An understanding of the balance between carbon and nitrogen assimilation in plants is key to future bioengineering for a range of applications. Metabolomic analysis of the model plant, Arabidopsis thaliana, using combined NMR-MS revealed the presence of two hemiterpenoid glycosides that accumulated in leaf tissue, to ∼1% dry weight under repeated nitrate-deficient conditions. The formation of these isoprenoids was correlated with leaf nitrate concentrations that could also be assayed in the metabolomic data using a unique flavonoid-nitrate mass spectral adduct. Analysis of leaf and root tissue from plants grown in hydroponics with a variety of root stressors identified the conditions under which the isoprenoid pathway in leaves was diverted to the hemiterpenoids. These compounds were strongly induced by root wounding or oxidative stress and weakly induced by potassium deficiency. Other stresses such as cold, saline, and osmotic stress did not induce the compounds. Replacement of nitrate with ammonia failed to suppress the formation of the hemiterpenoids, indicating that nitrate sensing was a key factor. Feeding of intermediates was used to study aspects of 2-C-methyl-D-erythritol-4-phosphate pathway regulation leading to hemiterpenoid formation. The formation of the hemiterpenoids in leaves was strongly correlated with the induction of the phenylpropanoids scopolin and coniferin in roots of the same plants. These shunts of photosynthetic carbon flow are discussed in terms of overflow mechanisms that have some parallels with isoprene production in tree species.F or centuries mankind has relied on carbon, prehistorically fixed from the atmosphere by plants and algae, as a source of fuel and other materials. This is a dwindling resource and, to develop renewable sources of chemicals, knowledge of the biochemical pathways that link photosynthesis to the accumulation of useful materials will be paramount. Terpenoids (isoprenoids) are examples of carbon-rich products that occur prolifically across the plant kingdom. They are not only end products of chloroplast assimilation of photosynthetic carbon but also, by virtue of the phytyl side chains of chlorophyll, key elements of the machinery of photosynthesis. Some terpenoids are ubiquitous and are classed as primary metabolites (sterols, carotenoids, and phytyl side chains of chlorophyll); others are hormones (gibberellins, abscisic acid, brassinosteroids, and strigolactones). However, the majority of terpenoids, especially those that can accumulate to high levels in plants, are classed as secondary metabolites, although many have roles in chemical ecology. Manipulation of flux through the primary terpenoid pathways and the accumulation of terpenoids via the secondary pathways both have potential for exploitation. Arabidopsis, the model plant, was, until recently, considered to be poorly endowed with terpenoids. For example, this species does not appear to contain oxidized sesqui-and diterpenes, compounds that are widespread in plants (1). There are some 30 terpene synt...

The ability to track changes in the levels of many metabolites in plants has great utility in a number of biological contexts. A metabolomics experiment usually requires the comparison of different varieties in either a functional genomics context or in response to perturbation by an external treatment. Such treatments can result in subtle changes in the final chemical signature of the plant tissue, and therefore, any unwanted variance produced in the generation of that tissue must be minimised. Procedures for plant growth, harvesting, preparation of extracts, and the subsequent collection of data have been optimised to minimise experimental variation within the dataset. This chapter describes in detail how to generate reproducible Arabidopsis tissue suitable for a typical plant metabolomics experiment. Issues concerned with tissue sampling, harvesting, and storage are also discussed.