Plants release strigolactones into the soil in order to attract friendly organisms and establish a symbiotic association with them. Unfortunately, these plant hormones are also perceived by parasitic weeds. A team of researchers led by Salim Al-Babili from the University of Freiburg has now identified important steps in the biosynthesis of strigolactones as well as coming up with unexpected discoveries. Does the new knowledge enable researchers to develop compounds that favour the establishment of symbioses? Will farmers in the future be able to drive parasites to suicide before they come into contact with maize, potatoes and other crops?
The roots of agricultural plants such as maize, millet, tomatoes and potatoes release tiny quantities of the hormone strigolactone into the soil in order to attract mycorrhizal fungi. The mycorrhizal fungi and the plants come into contact with each other and establish symbiotic associations in which the fungi provide the plant with minerals for which the plants pay with photosynthetic products. However, the strigolactones produced by the host plants also stimulate the germination of certain parasitic weeds, such as broomrape (Striga) and enchanter’s nightshade (Orobanche) species, and form structures which they use to tap into nutrients from the roots of the host. These parasitic weeds love the plants’ sugar juice but do not deliver anything in return. The infestation of crop plants with such root parasites can have disastrous consequences for agriculture, leading to losses of several billion euros per year - especially in Africa and Asia. “One possible strategy for combating these parasites is to use strigolactones. Over the last few years, biologists and chemists have increased their knowledge of these hormones. However, the individual steps of the strigolactone biosynthesis pathway have previously been largely unknown,” said Dr. Salim Al-Babili from the Institute of Biology II at the University of Freiburg.
Researchers need to obtain detailed insights into the intermediate steps in the biosynthesis of certain plant compounds in order to be able to interfere with biochemical processes and manipulate them to best suit the researchers’ needs. In addition, the identification of biosynthesis pathways can also lead to the identification of new active structure, as the research carried out by Dr. Al-Babili and his team shows. The identification of such structures would enable the agricultural industry to develop substances that specifically promote the “cooperation” between mycorrhizal fungi and agricultural plants or induce the root parasites to germinate without the host. This would lead to the death of the weed embryos.
The seeds of Striga and similar parasitic weeds have almost completely reduced their own storage substances during evolution, which is why they depend on tapping into a host’s nutrients once they have germinated. The early development of a radicle would lead to the death of the weed embryos. There are around 20 different known strigolactones, all of which are derivatives of carotenoids, which are common in plants and are building blocks for many other important molecules. Strigolactones have a somewhat complex chemical structure; they consist of three rings to which a fourth is connected by way of a so-called enol-ether bridge. “At first sight, strigolactones appear to be derived from three interconnected carotenoid rings, and the fourth one seems to derive from a different molecule,” said Al-Babili. “For many years it has been taught that numerous different reaction steps and hence enzymes are required for turning carotenoid molecules into strigolactones.” Some genes that play a role in the biosynthesis of these phytohormones were known from experiments with mutants; however, little has previously been known about their concrete function.
Al-Babili and his team decided to take a closer look at the most important enzymes involved in the biosynthesis of strigolactones. The pathway involves two carotenoid cleavage dioxygenases (CCD7 and CCD8) and the iron-binding protein D27, which is a carotene isomerase. They discovered that the spatial arrangement of the atoms of the precurser beta-carotene is the decisive factor in the biosynthesis of strigolactone and is determined by one of these three enzymes. “We were very surprised to find that the CCD7 enzyme is stereospecific,” said the biologist from Freiburg. Stereospecificity relates to the ability of enzymes to interact only with molecules with a specific conformation. Generally, plant carotenoids are found in all-trans conformation, which makes them look like a straight chain. However, the researchers from Freiburg found that CCD7 only cleaved carotenoids with a 9-cis configuration – i.e. a chain with a kink at the 9th atom. CCD7 cleaves this double bond, thereby leading to a shorter carotenoid, which is known as apo-carotenoid and also has a 9-cis configuration. In subsequent experiments, Al-Babili and his team found that the D27 isomerase converted the all-trans carotenoids into 9-cis carotenoids, which were then cleaved by CCD7. This finding led the researchers to change their mind about previously postulated synthesis steps that did not foresee such a 9-cis precursor.
What does the presence of the untypical 9-cis carotenoid precursor in the biosynthesis of strigolactones imply? Al-Babili and his team then focused on the enzyme CCD8 and came up with another unexpected finding. They found that this enzyme was able to recognize the stereoconfiguration of the apo-carotenoid. Upon exposure to all-trans carotenoids, the enzyme cleaved a double bond and the reactions somehow came to a halt. However, when the researchers exposed the enzyme CCD8 to a 9-cis-configured CCD7 product, chromatographic analysis revealed the presence of a previously unknown substance. It has several typical strigolactone structural elements, but it is significantly more simple in structure. CCD8 is thus rather exotic. As the chemical structure of the CCD8 product shows, the enzyme CCD8 catalyses several chemical reactions simultaneously, in contrast to most other enzymes: the enzyme incorporates three oxygens into a 9-cis carotenoid and performs a molecular rearrangement, linking carotenoids with strigolactones.
Al-Babili and his team named this compound carlactone in reference to its mixed nature: in chemical terms, carlactone is different from typical carotenoids whilst not yet being a typical strigolactone. However, it has strigolactone-like biological activities. It stimulates mycorrhizal fungi to establish contact with plants in order to establish a symbiotic association. In addition, it stimulates the germination of Striga seeds in the absence of a plant host. “In structural terms, carlactone consists of a minimal scaffold that is required in order for it to have the same effect as strigolactones,” said Al-Babili. “It is even simpler than theoretical studies had suggested for minimal functional strigolactone scaffolds.” The researchers from Freiburg now envisage using their findings for the generation of new compounds that are able to fulfil the different functions of strigolactones and be used in the agricultural industry. However, the researchers from Freiburg still have to obtain further insights before their vision can become reality.