Imaging the distribution of agrochemicals in plants by MALDI-MSI.

Pesticides are widely used in agriculture to control weeds, pests and diseases. Successful control is dependent on the compound reaching the target site within the organism after spray or soil application. This is influenced by a range of physiological and environmental factors. One such factor is uptake into foliar tissue and roots and subsequent movement through the plant. Uptake and translocation (and metabolism) can affect the speed and persistence of action, crop selectivity and weed spectrum, thus understanding these issues is of great importance. Conventional methods for determining uptake and movement of herbicides and pesticides include autoradiography, liquid scintillation and chromatographic techniques such as HPLC. Autoradiography using radiolabelled compounds provides the best indication of a compound's movement within the plant system which is an established technique but relies on radiochemical synthesis. The cost and time taken means that only a relatively few agrochemicals can be studied in this way.MALDI is a highly adaptable soft ionisation technique that was established in the late 1980's through the developmental research of Tanaka and Hillenkamp. The versatility of MALDI-MS has been extended in recent years with the advent of protein profiling and imaging directly from the surface of thin biological tissue sections. Research by Caprioli et al has resulted in the successful profiling and imaging of proteins from thin sections of healthy mouse brain and mouse brain containing tumor and the profiling of proteins from a neurotoxin induced rat modelof Parkinson's disease. Recently the imaging technique has been further developed by research groups to include the detection and imaging of small organic molecules on the surface of porcine epidermal tissue and the surface of rodent brain tissue. In preliminary studies undertaken to support the grant application for this project the detection and imaging of compounds on the surface of a plant leaf or inside a plant stem using MALDI-MS was demonstrated.In this project the technique of imaging matrix assisted laser desorption ionisation mass spectrometry has been used to examine the uptake and distribution of a range of compounds of agrochemical interest into growing plants. The agrochemicals of interest are azoxystrobin, mesotrione, fluazifop-p butyl and nicosulfuron. Azoxystrobin is a broad spectrum fungicide which inhibits mitochondrial respiration in fungi; the compound is absorbed through the roots and translocated in the xylem. Mesotrione is a selective phloem mobile herbicide which inhibits HPPD, an enzyme involved in carotene synthesis and results in bleaching of the leaves. Fluazifop-p butyl is absorbed as its ester, which is quickly hydrolysised to its carboxylic acid (fluazifop acid) that is mobile in the xylem and the phloem. Fluazifop acid works as a fatty acid synthesis inhibitor, by inhibition of acetyl CoA carboxylase (ACCase). Nicosulfuron is both xylem- and phloem-mobile which inhibits branched chain amino acid synthesis by means of the acetolactate synthase (ALS) enzyme.MALDI-MS images of mesotrione in tissue underlying the waxy cuticle layer following removal using a tape stripping method have been generated. MALDI-MS images have been generated from azoxystrobin following uptake via the roots in soya and sunflower plants. Nicosulfuron translocation following uptake via the0roots and foliar application has been determined in sunflowers and a phase I metabolite has been identified 48 hours following uptake via the roots and 24 hours following foliar application using MALDI-MSI. Nicosulfuron and azoxystrobin translocation has been determined in the same plant following application of the compounds to the plant stem. The methods developed were used to determine the extent of translocation of four sulfonylurea herbicides 24 hours following foliar application. Additionally, a preliminary method was assessed to determine whether MALDI-MSI can be used to assess translocation of agrochemicals.