Strigolactones are a recently identified class of plant hormones. These small, lipid-soluble molecules, derived from carotenoids (plant pigments), are produced by plants and released into their surroundings. They function as potent signaling molecules, directing plant growth and development while mediating interactions with other soil organisms.
Roles in Plant Development
Strigolactones shape a plant’s physical structure, particularly its shoot and root systems. Within the plant, these hormones transport from roots upwards to shoots, influencing branch development. They inhibit axillary bud outgrowth, which are dormant buds that can form side branches. This action helps maintain apical dominance, where the main stem grows more strongly than side branches. High levels of strigolactones suppress side shoot growth, channeling resources towards the main stem.
These hormones also influence a plant’s root architecture, crucial for nutrient and water uptake. Strigolactones affect primary root elongation and the formation of lateral roots, which are side roots. They reduce primary root length but can promote lateral root density under specific nutrient conditions, such as low phosphate availability. This modulation allows plants to adjust their root foraging strategies, optimizing their ability to explore the soil and acquire resources.
Interactions with Other Organisms
Strigolactones serve as chemical messengers, facilitating interactions with various soil organisms. These interactions can be beneficial, forming symbiotic relationships, or detrimental, involving parasitic organisms. Plants release strigolactones from their roots into the soil, where they are detected by other life forms.
A beneficial interaction involves arbuscular mycorrhizal fungi (AMF), which form symbiotic associations with many land plants. Strigolactones released by plant roots attract these fungi, guiding their hyphae towards the root surface. These hormones also stimulate the growth and branching of fungal hyphae, preparing them to colonize the plant roots. This partnership is mutually beneficial, as the fungi help the plant absorb nutrients, particularly phosphorus and nitrogen, from the soil, while the plant provides the fungi with carbohydrates.
Conversely, strigolactones are also exploited by parasitic weeds, such as Striga (witchweed) and Orobanche (broomrape) species, which threaten agricultural crops. These weeds have evolved to detect strigolactones in the soil, using them as a signal to germinate. When parasitic weed seeds detect strigolactone concentrations, it triggers their germination, ensuring germination only when a host plant is nearby. This chemical “trick” allows the parasitic seedlings to attach to the host root, drawing water and nutrients and impacting crop yield.
Agricultural Potential
Understanding the diverse roles of strigolactones offers opportunities for enhancing agricultural practices. Manipulating strigolactone signaling pathways can combat parasitic weed infestations. One approach involves “suicidal germination,” where synthetic strigolactone analogs are applied to fields in the absence of a host crop, prompting parasitic weed seeds to germinate and die without a host. Another strategy focuses on breeding crop varieties that produce lower amounts of strigolactones, thereby reducing the germination stimulus for parasitic weeds.
Beyond weed control, strigolactones can improve crop performance by optimizing nutrient acquisition. By modulating strigolactone levels, the symbiotic relationship between crops and arbuscular mycorrhizal fungi can be enhanced, leading to more efficient uptake of phosphorus and other nutrients. This could reduce the reliance on synthetic fertilizers, promoting more sustainable agriculture. Controlling strigolactone synthesis or perception could also allow for the optimization of plant architecture in crops. For example, regulating shoot branching could lead to plants with improved light capture, better spacing for mechanical harvesting, or increased yields by directing resources to desired plant parts.