SO FAR IN THIS SECTION, we’ve explored the amazing behavior and communication strategies of microbes and how they influence human cells in both friendly and hostile ways. In this chapter, we will briefly explore microbial influence on plants, which have an amazing inner world all their own.
While microbes help and hurt humans in an enormous number of ways, scientists are just now discovering the elaborate back-and-forth conversations among human cells. It is difficult to decipher small signals in the midst of complex human tissues and large blood vessels. With the simpler structure of plant cells and the more obvious ways they communicate with microbes, some startling conversations have been discovered that involve hundreds of back-and-forth signals.
These interactions are sequential and designed for the microbe and plant to accomplish amazing feats, such as inviting in an entire colony of microbes to help set up lifesaving factories where vital nitrogen is supplied. Another sequential outcome is multilevel, escalating attacks on each other in all-out warfare.
Many of us don’t realize how extensively plants interact with their surroundings. Plants gather information from the soil and the air. They signal to other plants with airborne chemicals and along fungal filaments. Plants sense signals from contact, light, smell, and possibly sound and magnetism. Internally, plant cells communicate with each other using water pressure, chemical signals, and electrical signals.
Plants engage in a wide range of communication that triggers defensive behaviors. They signal to other plants about predators. Responses to danger can include secreting poisons against insect eggs. They produce chemicals that attract predators of the insects that have laid eggs on their leaves. Plants can grow tumors to physically push eggs off leaves. They can time the arrival of toxins they produce to exact moments of the day, such as generating a toxin for mold just before morning dew.
NITROGEN FIXATION
Perhaps plants’ most elaborate communication is used to build nitrogen factories with microbes. Plants need nitrogen but can’t take it from the atmosphere. Nitrogen is essential for amino acids, proteins, and nucleotides in DNA and RNA. But for plants to use nitrogen, it first has to be altered to its usable form—a process called “fixing,” the conversion of nitrogen in the air into related nitrogenous compounds in soil.
Fixing can occur naturally by the action of lightning and volcanoes. Humans can provide nitrogen-rich fertilizer. And plants can build their own nitrogen factories using microbes that are able to fix nitrogen.
Multiple steps are needed to build nitrogen factories, which are accomplished with elaborate back-and-forth signaling among bacteria, fungi, and plant roots. Plants have to first determine which microbes are friends by sending signals and observing responses. When plants identify the correct microbes, they invite them in to create factories inside the plant. If enemies are found, they send attack molecules instead.
A fungus nitrogen factory in a corn plant cell. Electron micrograph. (USDA/Science Source)
Many plants form lifelong relationships with microbes to fix nitrogen. These plants most notably include legumes, such as peanuts, peas, soybeans, clover, and alfalfa. With the help of plant signals, bacteria form an elaborate visible structure, called a nodule, inside the plant. Fungi form smaller factories inside individual plant cells. Communication to build these factories occurs in a narrow region of soil directly near the root. Forming a factory is a business relationship between microbes and plants. Microbes receive carbon from plants to eat and plants receive the nitrogen in their roots that they need to survive.
BACTERIAL FACTORIES
First, plants send signals from the root or seed, and bacteria recognize the signal. Genes in bacteria, triggered by the plant’s signal, produce proteins, peptides, and amino acids. Plant receptors pick up the factors, even in extremely small amounts, with thin tubular protrusions called root hair. With the two signals recognizing each other, the root hair curls and forms a pocket for bacteria to enter into the plant through a special pathway. Together, they signal back and forth and create a path for microbes to grow into root hair and then deep into the plant.
At this point, the plant triggers new cells to build up an area inside the plant that will become the nitrogen factory. The bacteria multiply in the space created by the cells. The plant then builds a membrane around the entire region of plant cells and microbes. Plant cells near the edges of this membrane receive nitrogen as it is produced by the bacteria and pass it along to the rest of the cells. Microbes are given a huge supply of carbon in return and grow into a large colony, where they live for years.
Many different signals and “handshakes” are used in this process. Only a few are mentioned here. In fact, the plant can cancel the operation at hundreds of different points in the process. In this collaborative process, the plant uses calcium molecules to generate oscillations, which signal the bacteria that it’s time to enter the plant, and then where the bacteria should go once they’ve entered.
