Fungi inside healthy tissue of high-altitude trees

An endophyte is a fungus living inside the tissues of a plant that shows no symptoms of disease. This is neither rare nor exceptional: practically every plant examined turns out to host such fungi, and their role may be neutral, beneficial or latently pathogenic, depending on the plant's condition and the circumstances.

During a mycological survey of high-altitude forests in Xizang, Tibet, Kaili Pang's team isolated four endophytic strains from the asymptomatic tissues of the Tibetan cypress Cupressus gigantea and the silky rose Rosa sericea. On the basis of morphology and phylogenetic analysis of a five-gene dataset — ITS, LSU, tub2, tef1 and rpb2 — they were described as two new species: Parafenestella cupressicola and P. xizangensis, in the family Cucurbitariaceae, order Pleosporales.

The distinction rests on a combination of grounds. P. cupressicola forms a sister lineage to P. ulmi and P. ulmicola but differs in the dimensions of its conidiogenous cells and conidia. P. xizangensis shares conidial morphology with P. salicis but differs in its sequences. That is a typical situation in the taxonomy of microfungi: neither morphology alone nor sequence alone is enough.

The authors stress that the result expands the known host range and geographic distribution of Parafenestella, and that more endophytic diversity remains to be discovered in alpine ecosystems.

The choice of site is worth noting. High mountains are difficult and thinly explored ground for mycology, and at the same time places where isolation and harsh conditions favour lineages diverging. The Tibetan cypress, a long-lived endemic tree, is a particularly promising host in that context.

The boundary between an endophyte and a pathogen is fluid, and that is the most important caveat attached to such descriptions. The same fungus can live in tissue for years without symptoms and turn pathogenic once the plant weakens. Isolation from healthy tissue therefore says where the fungus was at the moment of sampling, not how it will behave.

Parafenestella is an instructive example here, because its known members are sometimes associated with dieback in broadleaved trees. Describing a new species as an endophyte is not a verdict on its harmlessness, only a record of the circumstances in which it was found.

What the paper establishes

Four strains cultured from surface-sterilised, asymptomatic plant tissues in Tibet formed two new species: Parafenestella cupressicola from a healthy Cupressus gigantea cone at 3,093 m, and P. xizangensis from bark of a healthy Rosa sericea branch at 3,163 m. Each is represented by two strains, while combined ITS, LSU, tub2, tef1 and rpb2 analysis recovered both clades with 100/1 ML/BI support. “Endophyte” here means isolation from inside asymptomatic tissue after surface sterilisation. It does not demonstrate benefit, permanent harmlessness or host specialisation, nor exclude a saprotrophic phase or latent pathogenicity. One tissue type and locality per species cannot delimit complete host ranges, distributions, abundance or conservation status.