From the Shauna Somerville Lab, June 2006A Novel Form of Resistance Prevents Pathogen Entry into Plant Cells A majority of host plants are resistant to the majority of pathogens encountered in their environment whether these are wind or rain borne spores that land on leaves or soil microbes happened upon by plant roots. Grape vines are not susceptible to the powdery mildew spores from diseased mustard plants in a neighboring field for example. The underlying mechanisms of this form of disease resistance have traditionally been little studied. The Paul Schulze-Lefert lab, Hans Thordal- Christensen lab and our lab, working collaboratively, identified mutants of the model plant Arabidopsis thaliana that were partially compromised in their resistance to the powdery mildew fungus, Blumeria graminis f. sp. hordei, which is normally a pathogen of barley. To date, several genes that had not previously been implicated in plant defenses have been identified by this approach. PEN3 encodes an ABC transporter of the pleiotropic drug resistance class (PDR8) (Stein et al. 2006). The phenotype of the pen3 mutants is illustrated below (left panel). The pen3 mutants allow the barley powdery mildew colonies to grow to a limited extent, producing longer and more branched hyphae, than on they do on wild-type plants. This enhanced growth is dependent on the establishment of feeding structures in epidermal cells that can draw nutrients from the host cells to support hyphal growth. In healthy, uninfected cells, the PEN3 protein appears uniformly distributed in the plasma membrane. A notable feature of the PEN3 protein is that it accumulates in a ring-like halo in the plasma membrane and in collar-like structures surrounding the fungal penetration peg as it attempts to enter a plant epidermal cell (middle panel). This pathogen-induced accumulation of the PEN3 protein suggests that the plant is able to sense the location of fungal penetration very precisely and mobilize resources to this site. Although the specific compounds exported or imported by the PEN3 transporter are unknown at present, it seems likely that PEN3 exports anti-microbial compounds to the cell wall space to prevent pathogen entry into host cells (right panel).
Two other genes that limit fungal ingress are PEN1, which encodes a syntaxin (SYP121), and PEN2 encoding a glycosyl hydrolase (Collins et al. 2003, Lipka et al. 2005). PEN2 may play a role in the synthesis of anti-microbial compounds, while the PEN1 syntaxin may contribute to vesicle- mediated secretion of materials to reinforce the cell wall. A third plant gene influencing the ability of powdery mildew fungi to penetrate across the cell wall is MLO, which acts as a negative regulator of penetration defenses (Consonni et al. 2006). The MLO gene was originally identified in barley and mlo mutations have been widely incorporated into commercial cultivars, providing durable, broad-spectrum resistance to powdery mildew over the past 30 years. A number of plant genes have been identified that contribute basal disease resistance, the residual resistance to pathogens observed in generally susceptible hosts. One of these genes is EDS1, which encodes a lipase-like protein although it does not exhibit lipase enzymatic activity (Wiermer et al. 2005). eds1 mutants are somewhat compromised in their resistance to the barley powdery mildew (right panel) and double mutants with pen3 support significantly enhanced growth of the barley powdery mildew. Our collaborators have shown that Erysiphe pisi, the powdery mildew of pea, grows even better than the barley powdery mildew and is able to asexually reproduce on the pen2 eds1 double mutant (Lipka et al. 2005). These results suggest that some of the known defenses, like EDS1-mediated defenses, also contribute to Arabidopsis resistance to inappropriate pathogens, like the barley and pea powdery mildews. They also highlight the differing extent that various inappropriate pathogens are constrained by Arabidopsis penetration defenses. Collectively, these results suggest that plants mount effective, broad spectrum defenses designed to block pathogen penetration across the cell wall and entry into host cells. Over evolutionary time, virulent powdery mildew species, such as Golovinomyces cichoracearum, which causes disease on Arabidopsis, must presumably have acquired the ability to avoid these defenses by stealth or to be insensitive to them. References
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