Effects of Combined Application of Potassium Silicate and Salicylic Acid on the Defense Response of Hydroponically Grown Tomato Plants to Ralstonia solanacearum Infection

Abstract

Bacterial wilt, caused by soilborne pathogenic bacterium Ralstonia solanacearum, is a serious and widespread disease that affects global tomato production. Both silicon (Si) and salicylic acid (SA) play important roles in enhancing tomato resistance against bacterial wilt, however, their combined effects on the defense responses of infected tomato plants remain unknown. Hence, the combined effects of Si and SA on physiological and biochemical parameters of R. solanacearum-infected tomato plants were investigated. The combination treatment of Si and SA significantly decreased disease incidences, lipoxygenase (LOX) activity and ethylene (ET) production. The combined treatments were more prominent in improving the morphological traits of root systems, such as root length, root surface area, average root diameter and root volume. The activities of polyphenol oxidase (PPO) and peroxidase (POD) and the concentrations of total soluble phenolics (TSPs) and lignin-thioglycolic acid (LTGA) derivatives were significantly increased in the plants with combined treatments. Si in combination with SA could significantly enhance neutral invertase (NI) and acid invertases (AI) activities in the leaves of tomato plants at 3 days post-infection (dpi) compared with application of Si alone. Three defense-related genes, PAL, POD and pathogenesis-related protein 1 (PR1), were significantly induced in Si+SA treatment at 7 dpi when compared with individual application of Si or SA. The expression level of salicylic acid-binding protein 2 (SABP2) was significantly higher for combination treatment when compared with treatment of Si or SA alone. The possible mechanisms involved in the synergistic effects of Si and SA on the control of tomato bacterial wilt were proposed. This study indicates that under hypertonic conditions, the combined application of 2.0 mM potassium silicate (K2SiO3) and 0.5 mM SA had a synergistic effect on the control of tomato bacterial wilt.

Uncovering the Role of Hormones in Enhancing Antioxidant Defense Systems in Stressed Tomato (Solanum lycopersicum) Plants

“…On the other hand, MeJA application significantly increased ET and NEACs biosynthesis in a dose-dependent manner when it was applied against microorganisms in tomato seeds and plants [144]. Interestingly, ERFs genes (ERF1, ERF5, and ERF.C4), well-known as stress response factors associated with ET signaling, were upregulated by SA, MeJA, and ABA [80,82,90,124]. In tomato fruit, exogenous application of hormones (MeJA, BRs, and ABA) showed a positive effect on ET during the tomato fruit ripening [12,44,186].…”

“…Moreover, some studies have indicated that, 24 h after SA application (0.2 and 1.0 mM), an increase in oxidative parameters is observed, which may trigger SAR [115,116]. For instance, SA application induces SAR in tomato plants against Alternaria solani [117,118], Fusarium oxysporum [119][120][121], Xanthomonas vesicatoria [122], Potato virus X [103], Tomato yellow leaf curl virus, Orobanche [123], Ralstonia solanacearum [124], Tomato mottle mosaic virus [125], and nematodes [126].…”

“…In this sense, some controversial information was described on the hormonal effects when they interact. SA application in tomato plants under salt or nematode stress (Ralstonia solanacearum) significantly reduced ET biosynthesis and EL and increased AEs [110,124]. Moreover, its application significantly increased some hormones (ABA, GA 3 , IAA, and JA) and reduced the OD caused by cold temperature growth [111,185] and R. solanacearum [124].…”

Recent Advances in the Bacterial Phytohormone Modulation of Plant Growth

“…Other researchers have also reported positive effects on plant growth with the application of SA and other compounds, such as potassium silicate (silicon), melatonin (MT) or the disaccharide trehalose in plants subjected to different types of stress. In the first case, silicon and SA were observed to play an important role in tomato plants in their resistance to the pathogenic effects of the Gram negative bacterium Ralstonia solanacearum [ 61 ], while the co-application of MT and SA improved growth in wheat plants subjected to salt stress. The co-application of MT and SA markedly improved plant functions such as the photosynthetic pigments content, photochemical reactions of photosynthesis, net photosynthetic rate, transpiration rate, stomatal conductance, as well as the osmoprotectants accumulation [ 62 ].…”