Strigolactone

Plants make special things in their roots.

These things help the plant find food. They call for tiny fungi to help. The fungi bring food from the soil.

These things also stop too many branches. They tell the plant to stay neat. This keeps the plant from growing too much.

But some bad plants use these signals. They find the roots to steal food. This can hurt the plant.

Scientists study these things to help crops grow. They want to keep the bad plants away.

Plants make special chemicals in their roots. We call these phytohormones. Phytohormones are signals that tell a plant how to grow.

Strigolactones do three main jobs. First, they help good fungi find the plant. These fungi live in the soil. They bring nutrients like phosphate to the roots.

Second, these chemicals stop too many branches from growing. They stop extra buds from growing on the stem. This helps the plant stay neat. Without them, a plant might grow too many branches.

Third, strigolactones can attract bad plants. Some parasitic plants, like Striga, look for these signals. When they find the signal, they start to grow. These bad plants steal food from the host plant.

Scientists study these chemicals to help farmers. They want to find ways to stop the bad plants. They might design new tools to protect crops in places like Africa. This could help many plants grow strong.

Strigolactones are a special group of chemical compounds made by plant roots. Because they act as signals, scientists call them phytohormones, which are plant hormones. These molecules are very important for how a plant lives and grows. They help the plant talk to other living things in the soil. They also control how the plant's body is shaped.

These chemicals work in three main ways. First, they help helpful fungi find the plant roots. These fungi, called arbuscular mycorrhizal fungi, give the plant nutrients like phosphate. Second, they act as branching inhibition hormones. This means they stop too many buds from growing on the stems. Third, they can accidentally signal parasitic plants like Striga lutea to grow. These parasites attach to the host plant's roots to take food.

Scientists first found strigolactones in 1966. They isolated them from the roots of cotton plants. Before this, people knew root extracts helped Striga seeds grow. But they did not know the specific molecule responsible. Later, researchers found other similar molecules like sorgolactone and alectrol. These also had a lactone group, so they were named strigolactones too.

Making these chemicals is a step-by-step process in the plant. It starts with a path called the carotenoid pathway. An enzyme called D27, or DWARF27, starts the first step. This step needs iron to work properly. Next, an enzyme called CCD7 helps separate the molecules. Then, another enzyme called CCD8 helps turn them into carlactone. Carlactone is the precursor, or the starting material, for all strigolactones.

Understanding strigolactones helps us understand how plants manage their energy. For example, they control a hormone called auxin to stop extra branching. They do this by blocking transport proteins named PIN1. This keeps the plant from growing too many branches at once. Scientists also study these chemicals to help farmers in Africa. They want to protect maize crops from parasitic weeds. By changing the strigolactone blend, they might make crops more resistant.

Strigolactones are a diverse group of chemical compounds produced by plant roots. Because these molecules act as signaling agents that regulate growth, scientists classify them as phytohormones, or plant hormones. These chemicals are vital for how a plant interacts with its environment and manages its own physical structure. They function as messengers that communicate with both beneficial organisms in the soil and the plant's own internal systems.

Chemically, all strigolactones share a common core structure. This structure consists of a tricyclic lactone, labeled as the A-B-C part, which is linked to a hydroxymethyl butenolide, known as the D part. While the A-B-C section varies between different species, the D ring remains quite constant. Researchers believe the biological activity of these molecules relies heavily on this D ring. In fact, studies show that if the C-D section of the molecule is modified, the molecule loses its ability to function.

Strigolactones drive three distinct physiological processes in nature. First, they trigger the germination of parasitic organisms, such as plants in the genus Striga. These parasites grow on the host plant's roots to survive. Second, they facilitate mutualistic associations with symbiotic fungi, specifically arbuscular mycorrhizal fungi. These fungi recognize the plant and provide essential nutrients like phosphate. Third, strigolactones act as branching inhibition hormones. They prevent excessive bud growth at stem terminals, which regulates the plant's overall shape.

The discovery of these molecules began in 1966 when they were first isolated from the roots of cotton plants. Before this, scientists knew that root extracts were necessary to start the germination of parasitic Striga lutea. The isolation of strigolactones finally identified the specific molecule responsible for this stimulation. Later, researchers identified similar compounds like sorgolactone and alectrol. Because these molecules also contained the characteristic lactone group, they were classified as strigolactones. Interestingly, strigolactones can induce parasitic plant germination in trace amounts, as little as 5 parts per million.

Plants create strigolactones through a complex biosynthetic pathway starting from carotenoids. The first step involves the isomerization of the 9th chemical bond of beta-carotene. This step is performed by the enzyme DWARF27, or D27, which requires iron as a cofactor. Next, the enzyme CCD7 catalyzes the separation of 9-cis-beta-carotene into 9-cis-aldehyde and beta-ionone. Then, the enzyme CCD8 rearranges the aldehyde to produce carlactone. Carlactone serves as the essential precursor for all strigolactones. Finally, proteins like MAX1 are thought to catalyze the oxidative steps that turn carlactone into more complex strigolactones.

There is also a deep connection between strigolactone synthesis and the production of abscisic acid (ABA). Both pathways share a common group of enzymes, specifically the 9-cis-epoxycarotenoid dioxygenase (NCED) family. Research on mutant plants showed that when NCED enzymes were defective, plants had low levels of both ABA and strigolactones in their roots. This suggests a shared enzymatic machinery between the two hormones. While the exact connection remains unclear, the correlation is a significant part of plant chemistry.

Strigolactones also manage plant architecture by interacting with another hormone called auxin. While auxin regulates secondary growth, strigolactones control how auxin is moved through the plant. When present in stem terminals, strigolactones block the expression of PIN1, which are transport proteins. In mutant plants lacking strigolactones, PIN1 is over-expressed, leading to excessive auxin transport and abnormal branching. This demonstrates how strigolactones act as a master regulator for the plant's physical development.