Summary
This project was initiated as a discovery project to investigate the potential of bacterial quorum sensing signals (QSS) as plant treatments to enhance pasture plant growth and health. It sought to establish a proof of concept for the effectiveness of QSS in improving plant growth and plant interactions with soil microbes. QSS are chemical signals (acyl homoserine lactones) synthesised by most gram negative bacteria. Bacteria use these signals to regulate behaviours that require bacterial cooperation, for example when infecting a host. This has been demonstrated to be important for infection of plants with pathogenic bacteria (for example soft rot bacteria) as well as symbiotic bacteria, (for example rhizobia). Different bacterial species produce different chemical variations of QSS.
The background for the work was established by previous research showing that purified or synthetic QSS from bacteria can alter gene expression in plants when applied to roots (Mathesius et al., 2003). This suggested that plants can perceive these bacterial signals, and perhaps use them as cues to detect bacteria by their QSS signature in the rhizosphere and to prepare for interactions with these bacteria, whether symbiotic or pathogenic. However, it had not previously been tested whether this ability to detect the signals and respond at a molecular level results in altered plant performance.
The gene expression studies showed that genes important in plant defence, metabolism and hormone control were affected by QSS. This suggested that QSS could
(i) alter plant defence responses and thus influence subsequent plant microbe interactions and
(ii) influence plant hormone metabolism that could result in changes to plant growth. These hypotheses were tested in selected pasture plant species: Lolium perenne (perennial ryegrass), Trifolium subterraneum (subclover) and Medicago truncatula (barrel medic).
Under sterile and non-sterile conditions, these plants or seeds were exposed to synthetic QSS that are known to be synthesised by soil/rhizosphere bacteria. Their subsequent germination, growth, nodulation (where appropriate) and interaction with pathogenic soil microbes was assessed. The project established a proof of concept that certain QSS can affect plant growth, germination and plant interaction with soil microbes. First, application of certain QSS accelerated seed germination under sterile conditions in all three plant species.
The effect depended on the concentration and structure of the QSS. Under non-sterile conditions, a smaller effect on germination was shown. However, this effect was transient and plant growth was no longer accelerated after one to three weeks of growth. Second, application of QSS as seed dressings or as a regular leaf spray altered plant growth. In some cases, leaf and root growth (weight) was significantly increased by the application of QSS. This effect again depended on the plant species, the structure of the QSS and the growth conditions and suggests specific recognition of QSS by plants. Third, pre-treatment of the two legumes, subclover and medic, with QSS influenced their ability to form nodules in symbiosis with rhizobia. Whereas QSS did not enhance nodulation under sterile growth conditions, nodulation could be enhanced in semi-sterile soil experiments. This also resulted in improved plant growth. The QSS synthesised by the specific rhizobial symbionts were most effective at enhancing nodulation, whereas other QSS were ineffective. Fourth, the effect of QSS on the interaction of plants with fungal or oomycete pathogens were tested. Pathogens selected for this study were those that have been shown to limit pasture plant growth and included Rhizoctonia solani AG 2.1, Phytophthora medicaginis and root knot nematodes (Meloidogyne javanica). The hypothesis behind this was that treatment with QSS could affect general defence responses in the plant that could then prepare the plant for a subsequent attack by pathogens.
The results showed no significant effect of QSS treatment on the development of root galls by root knot nematodes in medic, and a small reduction of gal numbers and plant growth in subclover. Both species were similarly infected by this pathogen. Rhizoctonia solani appeared to colonise both subclover and medic, but subclover showed no inhibition of growth in response to Rhizoctonia, whereas medic was significantly affected. Certain QSS enhanced growth of Rhizoctonia infected subclover roots. Certain QSS had a small beneficial effect on Rhizoctonia-infected medic when used as a regular leaf spray but not when used as a seed dressing. Phytophthora medicaginis severely infected medic and killed plants within 1-2 weeks. This was accompanied by production of a specific flavonoid in the roots that was used to monitor infection. Certain long chain QSS inhibited the accumulation of that flavonoid and alleviated early symptoms, but did not prevent the eventual devastating effect of P. medicaginis.
This project was designed to focus on the basic research on plant perception of QSS. It was only four years ago that the first reports emerged that indicted that plants can perceive and respond to QSS. Therefore, this research is still in a stage of discovery, with limited applications unless further research is carried out. To benefit from this research, QSS would need to be tested under field conditions with the local soil and pasture plants. The research done in this project could guide those tests by providing information about active QSS that could be tested, how they could be applied to the plants and what outcomes might be expected. However, since this project found that plant responses to QSS depended strongly on the exact structure and concentration of the QSS, as well as on the growth condition and plant species studied, the final outcome of the QSS-plant interaction in the field is difficult to predict.
Farmers interested in testing QSS for potential to enhance their pasture plant growths could benefit from the results. Many QSS are commercially available and could be tested as seed dressing before pasture plants are sown into the soil. This might enhance seed germination and could potentially benefit plants by being able to outgrow pathogens present in the top soil layers. The QSS could also be further tested by seed companies or research and development organisations. In addition, the observation that subclover was not affected by Rhizoctonia solani AG2.1 under the conditions used could be further investigated to determine the factors that could make subclover somewhat resistant. Since the application of certain QSS further enhanced growth of Rhizoctonia infected subclover, their potential in alleviating this important fungal pathogen disease could be further tested.
