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Infection of Norway spruce by Chrysomyxa rhododendri: ultrastructural insights into plant–pathogen interactions reveal differences between resistant and susceptible trees

Summary

Researchers studied how a rust fungus infects Norway spruce trees at the microscopic level, comparing healthy, resistant trees with ones that get severely infected. In resistant trees, the plant quickly builds protective barriers and accumulates compounds called tannins that slow down the fungus. Understanding these natural defenses could help foresters choose and grow spruce trees that better resist this devastating disease in high-altitude forests.

Background

Chrysomyxa rhododendri is a rust fungus causing severe needle infections in Norway spruce trees in European subalpine forests, leading to defoliation and reduced growth. Understanding the host-pathogen interaction at the cellular level is crucial for developing resistance strategies, particularly as little is known about the disease mechanisms in high-elevation tree pathogens.

Objective

This study aimed to characterize the ultrastructural development of C. rhododendri in Norway spruce needles from infection through aeciospore release, and to identify cellular and biochemical differences between resistant and susceptible tree genotypes using histology and transmission electron microscopy.

Results

In susceptible trees, C. rhododendri formed dense mycelium in intercellular spaces with subepidermal spermogonia and aecia, causing mesophyll cell degradation and chloroplast breakdown. In resistant tree needles, fungal hyphae were rare, with distinct cell wall modifications and accumulation of electron-dense material and tannin droplets around fungal structures. Yellow needle discoloration corresponded precisely to mycelial expansion and fungal carotenoid production.

Conclusion

The study reveals that highly resistant Norway spruce trees limit fungal growth through rapid enhancement of structural and chemical barriers including cell wall modifications and polyphenol accumulation, consistent with a hypersensitive response mechanism. These ultrastructural insights provide a foundation for understanding resistance mechanisms and improving selection of rust-resistant spruce genotypes for afforestation.
  • Published in:Tree Physiology,
  • Study Type:Experimental Research,
  • Source: PMID: 40459247
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