Abstract
Many Gram-negative bacteria use N-acyl-homoserine lactone (AHLs) as quorum sensing (QS) signalling molecules to monitor their population density and to regulate gene expression in a density dependent manner. Recently, it has been shown that AHLs are detected by the plants and they trigger plant defense responses. In this study, N-octanoyl-L-homoserine lactone (C8-HSL) has been used as resistant inducer against bacterial wilt disease of tomato caused by Ralstonia solanacearum. The present investigation focused on the role of defense related enzymes (phenylalanine ammonia lyase, peroxidase, polyphenol oxidase and lipoxygenase) in imparting resistance in tomato against R. solanacearum. Activities of these defense enzymes, increased in C8-HSL treated tomato plants, which were challenged with R. solanacearum. The transcripts accumulation studies for these enzymes were carried out using semi-quantitative reverse transcription PCR, with maximum mRNA accumulation in resistant cultivar upon treatment with C8-HSL. Quantitative real time-PCR (qRT-PCR) confirmed the maximum induction of all these four genes in C8-HSL treated plants. However, the expression of defense genes was higher in C8-HSL treated resistant cultivar than that of susceptible cultivar. Therefore, the results support the view that C8-HSL molecule enhances disease protection against R. solanacearum infection in tomato through the activation of defense genes. Key words: N-octanoyl-L-homoserine lactone, Ralstonia solanacearum, tomato, defense genes, quantitative real time-polymerase chain reaction (PCR).
Highlights
Many bacteria use small signalling molecule to communicate with each other and to co-ordinate their growth activities, this process is commonly referred to as quorum sensing (QS)
In the resistant tomato cultivar, the temporal pattern of phenylalanine ammonia lyase (PAL) enzyme revealed the maximum activity at 9 hpi (140 units) (Figure 1a), while in susceptible tomato cultivar, PAL enzyme revealed the maximum activity at 18 hpi (85 units) (Figure 1b)
In the resistant tomato cultivar, the temporal pattern of polyphenol oxidase (PPO) enzyme revealed the maximum activity at 12 hpi (72 units) (Figure 1e), while in susceptible tomato cultivar, PPO enzyme revealed the maximum activity at 24 hpi (35 units) (Figure 1f)
Summary
Many bacteria use small signalling molecule to communicate with each other and to co-ordinate their growth activities, this process is commonly referred to as quorum sensing (QS). Mathesius et al (2003) observed that the legume plant, Medicago truncatula are able to respond to nanomolar concentrations of synthetic and purified AHLs and these compounds elicit major changes in protein expression These changes suggest that bacterial QS signalling molecules might regulate the functions of these proteins, which include roles in defense responses of host plants, primary metabolism, plant hormonal response, transcriptional gene regulation, protein processing and activities of the cytoskeleton (Mathesius et al, 2003). The long chain AHL, oxo-C14-HSL, activates resistance towards different obligate bio-trophic pathogens such as Golovinomyces orontii and Blumeria graminis in Arabidopsis and barley, respectively (Schenk et al, 2014) All these studies demonstrate that AHLs can induce resistance in plant by activating the defense mechanism
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