Tobacco is a plant that belongs to the nightshade family, which includes tomatoes, potatoes, peppers, and eggplants.
Tobacco plants have evolved various defense mechanisms to protect themselves from herbivores, such as producing toxic chemicals, emitting volatile compounds, and activating hormonal pathways.
However, some tobacco plants have mutations that impair their defenses, making them more vulnerable to insect attack.
Surprisingly, these mutants also have some advantages over their normal counterparts, such as faster growth and higher genetic diversity.
The role of jasmonic acid in plant defense
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One of the key hormones involved in plant defense is jasmonic acid (JA), which regulates various reactions in plants, including defenses against herbivores and responses to environmental stress.
JA is synthesized from linolenic acid, a fatty acid found in plant membranes.
When a plant is wounded by an insect or a pathogen, JA levels increase rapidly and trigger the expression of genes that encode defensive proteins and metabolites.
Some of these defensive compounds are nicotine, trypsin inhibitors, protease inhibitors, and polyphenol oxidases, which can deter or harm the herbivores that feed on the plant.
However, JA production and signaling are not always beneficial for the plant. JA can also interfere with other hormonal pathways, such as those mediated by salicylic acid (SA) and ethylene (ET), which are involved in resistance to pathogens and abiotic stress.
JA can also reduce plant growth and reproduction by diverting resources from these processes to defense.
Therefore, plants need to balance the costs and benefits of JA-mediated defense according to the type and intensity of the threat they face.
Wild tobacco (Nicotiana attenuata) is a plant species native to the Great Basin Desert of the western United States.
It is also known as coyote tobacco because it is often consumed by coyotes as a stimulant or an anti-parasitic agent.
Wild tobacco has a complex and dynamic relationship with its herbivores, which include insects, mammals, and birds.
To cope with these diverse enemies, wild tobacco has developed a sophisticated defense system that involves JA and other hormones, as well as various chemical and physical traits.
However, not all wild tobacco plants have the same defense capabilities.
Researchers at the Max Planck Institute for Chemical Ecology in Jena have discovered that natural populations of wild tobacco contain plants with significant mutations in their JA biosynthesis or perception pathways.
These mutants are more susceptible to insect attack but grow faster than normal plants. They can also compensate for the reduced defenses due to robust genetic networks.
To study these mutants, the researchers developed a MAGIC (multiparent advanced generation intercross) population, which captures a large portion of the genetic diversity of the entire wild tobacco species in a breeding population with a homogenized genetic background.
By crossing 26 different parental lines from natural populations over several generations, they obtained 1,200 genetically diverse offspring that were screened for their JA levels and insect resistance.
The researchers found that about 10% of the MAGIC population had mutations that impaired their JA production or perception.
These mutants were more attractive and vulnerable to leafhoppers (Empoasca spp.), which are hemipteran insects that suck plant sap and transmit diseases.
However, these mutants also had higher growth rates and biomass than normal plants
. Moreover, these mutants were able to activate alternative defense pathways or increase the expression of other defensive genes to partially compensate for the loss of JA signaling.
The implications of wild tobacco mutants
The existence of wild tobacco mutants challenges the conventional view that natural selection favors plants with optimal defense against herbivores.
Instead, it suggested that there is a trade-off between defense and growth in plants and that different environmental factors can influence this balance.
For example, in years with low herbivore pressure, mutants with impaired JA signaling may have an advantage over normal plants because they can grow faster and produce more seeds.
However, in years with high herbivore pressure, these mutants may be eliminated because they cannot cope with the damage inflicted by insects.
The discovery of wild tobacco mutants also provides insights into the evolution and ecology of plant-insect interactions.
It showed that plants have a remarkable ability to adapt to changing conditions by modifying their defense strategies and exploiting their genetic diversity.
It also revealed that insects can influence plant evolution by exerting selective pressure on their hosts and inducing mutations in their defense genes.
By studying wild tobacco mutants in their natural habitat and in controlled experiments, researchers can learn more about how plants cope with herbivory and how insects respond to plant defenses.
This knowledge can help us understand the complex dynamics of natural ecosystems and improve the management of agricultural crops.
