Strigolactone

Strigolactones are a group of chemical compounds produced by a plant's roots. Due to their mechanism of action, these molecules have been classified as plant hormones or phytohormones. So far, strigolactones have been identified to be responsible for three different physiological processes: First, they promote the germination of parasitic organisms that grow in the host plant's roots, such as Striga lutea and other plants of the genus Striga. Second, strigolactones are fundamental for the recognition of the plant by symbiotic fungi, especially arbuscular mycorrhizal fungi, because they establish a mutualistic association with these plants, and provide phosphate and other soil nutrients. Third, strigolactones have been identified as branching inhibition hormones in plants; when present, these compounds prevent excess bud growing in stem terminals, stopping the branching mechanism in plants. Strigolactones are a group of chemical compounds produced by a plant's roots. Due to their mechanism of action, these molecules have been classified as plant hormones or phytohormones. So far, strigolactones have been identified to be responsible for three different physiological processes: First, they promote the germination of parasitic organisms that grow in the host plant's roots, such as Striga lutea and other plants of the genus Striga. Second, strigolactones are fundamental for the recognition of the plant by symbiotic fungi, especially arbuscular mycorrhizal fungi, because they establish a mutualistic association with these plants, and provide phosphate and other soil nutrients. Third, strigolactones have been identified as branching inhibition hormones in plants; when present, these compounds prevent excess bud growing in stem terminals, stopping the branching mechanism in plants. Strigolactones comprise a diverse group, but they all have core common chemical structure, as shown in the image to the right. The structure is based on a tricyclic lactone linked to a hydroxymethyl butenolide; the former is represented in the figure as the A-B-C part, while the latter is the D part of the molecule. It is important to note that most strigolactones present variations in the ABC part, but the D ring is quite constant across the different species, which led researchers to suspect that the biological activity relies on this part of the molecule. Different studies have demonstrated that the activity of the molecules is lost when the C-D section of the molecules is modified. Since strigolactones are involved in the signaling pathway required for germination of parasitic species (such as Striga sp.), they have been a proposed target to control pests and overgrowth of these parasitic organism. Using a molecule similar to strigolactones could be the key to designing a chemical and biological mechanism to stop the colonization of avplant's root by parasitic plants. Strigolactones were first isolated in 1966 from cotton plants, specifically from the roots. However its role in germination of other organisms was not determined until later. Previous studies with Striga lutea had already shown that root extracts from the host plants were necessary for the parasitic seed to start germinating, which made obvious that a substance produced in the roots was stimulating this process. The isolation of strigolactones lead to a series of tests that proved that this compound was the necessary molecule to induce germination of Striga species. Later on, similar compounds were proven to produce the same effect: sorgolactone and alecrol, both of them presented the characteristic lactone group, so they were classified as strigolactones. To induce germination of parasitic plants, strigolactones only needed to be present in trace amounts, in the order of 5 parts per million. The role of strigolactones as branching inhibitor hormone was discovered because of the use of a new set of mutant plants. These mutants presented excessive growth in the axillary buds, which induced their terminal stem to start branching abnormally. Previously, cytokinins were thought to be the only molecule involved in the regulation of stem branching, but these mutants presented normal production and signaling of cytokinins, leading to the conclusion that another substance was acting on the axillary buds. Different tests that consisted in inserting part of the mutants plants into wild specimens (and vice versa), were able to demonstrated that the mutants were either not able to recognize a signal molecule coming from the roots and the lower part of the plant, or not able to produce the require molecules to inhibit branching. This molecule, that was involved in branching regulation, was later identified to be a strigolactone. The conclusion was that, in presence of strigolactones, the plant would be prevented to overgrowth and develop excessive branches, but when is not present, the axillary bud will start inducing abnormal branching. Although strigolactones vary in some of their functional groups, their melting point is usually found always between 200 and 202 Celsius degrees. The decomposition of the molecule occurs after reaching 195oC. They are highly soluble in polar solvents, such as acetone; soluble in benzene, and almost insoluble in hexane.