Colletotrichum comes good

In a true Pavlovian, knee-jerk response way, whenever I hear – or see – the word Colletotrichum, I immediately think of the specific epithet lindemuthianum. Why? This harks back to hours of plant pathology classes decades ago (a testament to the power of teaching in that subject by Prof. Ivor Isaac* and esteemed colleagues). And that association is not a good one because C. lindemuthianum is a fungus that causes anthracnose of bean, which affects many types of ‘beans’ – e.g. pigeon pea, soya, mung bean, runner bean, black gram bean, and lima bean – globally. I therefore automatically and unconsciously ascribe pretty negative associations to Colletotrichum. Imagine my surprise, then, when I read that Colletotrichum tofieldiae [Ct] has been found in association with roots of Arabidopsis thaliana, and that the relationship appears to confer a benefit to the plant.

This association, in which the fungus lives as an endophyte (i.e. within the tissues of the ‘host’), was discovered in populations of Arabidopsis growing in the wild** in central Spain. And this certainly seems beneficial to the host because “Ct transfers the macronutrient phosphorus [Ed. – though probably in the less dangerous form of phosphate…] to shoots, promotes plant growth, and increases fertility only under phosphorus-deficient conditions”.*** Significantly, this relationship is not a mycorrhiza – arguably, the more usual fungus-flowering plant mutualism – but adds to the ever-growing list of fungal endophytisms.

What this research says to me is that you should never underestimate fungi: Their relationship with plants is double-edged. On the one hand they cause some of the most devastating plant diseases, which threaten future food security for all of us. On the other hand they engage in some of the most important mutualisms, e.g. mycorrhizas. Indeed, it is the association between early land plants and fungi hundreds of millions of years ago that is posited to have been one of the forces driving colonization of the land by plants, which has resulted in the marvellously diverse land flora we have today (and where the mycorrhizal association is found in 90% of its crop species). Plants and fungi, what a great team.

But all good things must come to an end, so to return to the beginning of this item and plant disease, and to Mebeaselassie Andargie and Jianxiong Li’s proposal for Arabidopsis thaliana to be “A Model Host Plant to Study Plant–Pathogen Interaction…”.

* See etymology for Verticillium isaacii in Patrik Inderbitzin et al.

** I imagine this fact – a ‘revelation’ perhaps? – might come as a shock to those who’d only ever thought of the laboratory/controlled environment growth cabinet as the natural home of Arabidopsis. But, yes, it’s a proper plant that had a life in the wild before being kidnapped therefrom and domesticated for plant research investigations (and still does, e.g. in central Spain, with or without its Ct partner…). And – shock-horror No.2 – it’s rather sobering to realise that this Ct endophytism is unlikely to have been chanced upon if all we knew of Arabidopsis was its life under sterile laboratory conditions…

*** I know what you’re thinking. Can’t we inoculate crops grown on P-fixing soils with Ct and avoid paying at least some of that phosphorus tax (see my previous AoBlog item,  “Life’s little certainties”)? Hmm, who knows, one to work on, maybe?