Pathogenic fungi use plants’ proteins against them.
Fungal diseases in plants cause huge economic problems for farmers worldwide, either by reducing crop yield or by killing plants outright. These disease-causing fungi produce an array of compounds, known as ‘virulence factors’ that they use to breach plant defences. The two are locked in a constant arms race, with the plant trying to produce defences that let it stay one step ahead of the pathogen. This is often described as the Red Queen Hypothesis.
A new paper by Djamei et al. has revealed the nature of one of the virulence factors of Ustilago maydis, the fungal agent that causes maize smut. U. maydis requires live plants to survive, secreting many protein effectors that suppress the plant’s defence response and alter its metabolic pathways to suit the fungus. Most of these proteins are of unknown function.
The researchers have identified an enzyme called chorismate mutase, Cmu1, which is involved in the conversion of chorismate into the amino acids tyrosine and phenylalanine. They demonstrate that the production of this enzyme is upregulated during plant colonisation and is secreted into plant cells. Mutants unable to produce the protein showed reduced virulence.
Post infection, a tagged version of Cmu1 could be seen in the fungal hyphae, the interface between the plant and fungus and the cytoplasm of the maize cells. The researchers showed evidence that Cmu1 is able to move between plant cells, most likely through plasmodesmata – narrow tubes that connect the cytoplasm of neighbouring plant cells.
This work also showed that Cmu1 is capable of forming a 1:1 complex (also known as a dimer) with ZmCm2, a chorismate mutase enzyme made by the maize cells. It appears that this dimer prevents the maize producing salicylic acid (SA) – a compound involved in mediating plant defences against pathogen attack.
Indeed, maize plants infected by U. maydis mutants unable to produce Cmu1 showed a ten times higher level of SA than those infected by the wild-type fungus. Chorismate is a substrate for SA biosynthesis and the authors of the paper propose that Cmu1, in conjunction with ZmCm2, alters the maize’s metabolism to lower the levels of chorismate available for SA production. However, given the large number of secreted proteins, it is likely that many more effectors are involved in altering the plant’s metabolic pathways.
Secreted chorismate mutases are encoded in the genomes of many fungal pathogens of plants and these virulence factors may prove to be a key part of the fungal armoury.
Djamei A, Schipper K, Rabe F, Ghosh A, Vincon V, Kahnt J, Osorio S, Tohge T, Fernie AR, Feussner I, Feussner K, Meinicke P, Stierhof YD, Schwarz H, Macek B, Mann M, & Kahmann R (2011). Metabolic priming by a secreted fungal effector. Nature, 478 (7369), 395-8 PMID: 21976020