Upregulation of chitinase and β-1,3-glucanase enzymes and modulation of biochemical defense response by fungal endophytes against Rhizoctonia solani in cotton

Cotton is more than just a fabric; it is a global agricultural engine, with India alone cultivating nearly half of the world's cotton fields. Yet, this vital crop faces a silent, underground enemy. A microscopic fungus called Rhizoctonia solani lives in the soil, waiting to attack young cotton seedlings. It causes a condition known as damping-off, where the stems of the seedlings rot at the base, causing them to collapse and die before they can even establish themselves. For decades, farmers have fought this enemy with chemical fungicides, but these chemicals can harm the soil and the beneficial microbes living within it, while the fungus itself can eventually learn to resist them. Nature, however, offers a different kind of solution. Hidden inside the tissues of healthy plants are microscopic fungi called endophytes. These are not invaders; they are permanent, harmless residents that live within the plant without causing disease. In many cases, these internal guests act as guardians, producing compounds that repel pests or triggering the plant's own immune system to stand ready for an attack.

A team of researchers at the Central Institute for Cotton Research in Nagpur, India, set out to see if they could use these internal guardians to protect cotton from the deadly soil fungus. They focused on three specific types of fungal endophytes that they had previously found living inside healthy cotton plants. The scientists wanted to know if they could wake up the cotton plant's natural defenses before the enemy even arrived. To do this, they took cotton seeds and soaked them in a liquid containing spores of these beneficial fungi, a process known as seed biopriming. This step essentially gave the seeds a head start, coating them with their new microscopic allies. They then planted these treated seeds in soil that had been infected with the disease-causing fungus, alongside seeds that had received no treatment. The goal was to observe how the treated plants fared against the infection and to understand exactly what happened inside the plant cells to make them stronger.

The results showed that the strategy worked remarkably well. The seeds treated with the beneficial fungi grew into seedlings that were far more likely to survive the attack than the untreated ones. In fact, the seeds treated with one specific endophyte, Daldinia eschscholtzii, showed the highest germination rates, with 91 percent of them sprouting successfully even in the presence of the disease, compared to only 67 percent for the untreated seeds in infected soil. But the story goes deeper than just survival rates. The researchers looked inside the tissues of the surviving plants to see what biochemical changes had occurred. They found that the treated plants were essentially in a state of high alert. They had accumulated significantly higher levels of proteins and sugars, which serve as the building blocks and fuel for the plant's defense machinery. The plants also produced much more phenolic compounds, which act like natural antiseptics and help strengthen the plant's cell walls, making it harder for the invading fungus to break through.

The study also measured the activity of specific enzymes, which are the tools the plant uses to fight back. The treated plants showed a surge in antioxidant activity, which helps neutralize the harmful chemicals that the disease fungus releases to damage the plant. One enzyme, called catalase, which helps break down toxic byproducts of the plant's own immune response, was particularly active in the plants treated with Daldinia eschscholtzii. Another enzyme, peroxidase, which is involved in hardening the plant's cell walls, was also highly active, especially in plants treated with a different endophyte called Trichoderma sulphureum. Perhaps most telling was the genetic evidence. The researchers examined the genes responsible for producing chitinase and beta-1,3-glucanase, two powerful weapons that plants use to dismantle fungal cell walls. In the plants treated with the beneficial fungi, the instructions for making these weapons were turned on with incredible speed and intensity. Within fifteen days of planting, the activity of the gene for chitinase had increased by as much as twenty-six times in the plants treated with Daldinia eschscholtzii, while the gene for beta-1,3-glucanase showed a nine-fold increase. This suggests that the beneficial fungi did not just fight the disease directly; they taught the cotton plant how to fight for itself.

The researchers observed that this defense response was not a single event but a coordinated effort that evolved over time. While the genetic signals were strongest in the early days of the plant's life, the biochemical defenses, such as the accumulation of sugars and proteins, continued to build up as the plants grew older. This indicates that the beneficial fungi primed the plant to be ready for an attack immediately, and then helped sustain that defense as the plant matured. The study did not find that the beneficial fungi eliminated the disease fungus entirely, but rather that they significantly reduced the damage it could cause. The plants treated with the endophytes were not immune, but they were resilient enough to survive and grow where untreated plants would have perished. This approach offers a promising alternative to chemical sprays, relying on the natural partnership between plants and the microscopic life they host. By simply soaking the seeds in a solution of these helpful fungi, farmers could potentially protect their crops using a method that is both effective and gentle on the environment, turning the cotton plant's own internal allies into its first line of defense against a persistent and destructive enemy.