A hidden defense switch: how apples fight back against devastating leaf spot disease

Glomerella leaf spot (GLS), caused by the fungus Colletotrichum gloeosporioides, is a major threat to apple production worldwide, causing severe defoliation and fruit necrosis. Current control relies heavily on chemical fungicides, raising environmental and public health concerns. In a significant step toward more sustainable solutions, researchers have uncovered a crucial genetic regulatory module that controls apple resistance to this destructive pathogen.

In apples, a small RNA molecule called microRNA482c (miR482c) was previously known to be triggered by fungal infection, but its precise role remained unclear. While the miR482 family typically suppresses immune responses by targeting resistance genes in other plants, its function in apples appeared to diverge. This raised a fundamental question: could miR482c be part of a unique defense mechanism specific to apples, and if so, how does it work? Based on these challenges, there is a need for in-depth research into the molecular pathways that underpin apple resistance to Colletotrichum gloeosporioides.

Now, a team from the Research Institute of Pomology at the Chinese Academy of Agriculture Sciences has published (DOI: 10.1093/hr/uhag199) their findings in Horticulture Research. The study identifies a novel regulatory module—MdERF1B-miR482c-MdCCR4-MdTLP1b—that orchestrates apple resistance. The authors demonstrate that a transcription factor, MdERF1B, can suppress miR482c, thereby activating a chain of immune responses.

The researchers discovered that miR482c does not target the typical NBS-LRR resistance genes, but instead directly cleaves the mRNA of a serine/threonine protein kinase, MdCCR4, a departure from its known role in other species. Overexpression of miR482c made susceptible apples even more vulnerable to the fungus, while silencing it enhanced resistance. Crucially, they found that MdCCR4 physically interacts with a pathogen-related protein, MdTLP1b, to activate defense. MdERF1B, a transcription factor, was shown to bind to the miR482c promoter, repressing its transcription. This means that when MdERF1B is active, it suppresses the suppressive miR482c, which in turn allows MdCCR4 and MdTLP1b to mount a robust immune response. Experiments with transiently transformed apple leaves and fruits confirmed that manipulating these genes directly affected disease severity.

The authors explained the significance of their work: “Our findings reveal a new layer of complexity in how apple trees defend themselves. We have shown that a transcription factor can control the expression of a microRNA, which then releases a protein kinase to work together with a defense protein. It is a multi-level regulatory cascade, and understanding this gives us promising new targets for breeding more resilient apple varieties.” They emphasized that this pathway provides a clearer picture of how resistance to this specific pathogen is achieved.

This discovery has significant implications for apple breeding and sustainable agriculture. By identifying key genes such as MdERF1B, MdCCR4, and MdTLP1b, researchers now have potential markers for selecting resistant cultivars or targets for genetic improvement. This could lead to the development of apple varieties that require fewer chemical fungicide applications, reducing the environmental impact and production costs associated with disease management. Ultimately, this research offers a path toward a more sustainable and resilient future for apple production.

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References

DOI

10.1093/hr/uhag199

Original Source URL

https://doi.org/10.1093/hr/uhag199

Funding information

This work was supported by the Asian regional cooperation special projects and Agricultural Science and Technology Innovation Program (CAAS-ASTIP-2016-RIP).

About Horticulture Research

Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2023. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.