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Fungal protein mutation explains why rice blast fungus causes widespread crop disease

The Pro1 protein make the fungus hardier and the release of its conidia favours dispersal in the wild.

Ahead of harvest season in Hasakeh, the wheat is drying up. — © AFP
Ahead of harvest season in Hasakeh, the wheat is drying up. — © AFP

In a recent study, Tokyo University researchers have revealed new findings about the Rice Blast Fungus. The research has uncovered that mutations in Pro1 (a mating-related protein).

This protein helps to express mating-related genes in different species of fungi – not only causes female sterility in this fungus but also offers it a ‘fitness advantage’ with increased chances of asexual spore release. In other words, the mutation functions to make the fungus sterile; however, this may also provide the fungus with an adaptive advantage.

This discovery reveals a conundrum behind the fungus’ adaptation and has significant implications for crop disease management and fungal evolution.

The Pro1 protein helps express mating-related genes in different species of fungi. But what happens when Pro1 malfunctions? Looking at the rice blast fungus, Pyricularia (Magnaporthe) oryzae the researchers attributed this to an increase the release of asexual spores.

This species of pathogenic fungi infects a variety of cultivates grains, such as rice, wheat, barley, and finger millet. The filamentous fungus infects as a spore that produces lesions or spots on parts of the rice plant.

Rice blast fungal spores. Image by Donald Groth - USDA Forest Service, Public Domain, CC Commons.
Rice blast fungal spores. Image by Donald Groth – USDA Forest Service, Public Domain

The infection cycle begins with asexual reproduction, where asexual spores called conidia attach to the surface of the leaves of the rice plant. This produces an infection-specific structure called an appressorium. This starts to penetrate the outermost cell layer of the leaf, resulting in visible lesions on the leaf surface.

When conditions are favourable, specialized structures called conidiophores emerge and produce more conidia, which disperse through the wind or atmospheric droplets to more rice plants.

Although the asexual mode of reproduction is the main driving force of the P. oryzae life cycle, sexual reproduction is also possible. The fungus appears to have strains equivalent to biological males and biological females. Yet most specimens collected from the fields show a loss of female fertility. The underlying genes and mechanisms responsible for the loss of sexual reproduction in P. oryzae have remained a mystery.

This is what led the researchers to describe the loss of sexual reproduction in the rice blast fungus as ‘advantageous’. This also overcomes the so-called paradox of sex’. This refers to the sexual mode of reproduction being energy intensive. Whereas asexual reproduction requires only one parent and allows for the more rapid generation of offspring.

To demonstrate this, the researchers used multiple genetic experiments to identify which genes were linked to female sterility. This led to the identification of mutations; these are mutations leading to a dysfunctional Pro1 protein. Not only does this make the fungus hardier, the release of asexual conidia favours dispersal in the wild.

The research has been published in the journal iScience and it is titled “Dysfunctional Pro1 leads to female sterility in rice blast fungi”.

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Written By

Dr. Tim Sandle is Digital Journal's Editor-at-Large for science news. Tim specializes in science, technology, environmental, business, and health journalism. He is additionally a practising microbiologist; and an author. He is also interested in history, politics and current affairs.

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