There are, always, too many pathogens in the world. Often too many to catalog and keep track of and they are constantly changing, whether bacterial, viral, fungal, or even other options. When it comes to plants and agricultural output, the worst kinds of pathogens are those that can infect a variety of important crops grown around the world. This is made easier for some targets when there are several relevant plant species that are closely related to each other, such as the Brassica genus. Common household foodstuffs such as cabbage, broccoli, and the general mustard seed can be found within and any pathogen that targets Brassica as a whole has a myriad of targets to munch on.
The Alternaria Problem
And Alternaria brassicae, source of the Alternaria blight, is just one of those very pathogens. A fungal one, in fact, that is highly infectious and causes tissue necrosis. A contributor to this is the fact that it can infect its target plants at any stage of their life cycle, including as seeds, and can spread via spores both through the soil and the very air. Once attached, they wait until the plant has developed or is already at a stage where it has formed stomata on the bottom of their leaves for gas exchange.
The fungus then extends germ tubes through the stomatal openings and directly into the less protected inner sides of the epidermal cells. The fungal invasion spreads from there, forming a visual phenotype of black lesions on the surface of the plant. While the disease’s takeover and initial incorporation can be managed with certain fungicides, there is a definite desire to lessen the need for such items and to give the plants a more direct approach for beating back their aggressor.
Non-Host Is The Best Host
Scientists at Assam Agricultural University in India were investigating this fungal disease in Brassica species and specifically brown mustard plants that farmers were growing across the region. They noticed that a field of chickpeas planted right next to a field of brown mustard entirely infected with Alternaria blight were not impacted at all and had complete resistance to infection. This led them to try a molecular and biochemical comparison between the two species to see what gives chickpeas their non-host resistance and sets them apart from the Brassica genus.
Non-host resistance (NHR) refers to how almost all pathogens have a particular host range that they can infect and can’t do anything to the species outside of that range. This is usually due to their methods of infection, such as via particular receptors or effectors in their host species, not being present on the cells of other species, though there are additional forms of resistance that overall prevent the pathogen from being successful even if applied to the resistant plant. These factors are commonly divided into preinvasion and postinvasion resistance, as very different methods are applied for each.
The researchers wanted to know exactly what genes and traits give chickpeas their non-host resistance and whether any of those could be meaningfully given to Brassica species through biotechnological means. What they found is that, for one, chickpeas have a robust pathogen detection network and overall defense system that allows them to quickly target and destroy any Alternaria fungal pathogens that might try to invade. The signaling cascade of networked genes appears to play a critical role in this effort and is something that could potentially be transferred to another species.
Targeted Extreme Measures
Additional techniques to prevent infection include closing the stomata upon first detection of a pathogen, thickening of the cell wall and deposition of more epicuticular waxes on the plant’s surface, and also the production of more secondary metabolites that interfere with the fungal activities. These metabolites, called phytoalexins, are antifungal agents that prevent the creation of the germ tube and general functioning of the fungus itself.
If infection does happen somehow, it was shown that chickpeas also have a strong postinvasion setup that includes programmed cell death for infected cells and the formation and spread of reactive oxygen species (ROS) that damage the fungal tissues directly, along with helping complete the cell death commands. This proved more than enough to drive off or kill any fungal tubes that managed to penetrate into the chickpea leaf cells and so no further preventive measures were needed or observed.
The Task At Hand
Many previous studies have shown at least some effectiveness in conveying non-host resistance after the transgenic transfer of such resistance genes, including against fungal pathogens. The team plans to continue their research on chickpea NHR and test out some of the more promising gene options in the future and see if a Brassica species like brown mustard can be made wholly resistant to infection with Alternaria blight.
If they can manage such an accomplishment many farmers around the world, especially subsistence farmers that rely on their annual Brassica crop production for food, will be protected against future crop failures caused by the dastardly Alternaria brassicae fungus.
Photo CCs: CSIRO ScienceImage 3196 Chickpeas in glasshouse from Wikimedia Commons