Sphingolipids and Plant Defense/Disease: The “Death” Connection and Beyond

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

Construction of a genome-wide genetic linkage map and identification of quantitative trait loci for powdery mildew resistance in Gerbera daisy

Powdery mildew (PM) is a common fungal disease in many important crops. The PM caused by Podosphaera xanthii has been the most challenging problem in commercial Gerbera (Gerbera hybrida) production globally, often leading to severe losses of crop yield and quality. A small number of PM-resistant breeding lines and cultivars have been reported in Gerbera, but the underlying genetics for PM resistance in Gerbera is largely unknown. Scarcity of genomic resources such as genetic linkage maps and molecular markers has severely hindered the effort to understand the genetic basis and locate loci controlling PM resistance in Gerbera. This study aimed to construct a genome-wide genetic linkage map, identify quantitative trait loci (QTL), and molecular markers for PM resistance in Gerbera. A segregating mapping population was developed by crossing PM-resistant and -susceptible Gerbera breeding lines, genotyped by sequencing, and phenotyped for PM resistance. A genome-wide genetic linkage map constructed with 791 single polymorphic site (SNP) markers spans 1912.30 cM across 27 linkage groups (LG) and reaches a density of 1 marker per 2.42 cM. One major consistent QTL was discovered in LG16, explaining more than 16.6% of the phenotypic variance for PM resistance. The QTL was tagged with two flanking SNP markers. The availability of this genetic linkage map will be very useful for locating and tagging QTLs for other important traits in Gerbera, and the newly discovered QTL and SNP markers will enable development of molecular markers for improving Gerbera for resistance to PM

Silicon modulates multi-layered defense against powdery mildew in Arabidopsis

Silicon (Si) has been widely employed in agriculture to enhance resistance against pathogens in many crop plants. However, the underlying molecular mechanisms of Si-mediated resistance remain elusive. In this study, the Arabidopsis-powdery mildew pathosystem was employed to investigate possible defense mechanisms of Si-mediated resistance. Because Arabidopsis lacks efficient Si transporters and thus is a low Si-accumulator, two heterologous Si influx transporters (from barley and muskmelon) were individually expressed in wild-type Arabidopsis Col-0 and a panel of mutants defective in different immune signaling pathways. Results from infection tests showed that while very low leaf Si content slightly induced salicylic acid (SA)-dependent resistance, high Si promoted PAD4-dependent but largely EDS1- and SA-independent resistance against the adapted powdery mildew isolate Golovinomyces cichoracearum UCSC1. Intriguingly, our results also showed that high Si could largely reboot non-host resistance in an immune-compromised eds1/pad4/sid2 triple mutant background against a non-adapted powdery mildew isolate G. cichoracearum UMSG1. Taken together, our results suggest that assimilated Si modulates distinct, multi-layered defense mechanisms to enhance plant resistance against adapted and no-adapted powdery mildew pathogens, possibly via synergistic interaction with defense-induced callose.https://doi.org/10.1186/s42483-020-00048-

Enhanced resistance in Theobroma cacao against oomycete and fungal pathogens by secretion of phosphatidylinositol-3-phosphate-binding proteins

The internalization of some oomycete and fungal pathogen effectors into host plant cells has been reported to be blocked by proteins that bind to the effectors' cell entry receptor, phosphatidylinositol-3-phosphate (PI3P). This finding suggested a novel strategy for disease control by engineering plants to secrete PI3P-binding proteins. In this study, we tested this strategy using the chocolate tree Theobroma cacao. Transient expression and secretion of four different PI3P-binding proteins in detached leaves of T. cacao greatly reduced infection by two oomycete pathogens, Phytophthora tropicalis and Phytophthora palmivora, which cause black pod disease. Lesion size and pathogen growth were reduced by up to 85%. Resistance was not conferred by proteins lacking a secretory leader, by proteins with mutations in their PI3P-binding site, or by a secreted PI4P-binding protein. Stably transformed, transgenic T. cacao plants expressing two different PI3P-binding proteins showed substantially enhanced resistance to both P. tropicalis and P. palmivora, as well as to the fungal pathogen Colletotrichum theobromicola. These results demonstrate that secretion of PI3P-binding proteins is an effective way to increase disease resistance in T. cacao, and potentially in other plants, against a broad spectrum of pathogens.This is the publisher’s final pdf. The article is copyrighted by Society for Experimental Biology, Association of Applied Biologists and John Wiley & Sons Ltd. and published by Wiley Open Access. It can be found at: http://onlinelibrary.wiley.com/journal/10.1111/%28ISSN%291467-7652Keywords: phosphatidylinositol-3-phosphate-binding protein, oomycetes, disease resistance, fungi, Theobroma cacao, effectorsKeywords: phosphatidylinositol-3-phosphate-binding protein, oomycetes, disease resistance, fungi, Theobroma cacao, effector

Comparative genome analyses reveal sequence features reflecting distinct modes of host-adaptation between dicot and monocot powdery mildew