In those cells that are providing directions, calcium is pumped in channels from the plant’s outer cell membrane to the cell’s nucleus and back, causing rising and falling calcium levels. These oscillations produce a “yellow brick road” to help the microbes identify the path to the factory. As the microbes move toward the factory location, calcium oscillations appear in adjacent cells, from the root hairs all the way to the site of new cells built for the factory. Signals occur along the entire route.
FUNGAL SIGNALS
Many plants have a symbiotic relationship with fungi as well. Fungi are classified as both single-celled (e.g., yeast) and multicellular microbes. In the evolutionary process, fungi that had already been well established in soil helped plants transition from water onto land. They were able to help plants in multiple ways because the long, thin fungus filaments formed “wires” between most plants, and these provided conduits for sending nourishment and signals.
The process fungi use to form nitrogen factories in plants is slightly different from that used by bacteria, but it has the same general outline. As well as fixing nitrogen, fungal factories can bring phosphorus and other nutrients for the plants. Fungus can establish these factories in 80 percent of all plants, including those that make up a large amount of our food supply, such as cereals, fruits, and vegetables.
Fungi are invited into plants with similar signaling. They then set up nodules that serve as factories to fix nitrogen inside individual root cells. These factories can be microscopic, as opposed to the large observable nodules produced by bacteria. After back-and-forth signaling, thin fungal filaments, called hyphae, contact the root and enter through the top layer of cells. Once inside the plant, fungi grow more filaments, which serve as the small factories to start manufacturing nutrients. As they do for bacteria, plants make major renovations to accommodate the fungal factories. Signals and nutrients go back and forth between plants and fungi.
The filament branches form elaborate networks. They stretch for miles, connecting even unrelated plants to one another, across a forest, for example. These signaling channels share nutrients and warn of danger. Plants are active in signaling along these fungal filaments and can cut off fungi in some situations, while depending on them for nutrients at other times. Varieties of fungi mostly live peacefully in plants, with one large old tree found to have 2,500 different fungal species. Each root system can have more than a hundred different species of nitrogen-fixing fungi. Fungi recycle the plants they live in by eating them after they die, providing rich nutrients for a new generation of plant life.
WARFARE BETWEEN PLANTS AND MICROBES
Communication between microbes and plants can be pleasant and helpful, or very destructive.
Plant cells are larger and more complex than most microbes, but microbes are mobile and can easily mobilize into an army of many to effectively attack their hosts. Both bacteria and plants produce new, precise proteins to attack each other’s specific cellular processes.
Just as B lymphocytes were described as editing their own DNA to produce antibodies when fighting microbe invaders, plants use a similar self-editing method to produce proteins to kill particular microbes. These proteins become prominent weapons against enemy bacteria, viruses, and fungi.
Plants also use small RNA molecules in several different ways for warfare. Plant RNAs can silence particular genes that would ordinarily produce proteins used by microbes as weapons. Such RNA molecules can alter messenger RNA in microbes to interrupt the production of attack proteins. These plant RNAs can also directly interfere with ribosomes that manufacture proteins. Plants use RNA to target very specific pieces of genetic material in microbes. Microbes fight back by suppressing the plant RNA molecules. Both plants and microbes use these small silencing RNAs to fight each other, as well as enemy viruses.
Plants also have receptors that recognize particular shapes of microbial molecules used in warfare. These patterned receptors trigger particular proteins to fight the microbes. Bacteria and viruses both counter with their own molecules that stop plant proteins. Plants then use an even more specific process of producing antibody-like attack molecules that are designed for a particular microbial species.
A plant must defend against thousands of different microbes and new unknown types as the microbes continue to evolve. Plants can also use long-range communication along their fungal filament branches to help defend themselves against local microbes. As microbes evade and counter the attacks, plants up the ante and use even stronger methods.
For example, plants utilize pieces of bacteria and viruses to target and kill precise microbial species. The pieces of the microbe are placed in these plant attack molecules, which then match the piece they are carrying with molecular patterns on the microbe they are chasing. When they find the precise matching molecule on the microbe, they cut it, killing the microbe. This is somewhat similar to the way human cells present small pieces of microbes to T cells to enable the T cells to track precise microbes and kill them.
Still, the battle isn’t always over, even with such precise mechanisms. When these cutting machines chase viruses, for example, the viruses can hide in tiny pouches in the plant cell’s outer membrane. Plants then respond by making special machinery resembling arms that can reach into these pouches with cutting devices to kill the virus. As a last resort, plants retain a nuclear option. They can kill their own cells that are losing, thereby killing all the microbes inside the cell.