By Céline Caseys
Garlic. Holy water. Crucifix. Silver chains. A stiff wooden stake. A mirror. That’s the toolkit you’d probably pack up for a close encounter with a vampire.
What if the problem was made worse? You can’t move because your feet are glued to the floor. You can’t run, can’t hide. It hardly seems like a fair fight. Maybe would you adapt your defense strategy?
This is a perfect metaphor for the challenge plants face, as they fight sucking predators. But unlike the fantasy-films-inspired defense kit that requires putting a rough and physical fight, plants fight back with a collection of biochemical tools, an internal arsenal that allows them to engage a motionless battle against ruthless fluid suckers.
Planthoppers are such plant vampires. They are pretty but small insects that often look like leaves — but they are far from innocuous. These herbivores feed by draining the plant of its valuable fluids, stealing water and nutrients. And like mosquitos infected with malaria, the draining of a little fluid, in this case sap (plant’s blood), is by far not the only threat. Insects also transmit disease from one plant to another by piercing with their infected fangs into their next victim.
So how do plants defend themselves? Is there a plant equivalent to the vampire-poisoning garlic and holly water? That’s the serious question that Liang Hu and colleagues from Wuhan University in China set out to answer by studying brown planthopper and rice.
Brown planthoppers are some of the most devastating pests in rice, especially lowland rice grown in parts of Asia and South America. This insect sucks the life out of rice fields, decreasing the yield of a crop that feeds a large part of the world’s population.
The Plant Immune System
In addition to various physical and chemical defense mechanisms, plants have an immune system. To initiate these defense programs, plants detect clues from their environment that indicate the presence of insects and pathogens. The plant immune system can sense the attack at the cellular level, sends a signal around affected cells, and then deploys a defense strategy. One plant trick consists of resistance genes that encode molecular sentinels that detect danger and alert other cells of an immediate threat.
The Genes That Contribute To The Fight
Hu and colleagues (2017) identified part of rice’s arsenal against the planthopper. The work published in The Plant Cell describes a resistance gene against the planthopper, cleverly named Brown PlantHopper resistance 14 (BPH14). This resistance gene consists of three major parts (called domains) that contain the genetic information for producing a resistance protein. To reveal what each of these domains does, the researchers genetically engineered rice plants so that they would only produce one BPH14 domain, or combinations of different domains. With this approach they could test which part of the genetic blueprints actually matter, or if the whole set is required.
They showed that two of the parts play an important role. One, called the “coiled-coil” (CC) domain and the other called the “nucleotide binding” (NB) domain provide the resistance. The CC domain recognizes compounds (called effectors) that signal the presence of the insect. In the Brown Planthopper, a chemical secreted from the insect’s salivary glands binds the CC domain and tells the plant that the insect is feeding. The NB domain is a molecular switch that helps transmit the warning into the plant cell itself.
Once activated, BPH14 interacts with two other proteins we can think of as “networking agents” named WRKY46 and WRKY72 (scientists pronounce this as “worky”). These two proteins are responsible for turning genes on and off by directly interacting with DNA. Here they turn on a set of genes involved in mounting an effective defense response.
A Well-Orchestrated Defense Plan
Like animals, plants have hormones, small mobile molecules created by the plant that travel to different tissues and control important processes. In this case the WRKY transcription factors cause an increase in two hormones, called salicylic acid and jasmonic acid. These hormones serve as “first-aid” response while further activating and boosting the defenses. They patch the damage and increment the defense against further damage.
The plant also produces other chemicals to fight back, namely a class of molecules called reactive oxygen species. These are highly reactive compounds that can cause a lot of cell damage. They need to be held in check, but can also be put to good use in defense. They can work to destabilize and stress the attacker, but essentially serve as an internal signal, telling other plant cells that there is an attack and to ignite a defense.
In addition to the chemical defenses, the plant fights back physically. Another piece of the defense plan orchestrated by the WRKY proteins is the buildup of callose. Callose is a sugar-based polymer that the plants use as band-aid to build temporary barriers in the damaged area under attack. It is secreted within minutes and forms a wall in the plant‘s nutrient rich arteries where it limits the loss of sap.
The plants defend themselves through a long run plan by sensing the attacker and fighting back, both with chemical weapons and physical barriers. That’s how BPH14 activated defenses thwart planthopper predation. The insects do not grow and survive well on plants that know how to defend themselves. Be warned insects!
Understanding the “garlic and holy water” of plant resistance to planthopper is a great start to breeding future varieties that are ‘trained’ to fight back. Stronger genetics means less use of chemicals to control insects, less environmental pressure, and better yield for farmers in developing countries that may simply not have access to agricultural insecticides. It shows the importance of understanding basic biological questions in how plants fight back when under attack.
Department of plant sciences
University of California, Davis
Hu L, Wu Y, Wu D, Rao W, Guo J, Ma Y, Wang Z, Shangguan X, Wang H, Xu C, Huang J. The Coiled-Coil and Nucleotide Binding Domains of BROWN PLANTHOPPER RESISTANCE14 Function in Signaling and Resistance against Planthopper in Rice. The Plant Cell. 2017 Dec 1;29(12):3157–85
Herman M, Williams M. Fighting for their lives: plants and pathogens. Teaching Tools in Plant Biology: Lecture Notes. Plant Cell, doi/10.1105/tpc.112.tt061