If I asked you to name a microbe that can cause disease in plants you are likely to name a species of bacteria, fungus or virus. But would you name an oomycete?

An A. thaliana leaf infected with downy mildew. The oomycete H. arabidopsidis is shown in blue. Image by Caillaud et al. (2014) licenced under CC-BY.

At first glance oomycetes look quite like fungi because they grow by producing branching filaments—similar to fungal mycelia—through which they absorb nutrients from their surroundings. Oomycetes used to be classified as fungi but, in fact, they are not closely related.

The cell wall that surrounds oomycete cells provides a clue of where they fit in the tree of life: it is made of cellulose, not chitin like fungi, suggesting that oomycetes are more closely-related to algae or plants. This is supported by other features of the lifestyle of oomcyetes and analyses of their genetic information, which place them in the same Phylum as brown algae and diatoms (1).

Oomycetes are responsible for causing many diseases in crop plants, including late potato blight and downy mildew in grape. A severe outbreak of late potato blight in the 1840s led to the Irish Potato Famine, which killed a million people and forced many others to leave the country. Despite this, the oomycete that causes late potato blight—Phytophera infestans—is still sometimes called a fungus.

Understanding how oomycetes infect plants could help us to develop disease-resistant crop varieties to help control these diseases. A recent paper published in PLOS Pathogens studied the relationship between an oomycete called Hyaloperonospora arabidopsidis and its host, the model plant Arabidopsis thaliana (2).

H. arabidopsidis infects A. thaliana leaves producing yellow and brown patches to appear on the leaves, known as downy mildew. To get nutrients from its host, the oomycete produces a special structure called a haustoria that invades into plant cells. The haustoria is separated from the plant cell cytoplasm by a membrane that joins the plant cell membrane. The plant launches several defences to fend off the oomycete and the haustoria gradually becomes encased by a polymer called callose, but it is not clear how this works.

Caillaud et al. found that a plant protein called PDLP1 is found in the membrane surrounding haustoria and is involved in plant defence against the oomycete (2). The experiments suggest that its role in plant defence is to promote the deposition of callose around the haustoria.

PDLP1 belongs to a family of proteins that are found around the channels “or bridges” that connect neighboring plant cells, known as plasmodesmata. Plasmodesmata allow proteins and other molecules to move between plant cells, but sometimes this is not desirable and so the plasmodesmata can be closed to isolate individual cells. To close a plasmodema, the PDLP proteins promote callose deposition around it. Therefore, the experiments suggest that callose deposition around haustoria and plasmodesmata may happen in a similar way and involve the same proteins.

References:

1. Wikipedia: oomycete (retrieved 25/11/14)

2. Caillaud, M., Wirthmueller, L. et al. (2014) The Plasmodesmal Protein PDLP1 Localises to Haustoria- Associated Membranes during Downy Mildew Infection and Regulates Callose Deposition. PLOS Pathogens.