Plants and Microbes

Much is known about the evolution of plant immunity components directed against specific pathogen strains: They show pervasive functional variation and have the potential to coevolve with pathogen populations. However, plants are effectively protected against most microbes by generalist immunity components that detect conserved pathogen-associated molecular patterns (PAMPs) and control the onset of PAMP-triggered immunity. In Arabidopsis thaliana, the receptor kinase flagellin sensing 2 (FLS2) confers recognition of bacterial flagellin (flg22) and activates a manifold defense response. To decipher the evolution of this system, we performed functional assays across a large set of A. thaliana genotypes and Brassicaceae relatives. We reveal extensive variation in flg22 perception, most of which results from changes in protein abundance. The observed variation correlates with both the severity of elicited defense responses and bacterial proliferation. We analyzed nucleotide variation segregating at FLS2 in A. thaliana and detected a pattern of variation suggestive of the rapid fixation of a novel adaptive allele. However, our study also shows that evolution at the receptor locus alone does not explain the evolution of flagellin perception; instead, components common to pathways downstream of PAMP perception likely contribute to the observed quantitative variation. Within and among close relatives, PAMP perception evolves quantitatively, which contrasts with the changes in recognition typically associated with the evolution of R genes.

The term witches' broom comes from the German word Hexenbesen, which means to bewitch (hex) a bundle of twigs (besom).

   Witches' brooms occur on many different woody plant species, including deciduous trees such as hackberry, maple, and willow, and conifers such as pine and spruce. There may be only one broom in a tree, or they may be many scattered throughout the tree. In some cases, the brooms are quite large in size and are easily spotted. In others, they are small and well-hidden.

   A number of stresses, both biological and environmental, can lead to the formation of brooms. Organisms such as fungi, phytoplasmas (bacterial-like organisms), mites, aphids, and mistletoe plants can cause abnormal growth when they attack a host tree. Environmental stresses that injure the growing points of branches can also trigger the formation of brooms. Some brooms appear to be caused by genetic mutations in the buds of the branches. Unlike brooms caused by living organisms, there is usually just one broom per tree when the cause is a genetic mutation.

On April 25, the Romans celebrated Robigalia, an ancient agricultural festival held in honor of Robigo (or Robigus, the gender was uncertain), the goddess of blight, red rust, or mildew. The main ritual was the sacrifice of a red dog (color of rust) to protect grain crops from disease. Today, Robigus (wheat rust) is back with a vengance and still threatens wheat crops worldwide. Perhaps April 25, Robigalia day, should be wheat rust awareness day?

See also http://en.wikipedia.org/wiki/Robigalia

Here are my 2 maps for this discussion (1). I used linear regression to predict corn yield for each county in the US, using time (year) as the independent variable. I used the years 1950 to 1969 to create the model, and to predict corn yield in 1970. This should be a reasonable estimate of the ‘expected yield’ for 1970 for each county, if it had been a ‘normal year’.

(1) Blight is right: genetic uniformity was to blame. http://agro.biodiver.se/2012/04/blight-is-right-genetic-uniformity-was-to-blame/

Arbuscular mycorrhizal fungi (AMF) represent a monophyletic fungal lineage (Glomeromycota) that benefits terrestrial ecosystems worldwide by establishing an intimate association with the roots of most land plants: the mycorrhizal symbiosis. This relationship results in an improved acquisition of nutrients (e.g., phosphate and nitrates) from the soil by the plant partners and, in exchange, allows the AMF to obtain the photosynthetically fixed carbon sources (e.g., sugars) necessary for their survival and propagation [1], [2] (Figure 1). This fungal lineage is known to impact the function and biodiversity of entire ecosystems by producing extensive underground networks, composed of hyphae and spores, that interconnect a number of unrelated individual plant species [1], [2]. These networks also function as a significant sink for atmospheric carbon dioxide, and represent significant underground “nutrient highways” that benefit entire plant and microbial communities. Indeed, AMF spores and hyphae are also a valuable source of food for many soil microorganisms (i.e., bacteria, other fungi, and nematodes), and because of their many beneficial effects on terrestrial ecosystems, AMF are widely used in organic agriculture and plant nurseries to improve the growth of economically important species.

