Corn smut fungus weakens maize’s defences by making it ‘see red’
A smut fungus causes infected maize plants to waste their resources and make a red pigment, rather than strengthen their cell walls.
The production of agricultural crop plants is severely hindered by bacteria, viruses, and fungi that have developed their own strategies to colonize these plants and obtain nutrients from them. Some pathogens kill the plants they colonize, but ‘biotrophic pathogens’ employ sophisticated strategies to manipulate the host plant without killing it.
During the past decade it has been recognized that the interactions between plants and biotrophic pathogens are largely governed by effector proteins — which are typically released by the pathogen after it makes contact with the host. These effector proteins can either stay in the space between the plant cells and pathogen cells, or actually enter inside the plant cells.
The fungus Ustilago maydis is one such biotrophic pathogen that colonizes maize plants and causes a disease called corn smut. Hallmarks of this infection are the formation of large plant tumors and the production of a red pigment, called anthocyanin, in infected plant tissues.
Now, Shigeyuki Tanaka and co-workers reveal that an effector called Tin2, which is released by the corn smut fungus, causes the production of this anthocyanin pigment. Tin2 moves inside plant cells, where it blocks the breakdown of a protein-modifying enzyme that is necessary to ‘switch on’ the production of anthocyanin. When a mutant fungus that lacks Tin2 infects a maize plant, no anthocyanin is induced and the pathogen fails to reach the vascular tissue, where it would normally get most of its nutrients. Tanaka and co-workers revealed that, in these infections, this vascular tissue was strongly reinforced with a compound, called lignin, suggesting that the plant fortifies these cell walls to block access by the mutant fungus.
Since the building blocks needed to make lignin are also required for making anthocyanins, Tanaka and co-workers suggest a model whereby Tin2 compromises the ability of plants to protect themselves by diverting resources away from making lignin. In line with this speculation, the corn smut fungus was shown to cause stronger disease symptoms in maize plants with mutations that prevent them from producing lignin.
The Tin2 effector of the corn smut fungus appears to target a critical protein in maize that can shift the balance of the plant’s metabolic pathways in favor of the pathogen. Further, since anthocyanin production is also observed after infections of plants with other microbes, these findings may have uncovered a microbial strategy to enhance virulence that is also employed by other plant pathogens.
To find out more
Listen to Regine Kahmann explain how corn smut attacks maize in episode 8 of the eLife podcast.
Read the eLife research paper on which this story is based: “A secreted Ustilago maydis effector promotes virulence by targeting anthocyanin biosynthesis in maize” (January 28, 2014).
eLife is an open-access journal that publishes outstanding research in the life sciences and biomedicine.
