Exploring Microbial Biocontrol Agents for Kauri Dieback Disease Prevention

This research project screened microbial isolates from the leaves and roots of five kauri companion plants against the mycelia and oospores of Phytophthora agathidicida. Through screening, BCAs were discovered that inhibited mycelial growth, oospore germination and viability, and their secreted secondary metabolites were identified.</p><p>Fourteen of the functionally isolated bacterial and fungal isolates demonstrated short-range inhibition of P. agathidicida mycelia, and eight demonstrated long-range inhibition.

Two fungal isolates (KAL1a11, short-range; KOL2a11, long-range) and two bacterial isolates (WL3b11, short-range; TR2a11, long-range) were selected for further screening against oospores. There was some variability in the screening results, however the two bacterial isolates showed considerable promise as BCAs, where oospores co-cultured with Rahnella ecdela strain WL3b11 showed a decrease in live stained oospores and an increase in dead and dual stained (damaged) oospores.

Oospores treated with Pseudomonas lactucae strain TR2a11 supernatant showed minimal, stressed germination. AntiSMASH analysis of the genomes of each of these bacteria showed biosynthetic gene clusters (BGCs) with predicted natural products from classes containing known biocontrol compounds. P. lactucae TR2a11 contained two novel BGCs for hydrogen cyanide, a β-lactone, and a terpene precursor.

R. ecdela WL3b11 also contained a terpene precursor and a β-lactone BGC, as well as an azole-containing RiPP. Both genomes also contained multiple secreted carbohydrate active enzymes (CAZymes) from glycosyl hydrolase families with lytic activity against known P. agathidicida cell wall components.</p><p>Sourcing biocontrol agents from companion plants already present within the kauri forest environment leads to easier implementation of any potential treatments with the BCAs or their natural products.

Both P. lactucae TR2a11 and R. ecdela WL3b11 show promise as BCAs that could be implemented as a probiotic inoculant in P. agathidicida infected kauri forests to aid in resistance to infection, and to help prevent the spread of the disease.</p>