Collaborative Research: Unraveling secrets of Rapid Ohi'a Death resistance: assessing carbohydrates and chemical defenses in the response of Hawaiian forests to a fungal pathogen

Invasive pathogens are an increasing threat to forests worldwide, causing tree mortality and reducing ecosystem health. To better predict the impact of pathogens on forests, fundamental knowledge of how trees physiologically respond to stress caused by pathogens is critical. Just like animals store fat, trees store carbohydrates for later use.

Carbohydrate stores are then used by trees to support growth, but they can also be used to build chemical compounds that provide defense against pathogens. The relationship between a tree’s storage of carbohydrates and investment in defensive compounds remains relatively unexplored, but this information is key to understanding a tree’s resistance to pathogens.

This project integrates two aspects of a tree’s physiology—carbohydrates and chemical defenses—to identify how trees buffer against stress caused by pathogens. Results will provide insight into the ability of Hawaiʻi’s ʻōhi‘a trees to resist a novel fungal pathogen that is causing widespread mortality. ʻŌhi‘a is the most abundant native forest tree throughout Hawaiʻi and an integral part of the island’s ecology, economy, and culture.

However, without a better understanding of ʻōhi‘a tree biology and its response to stress, the impacts of the fungal pathogen will remain unresolved and disease mitigation measures will be hindered. This research will aid the conservation and management of tropical forests. Drawing on a shared interest in improving research with native knowledge and increasing the recruitment and retention of minority groups in the sciences, the team will support an authentic independent undergraduate research experience for Native Hawaiian and Pacific Islander students.

In Hawaiian forests, a novel destructive fungal pathogen, Rapid ʻŌhiʻa Death (ROD; Ceratocystis spp.), threatens the keystone tree species Metrosideros polymorpha Gaud. (‘ōhi‘a). To protect these native forests, there is an urgent need to identify resistant phenotypes and the mechanisms responsible for enhanced survival. To meet this challenge and improve the ability to predict resistance of ʻōhiʻa trees to ROD, the team will couple field and greenhouse-based experiments to quantify the relationship between nonstructural carbohydrates (NSCs, i.e., sugars and starch) and the physical and chemical defense response to infection.

In Aim 1, the team will identify the seasonal minima and maxima of both NSC reserves and constitutive defensive chemistry in different tree organs of mature ‘ōhi‘a trees. In Aim 2, the team will artificially manipulate stem NSC reserves in mature ‘ōhi‘a trees to directly test the role of NSC reserve status in determining the induced defense response and its role in providing resistance to ROD infection.

In Aim 3, a greenhouse experiment will be conducted using ‘ōhi‘a saplings with genotypes across different putative resistance categories to assess genotype × environment effects on NSC-defense responses to ROD. These synergistic aims will enhance knowledge of ʻōhi‘a tree biology with crucial applications for its conservation and management in the face of unprecedented biotic stress.

The knowledge gained will fundamentally change the way biotically-driven tree mortality is modeled and impact a range of disciplines from plant pathology to silviculture. The project will also provide an undergraduate research experience for Native Hawaiian and Pacific Islander students.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.