The biochemical profile of plants determines their nutritional value and levels of plant secondary defences, and it can be altered by both abiotic (e.g. temperature, light, nutrient availability) and biotic (e.g. herbivory and plant pathogens) factors. These chemical compounds are well known to have significant effects on insect performance and population dynamics, both of which are key determinants of the establishment success and effectiveness of weed biocontrol agents. Hence an understanding of the biochemical phenotypes (observable characteristics) of invasive alien plants, and the biotic and abiotic factors that determine that particular phenotype, could provide valuable information to help decisions on the selection of biocontrol agents, and could even help improve establishment rates and agent impact.
In a recent publication, two of our weed biocontrol researchers, Simon Fowler and Ronny Groenteman, in collaboration with researchers at Massey University (Paul Barrett, who led the review, Andrea Clavijo-McCormick) and AgResearch (Arvind Subbaraj), review how plant metabolomics could be applied to weed biocontrol programmes to improve agent establishment, effectiveness and safety. According to Ronny, this paper explores how and why invasive alien plants are likely to exhibit altered biochemical phenotypes in their introduced range, and how the biochemical phenotype of plants exposed to variable environments with their complex and interacting biotic and abiotic influences might affect the performance of insect herbivores. The authors also identify a range of potential applications of plant metabolomics that could benefit weed biocontrol programmes. Here we provide a summary of the review paper, but first, what is metabolomics?
Plant metabolomics is the study of plant biochemistry at the molecular level, elucidating both known and unknown metabolites (intermediate or end products of metabolism) in biological samples. This technique uses analytical chemistry, bioinformatics and multivariate statistics to characterise the biochemical profile of a plant and identify biochemical phenotypes.
Plant metabolites play an important role in a plant’s growth, development, and response to environmental conditions. They include a wide array of primary metabolites essential for growth and reproduction, and secondary metabolites essential for interactions and defence. Since abiotic and biotic influences can significantly alter the metabolic profiles of plants, it would not be unexpected for invasive plant species to exhibit altered metabolite profiles in their introduced ranges compared to their native range. This is potentially significant for weed biocontrol programmes, since alterations to plant chemical profiles (in terms of their nutritional value and their secondary chemical defences) can have a direct influence on insect survival and development.
Exactly how abiotic and biotic stressors influence the balance of secondary metabolite mixtures produced by plants remains largely unexplored. Subtle changes to a particular mix of plant metabolites could affect host plant selection and/or utilisation, either directly via deterrents or toxicity, or by changing the volatile compounds that trigger host finding or avoidance behaviour in insect herbivores. Hence, subtle changes to the chemical composition of a small number of compounds could affect the performance of a specialist insect or pathogen on a specific plant biochemical phenotype in its invaded range. Realistically, it would be difficult and costly to test individual abiotic and biotic factors, and all possible combinations, to predict the effects of altered chemical profiles of invasive plants on introduced biocontrol agents. Using an approach such as metabolomics is promising for determining the combined effects of biotic and abiotic factors on the resulting plant metabolic phenotype.
Potential applications of metabolomics in weed biocontrol
Matching target plant metabolomes at agent collection and release sites
There are many documented reasons why weed biocontrol agents fail to establish or are ineffective when they do, but it is not often considered that establishment failure could be due to changes to the biochemical phenotype of the invasive weed in its introduced range. Since abiotic and biotic fluctuations occur across and within geographical regions following seasonal patterns, changes in plant biochemistry could positively or negatively affect insect herbivore performance.
Being able to determine variation in the metabolomes of invasive plants in their native and introduced ranges would enable biocontrol practitioners to select collection and release sites where the target weed has similar metabolite profiles. This would reduce the need for agents to overcome any plant defensive or nutritional challenges, especially when they are vulnerable during the early stages of establishment.
Determining the greatest plant defensive response to a single agent
Predicting the performance and impact of a biocontrol agent in a new environment is often difficult. The authors suggest that metabolomic analysis to gain an understanding of plant response to a particular insect species or feeding guild could provide additional information for the agent selection process.
Plants elicit defences which can be specific to a particular species of herbivore or plant pathogen, or to a particular feeding guild, meaning essentially they produce an appropriate cocktail of compounds directed at the attacker or attackers,” explained Ronny. “Under controlled conditions, we may expect a single attacking herbivore species to elicit a particular profile of chemical compounds, so by using metabolomics, we can measure this response both quantitatively and qualitatively. Taking this a step further, a large defensive metabolite response to a certain herbivore or guild may indicate a plant suffering significant stress which could help select candidate biocontrol agents with potential for the greatest impact on the target weed.”
Identifying supplementary or additional agents
Insect herbivores and pathogens are applied as weed biocontrol agents individually or in combination, and often a suite of agents is released against one target weed. There is a small amount of evidence to suggest that in some plant–insect systems, the plant chemical response induced by one herbivore may facilitate the preference and/or performance of another herbivore. Alternatively, the induced plant response to one herbivore may reduce the preference and/or performance of another.
Studying these associations using metabolomics could help with matching agents that work well together. It could also help explain why a certain agent may be failing if a co-agent has not been introduced in tandem, or it can signal which agents should never be combined.
Augmenting host range and specificity testing
To determine whether potential weed biocontrol agents are safe for release, their host specificity is determined through rigorous testing of phylogenetically related plant taxa from the invaded range. In the case of specialised insects, the assumption is that the host range will be restricted to plant taxa with similar morphological and biochemical characteristics.
The identification of biochemical similarity of plant species through broad spectrum metabolite testing could augment host specificity testing protocols currently focused on phylogenetic relatedness. This could improve host range testing by including species that could be at risk due to similarity in their biochemical profile to the target weed, and which would otherwise have been overlooked because they are phylogenetically distant from the target weed. Also, this could be particularly valuable when phylogenetically related or other plants of interest are difficult to obtain, and metabolomics could be used as a proxy ‘plant’.
In conclusion, although the field of metabolomics has yet to develop to an operational level, the potential for adding metabolomic analysis to the existing weed biocontrol toolkit could provide weed biocontrol projects with a suite of applications for greater effectiveness and safety. This would be multi-faceted, complex and even costly, however metabolomics is a powerful technology that could be at the core of beneficial collaborative research between biocontrol practitioners, plant biochemists and plant-insect ecologists.
Further reading
Barrett DP, Fowler S, Subbaraj AK, Groenteman R, Clavijo-McCormack A 2021. Metabolomic analysis of host plant biochemistry could improve the effectiveness and safety of classical weed biocontrol. Biological Control 160: 104663.
https://doi.org/10.1016/j.biocontrol.2021.104663
