Powdery mildew fungal pathogens penetrate the plant cell wall and develop a feeding structure called the haustorium to steal photosynthetate from the host cell. Here, we report that the broad-spectrum mildew resistance protein RPW8.2 from Arabidopsis thaliana is induced and specifically targeted to the extrahaustorial membrane (EHM), an enigmatic interfacial membrane believed to be derived from the host cell plasma membrane. There, RPW8.2 activates a salicylic acid (SA) signaling-dependent defense strategy that concomitantly enhances the encasement of the haustorial complex and onsite accumulation of H 2 O 2 , presumably for constraining the haustorium while reducing oxidative damage to the host cell. Targeting of RPW8.2 to the EHM, however, is SA independent and requires function of the actin cytoskeleton. Natural mutations that impair either defense activation or EHM targeting of RPW8.2 compromise the efficacy of RPW8.2-mediated resistance. Thus, the interception of haustoria is key for RPW8-mediated broad-spectrum mildew resistance.

Sphingolipids comprise a major class of structural materials and lipid signaling molecules in all eukaryotic cells. Over the past two decades, there has been a phenomenal growth in the study of sphingolipids (i.e., sphingobiology) at an average rate of ∼1000 research articles per year. Sphingolipid studies in plants, though accounting for only a small fraction (∼6%) of the total number of publications, have also enjoyed proportionally rapid growth in the past decade. Concomitant with the growth of sphingobiology, there has also been tremendous progress in our understanding of the molecular mechanisms of plant innate immunity. In this review, we (i) cross examine and analyze the major findings that establish and strengthen the intimate connections between sphingolipid metabolism and plant programmed cell death (PCD) associated with plant defense or disease; (ii) highlight and compare key bioactive sphingolipids involved in the regulation of plant PCD and possibly defense; (iii) discuss the potential role of sphingolipids in polarized membrane/protein trafficking and formation of lipid rafts as subdomains of cell membranes in relation to plant defense; and (iv) where possible, attempt to identify potential parallels for immunity-related mechanisms involving sphingolipids across kingdoms.

The RPW8 locus of Arabidopsis thaliana confers broad-spectrum resistance to powdery mildew pathogens. In many A. thaliana accessions, this locus contains two homologous genes, RPW8.1 and RPW8.2. In some susceptible accessions, however, these two genes are replaced by HR4, a homolog of RPW8.1. Here, we show that RPW8.2 from A. lyrata conferred powdery mildew resistance in A. thaliana, suggesting that RPW8.2 might have gained the resistance function before the speciation of A. thaliana and A. lyrata. To investigate how RPW8 has been maintained in A. thaliana, we examined the nucleotide sequence polymorphisms in RPW8 from 51 A. thaliana accessions, related disease reaction phenotypes to the evolutionary history of RPW8.1 and RPW8.2, and identified mutations that confer phenotypic variations. The average nucleotide diversities were high at RPW8.1 and RPW8.2, showing no sign of selective sweep. Moreover, we found that expression of RPW8 incurs fitness benefits and costs on A. thaliana in the presence and absence of the pathogens, respectively. Our results suggest that polymorphisms at the RPW8 locus in A. thaliana may have been maintained by complex selective forces, including those from the fitness benefits and costs both associated with RPW8.