased the tolerance of tomato to drought stress via the significant role of PopW in numerous physiological processes. PopW maintained root vigour and the photosynthetic system by elevating root intension and the chlorophyll content, reduced oxidative stress by improving the antioxidant enzyme activity, protected cell integrity by enhancing the proline content and decreasing the MDA content, and promoted stomatal closure by triggering ABA accumulation in plant leaves under drought-stress conditions. Together, these results illustrate that PopW could reduce water loss and increase drought tolerance in tomato under dehydration-stress conditions