December 23, 2003 | A. Elizabeth Arnold†‡, Luis Carlos Mejía†, Diamond Kyllo†, Enith I. Rojas†, Zuleyka Maynard†, Nancy Robbins†, and Edward Allen Herre†
This study investigates the role of fungal endophytes in protecting a tropical tree, *Theobroma cacao*, from pathogen damage. Endophytes are fungi that live harmlessly within the tissues of plants, and their interactions with hosts are poorly understood. The researchers found that *T. cacao* is colonized by a diverse array of endophytes, which are horizontally transmitted and accumulate over the lifetime of the leaf. These endophytes significantly reduced leaf necrosis and mortality when the tree was challenged with a major pathogen, *Phytophthora*. The protection provided by endophytes was localized to infected tissues and was greater in mature leaves, which have less intrinsic defense against pathogens. In vitro experiments suggested that host affinity is mediated by leaf chemistry, and that protection may be mediated by direct interactions between endophytes and foliar pathogens. This study demonstrates that diverse, horizontally transmitted endophytes can play an important but previously underappreciated role in host defense.This study investigates the role of fungal endophytes in protecting a tropical tree, *Theobroma cacao*, from pathogen damage. Endophytes are fungi that live harmlessly within the tissues of plants, and their interactions with hosts are poorly understood. The researchers found that *T. cacao* is colonized by a diverse array of endophytes, which are horizontally transmitted and accumulate over the lifetime of the leaf. These endophytes significantly reduced leaf necrosis and mortality when the tree was challenged with a major pathogen, *Phytophthora*. The protection provided by endophytes was localized to infected tissues and was greater in mature leaves, which have less intrinsic defense against pathogens. In vitro experiments suggested that host affinity is mediated by leaf chemistry, and that protection may be mediated by direct interactions between endophytes and foliar pathogens. This study demonstrates that diverse, horizontally transmitted endophytes can play an important but previously underappreciated role in host defense.