Powdery mildew (PM) is one of the most important and widespread plant diseases caused by biotrophic fungi. Notably, while monocot (grass) PM fungi exhibit high-level of host-specialization, many dicot PM fungi display a broad host range. To understand such distinct modes of host-adaptation, we sequenced the genomes of four dicot PM biotypes belonging to Golovinomyces cichoracearum or Oidium neolycopersici. We compared genomes of the four dicot PM together with those of Blumeria graminis f.sp. hordei (both DH14 and RACE1 isolates), B. graminis f.sp. tritici, and Erysiphe necator infectious on barley, wheat and grapevine, respectively. We found that despite having a similar gene number (6620–6961), the PM genomes vary from 120 to 222 Mb in size. This high-level of genome size variation is indicative of highly differential transposon activities in the PM genomes. While the total number of genes in any given PM genome is only about half of that in the genomes of closely related ascomycete fungi, most (~ 93%) of the ascomycete core genes (ACGs) can be found in the PM genomes. Yet, 186 ACGs were found absent in at least two of the eight PM genomes, of which 35 are missing in some dicot PM biotypes, but present in the three monocot PM genomes, indicating remarkable, independent and perhaps ongoing gene loss in different PM lineages. Consistent with this, we found that only 4192 (3819 singleton) genes are shared by all the eight PM genomes, the remaining genes are lineage- or biotype-specific. Strikingly, whereas the three monocot PM genomes possess up to 661 genes encoding candidate secreted effector proteins (CSEPs) with families containing up to 38 members, all the five dicot PM fungi have only 116–175 genes encoding CSEPs with limited gene amplification. Compared to monocot (grass) PM fungi, dicot PM fungi have a much smaller effectorome. This is consistent with their contrasting modes of host-adaption: while the monocot PM fungi show a high-level of host specialization, which may reflect an advanced host-pathogen arms race, the dicot PM fungi tend to practice polyphagy, which might have lessened selective pressure for escalating an with a particular host.Wu, Y., Ma, X., Pan, Z. et al. Comparative genome analyses reveal sequence features reflecting distinct modes of host-adaptation between dicot and monocot powdery mildew. BMC Genomics 19, 705 (2018)

Expression of the Membrane-Associated Resistance Protein RPW8 Enhances Basal Defense Against Biotrophic Pathogens

The powdery mildew resistance genes RPW8.1 and RPW8.2 from Arabidopsis differ from the other isolated plant resistance (R) genes in their predicted protein domains and their resistance spectrum. The two homologous RPW8 genes encode small proteins featuring a predicted amino-terminal transmembrane anchor domain and a coiled-coil domain and confer resistance to a broad spectrum of powdery mildews. Here, we show that Arabidopsis plants expressing the RPW8 genes have enhanced resistance to another biotrophic pathogen, Hyaloperonospora parasitica, raising the possibility that the RPW8 genes may function to enhance salicylic-acid-dependent basal defenses, rather than as powdery-mildew-specific R genes. When overexpressed from their native promoters, the RPW8 genes confer enhanced resistance to the Cauliflower mosaic virus, but render plants more susceptible to the necrotrophic fungal pathogens Alternaria and Botrytis spp. Furthermore, we show that the RPW8 proteins appear to be localized to the endomembrane system, overlapping with the endoplasmic reticulum–associated small GTPase SAR1, and accumulate to higher levels in response to application of exogenous salicylic acid, one of the signaling molecules of plant defense

Specific Targeting of the Arabidopsis Resistance Protein RPW8.2 to the Interfacial Membrane Encasing the Fungal Haustorium Renders Broad-Spectrum Resistance to Powdery Mildew[W][OA]

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 H2O2, 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

Enhanced transcription of the Arabidopsis disease resistance genes RPW8.1 and RPW8.2 via a salicylic acid-dependent amplification circuit is required for hypersensitive cell death.

The Arabidopsis disease resistance (R) genes RPW8.1 and RPW8.2 couple the recognition of powdery mildew pathogens of this plant with the subsequent induction of a localized necrosis, or hypersensitive response (HR). The HR restricts the spread of the infection and renders the plant resistant. One-third of Arabidopsis plants transformed with a genomic fragment containing RPW8.1 and RPW8.2 developed spontaneous HR-like lesions (SHL) in the absence of pathogens. We demonstrate that SHL occurs in transgenic lines that contain multiple copies of the transgene and express RPW8.1 and RPW8.2 at high levels. SHL is associated with salicylic acid (SA) accumulation, and at the site of the lesion, there is increased expression of RPW8.1, increased production of H2O2, and increased expression of pathogenesis-related genes. These lesions are physiologically similar to the pathogen-induced HR mediated by RPW8.1 and RPW8.2. Significantly, environmental conditions that suppress SHL suppress the transcription of RPW8.1 and RPW8.2 and also suppress resistance to powdery mildews, even in transgenic lines containing RPW8.1 and RPW8.2 that normally do not express SHL. Furthermore, treatment with SA increases the transcription of RPW8.1 and RPW8.2, induces SHL, and enhances resistance to powdery mildews. We conclude that HR requires the transcription of RPW8.1 and RPW8.2, which is regulated independently of the pathogen by SA-dependent feedback amplification