Systemic acquired resistance (SAR) develops in response to local microbial leaf inoculation and renders the whole plant more resistant to subsequent pathogen infection. Accumulation of salicylic acid (SA) in noninfected plant parts is required for SAR, and methyl salicylate (MeSA) and jasmonate (JA) are proposed to have critical roles during SAR long-distance signaling from inoculated to distant leaves. Here, we address the significance of MeSA and JA during SAR development in Arabidopsis thaliana. MeSA production increases in leaves inoculated with the SAR-inducing bacterial pathogen Pseudomonas syringae; however, most MeSA is emitted into the atmosphere, and only small amounts are retained. We show that in several Arabidopsis defense mutants, the abilities to produce MeSA and to establish SAR do not coincide. T-DNA insertion lines defective in expression of a pathogen-responsive SA methyltransferase gene are completely devoid of induced MeSA production but increase systemic SA levels and develop SAR upon local P. syringae inoculation. Therefore, MeSA is dispensable for SAR in Arabidopsis, and SA accumulation in distant leaves appears to occur by de novo synthesis via isochorismate synthase. We show that MeSA production induced by P. syringae depends on the JA pathway but that JA biosynthesis or downstream signaling is not required for SAR. In compatible interactions, MeSA production depends on the P. syringae virulence factor coronatine, suggesting that the phytopathogen uses coronatine-mediated volatilization of MeSA from leaves to attenuate the SA-based defense pathway.

Phytopathogens secrete effector proteins to manipulate their hosts for effective colonization. Hemibiotrophic fungi must maintain host viability during initial biotrophic growth and elicit host death for subsequent necrotrophic growth. To identify effectors mediating these opposing processes, we deeply sequenced the transcriptome of Colletotrichum higginsianum infecting Arabidopsis. Most effector genes are host-induced and expressed in consecutive waves associated with pathogenic transitions, indicating distinct effector suites are deployed at each stage. Using fluorescent protein tagging and transmission electron microscopy-immunogold labelling, we found effectors localised to stage-specific compartments at the host-pathogen interface. In particular, we show effectors are focally secreted from appressorial penetration pores before host invasion, revealing new levels of functional complexity for this fungal organ. Furthermore, we demonstrate that antagonistic effectors either induce or suppress plant cell death. Based on these results we conclude that hemibiotrophy in Colletotrichum is orchestrated through the coordinated expression of antagonistic effectors supporting either cell viability or cell death.

Pseudoperonospora cubensis, an obligate oomycete pathogen, is the causal agent of cucurbit downy mildew, a foliar disease of global economic importance. Similar to other oomycete plant pathogens, Ps. cubensis has a suite of RXLR and RXLR-like effector proteins, which likely function as virulence or avirulence determinants during the course of host infection. Using in silico analyses, we identified 271 candidate effector proteins within the Ps. cubensis genome with variable RXLR motifs. In extending this analysis, we present the functional characterization of one Ps. cubensis effector protein, RXLR protein 1 (PscRXLR1), and its closest Phytophthora infestans ortholog, PITG_17484, a member of the Drug/Metabolite Transporter (DMT) superfamily. To assess if such effector-non-effector pairs are common among oomycete plant pathogens, we examined the relationship(s) among putative ortholog pairs in Ps. cubensis and P. infestans. Of 271 predicted Ps. cubensis effector proteins, only 109 (41%) had a putative ortholog in P. infestans and evolutionary rate analysis of these orthologs shows that they are evolving significantly faster than most other genes. We found that PscRXLR1 was up-regulated during the early stages of infection of plants, and, moreover, that heterologous expression of PscRXLR1 in Nicotiana benthamiana elicits a rapid necrosis. More interestingly, we also demonstrate that PscRXLR1 arises as a product of alternative splicing, making this the first example of an alternative splicing event in plant pathogenic oomycetes transforming a non-effector gene to a functional effector protein. Taken together, these data suggest a role for PscRXLR1 in pathogenicity, and, in total, our data provide a basis for comparative analysis of candidate effector proteins and their non-effector orthologs as a means of understanding function and evolutionary history of pathogen effectors.

The arbuscular mycorrhizal (AM) symbiosis, formed by land plants and AM fungi, evolved an estimated 400 million years ago and has been maintained in angiosperms, gymnosperms, pteridophytes, and some bryophytes as a strategy for enhancing phosphate acquisition. During AM symbiosis, the AM fungus colonizes the root cortical cells where it forms branched hyphae called arbuscules that function in nutrient exchange with the plant. Each arbuscule is enveloped in a plant membrane, the periarbuscular membrane, that contains a unique set of proteins including phosphate transporters such as Medicago truncatula MtPT4 [Javot et al., (2007) Proc Natl Acad Sci USA 104:1720–1725], which are essential for symbiotic phosphate transport. The periarbuscular membrane is physically continuous with the plasma membrane of the cortical cell, but MtPT4 and other periarbuscular membrane-resident proteins are located only in the domain around the arbuscule branches. Establishing the distinct protein composition of the periarbuscular membrane is critical for AM symbiosis, but currently the mechanism by which this composition is achieved is unknown. Here we investigate the targeting of MtPT4 to the periarbuscular membrane. By expressing MtPT4 and other plasma membrane proteins from promoters active at different phases of the symbiosis, we show that polar targeting of MtPT4 is mediated by precise temporal expression coupled with a transient reorientation of secretion and alterations in the protein cargo entering the secretory system of the colonized root cell. In addition, analysis of phosphate transporter mutants implicates the trans-Golgi network in phosphate transporter secretion.

