The plant surface is a complex molecular battlefield during plant-pathogen or plant-pest interaction. During infection, plant cells produce a group of proteins, coded by non-homologous genes, named Pathogenesis Related (PR) Proteins. Seventeen PR-proteins families have been identified based on biological activity, which can range from cell-wall/membrane degrading enzymes, to protease inhibitors, and proteins related to oxidative metabolism . Each PR-protein family has a specific role during plant-pathogen interaction. Some of them act as "attack" molecules to damage the pathogen, while others act as "defence" molecules, to protect plant cells from the molecular attack of pathogens. Villamil and Hoorn  review aspects of this "zig-zag" model of plant-pathogen interaction.
Xylanase inhibitor proteins (XIP) are potential "defence" molecules, which could act to prevent plant cell wall degradation by fungal hydrolytic enzymes. They have sequence similarity to glycoside hydrolases of family 18 (GH18) that are plant class III chitinases (PR-8). The GH18 family includes naturally inactive chitinases showing (β/α)8 topology that are predicted to show no catalytic activity due to mutations in the catalytic domain. Some of these proteins have been identified as inhibitors of xylanases (belonging to glycoside hydrolase families GH10 and GH11). In wheat, a chitinase-like xylanase inhibitor protein (XIP-I) had its structure elucidated and its mechanism of inhibition proposed [3, 4]. Structural features of these (β/α)8 chitinase-like xylanase inhibitors, as well its interaction with xylanases, has been reviewed recently .
Asian rust (Phakopsora pachyrhizi) is a new devastating disease, which has affected the cultivation of soybean (Glycine max (L.) Merril L) in Brazil. It was first detected in the country by 2001 and, due to favourable climatic conditions for fungal transmission, the productivity of the soybean crop, in yield/ha, declined by 17% from 2003 to 2005 [5, 6]. Since the appearance of Soybean rust in Brazil, chemical fungicides from the group of Triazoles, Strobilurins and Benzimidazoles have been used for the control of this disease. However, the use of these fungicides is related to neurological, immunological and reproductive disorders in mammals, as well as causing arrest of mitosis [7, 8]. Alternative, less environmentally-damaging methods for control of this pathogen that do not pose risks to human health are urgently required.
In this paper we report cloning, heterologous expression and enzymatic features of a new chitinase-like xylanase inhibitor protein (XIP) from coffee (Coffea arabica) (CaclXIP - Coffea arabica Chitinase-like Xylanase Inhibitor Protein), originally identified in the coffee genome  as a Class III Chitinase. CaclXIP showed only residual chitinolytic activity, but was an effective inhibitor of Acrophialophora nainiana xylanases, which are important enzymes to phytopathogenic fungi virulence. When assayed towards P. pachyrhizi (Asian rust), CaclXIP was able to arrest spore germination. As far as we know, this is the first time that a XIP-like molecule has been related to such biological activity. This work suggests that CaclXIP may be an eligible candidate for biotechnological approaches to control Asian rust. Such work is also trying to shed new light on the functional versatility of GH18 members and, consequently, the implication of such plurifunctionality for genome annotations and prediction of gene function.