Five major crops that feed the world’s population are challenged by the fungal/oomycete diseases shown. All global harvests are from 2009/2010 (www.fao.org or FAOSTAT). *Figures are estimates and are based on 100% of milled rice and 100% of potato harvest being used for human consumption. However, 34% of the global wheat harvest is produced in developing countries, of which 100% is assumed to be used for human food, but only 45% of the total wheat harvest produced from developed countries is used for human food (the rest is used as animal feed and biofuel). Of the global maize harvest, 32% is produced in developing countries and 100% is assumed to be used for food and in the US 2.5% of their total harvest of 333m tonnes of maize (41% of global harvest) is used directly as food. Of the global soybean harvest, 36% is from US and Canada (10% of this is used as food, the rest goes into processed food and animal feed) and 64% is produced in developing nations, where 100% is assumed to be used as food. These figures do not take into account post‐harvest storage losses.

**This is 8.5% ‐ 61.2% of the world’s population, based on the 2011 population estimate of 7 billion people.

Supplementary Material of Fisher et al. Emerging fungal threats to animal, plant and ecosystem health. Nature (2012) http://www.nature.com/nature/journal/v484/n7393/extref/nature10947-s1.pdf

Although the title is reminiscent of an Indiana Jones film, this is an interesting and well-produced biography of Harry Marshall Ward, one of the first people to work on physiological plant pathology. A review in the Annals of Botany is highly appropriate because he was a joint author of the first paper ever published in the journal (Ward HM, Dunlop J. 1887. On some points on the histology and physiology of the fruits and seeds of Rhamnus. Annals of Botany 1: 1–26) and the first book reviewed was his translation of Sachs' Lectures on the Physiology of Plants. Both these are typical of Ward's work in that they are concerned with how plants function.

As a common protein modification, ubiquitination is used for regulating the fate of protein targets, notably in terms of stability. In recent years, it has emerged to play key roles in the regulation of plant defense responses. Given its flexibility and critical roles in signaling, primarily in the control of protein turnover, ubiquitination is probably targeting many major immune regulators for modification or degradation. In this review, we summarize the latest findings on how different components of the ubiquitination pathway are involved in NB-LRR R protein-mediated immunity.

Plant innate immunity is activated on the detection of pathogen-associated molecular patterns (PAMPs) at the cell surface, or of pathogen effector proteins inside the plant cell1, 2, 3, 4. Together, PAMP-triggered immunity and effector-triggered immunity constitute powerful defences against various phytopathogens. Pathogenic bacteria inject a variety of effector proteins into the host cell to assist infection or propagation. A number of effector proteins have been shown to inhibit plant immunity5, but the biochemical basis remains unknown for the vast majority of these effectors. Here we show that the Xanthomonas campestris pathovar campestris type III effector AvrAC enhances virulence and inhibits plant immunity by specifically targeting Arabidopsis BIK1 and RIPK, two receptor-like cytoplasmic kinases known to mediate immune signalling6, 7, 8. AvrAC is a uridylyl transferase that adds uridine 5′-monophosphate to and conceals conserved phosphorylation sites in the activation loop of BIK1 and RIPK, reducing their kinase activity and consequently inhibiting downstream signalling.

Systemic acquired resistance (SAR) is a state of heightened defense to a broad spectrum of pathogens that is activated throughout a plant following local infection. Development of SAR requires the translocation of one or more mobile signals from the site of infection through the vascular system to distal (systemic) tissues. The first such signal identified was methyl salicylate (MeSA) in tobacco (Nicotiana tabacum). Subsequent studies demonstrated that MeSA also serves as a SAR signal in Arabidopsis (Arabidopsis thaliana) and potato (Solanum tuberosum). By contrast, another study suggested that MeSA is not required for SAR in Arabidopsis and raised questions regarding its signaling role in tobacco. Differences in experimental design, including the developmental age of the plants, the light intensity, and/or the strain of bacterial pathogen, were proposed to explain these conflicting results. Here, we demonstrate that the length of light exposure that plants receive after the primary infection determines the extent to which MeSA is required for SAR signaling. When the primary infection occurred late in the day and as a result infected plants received very little light exposure before entering the night/dark period, MeSA and its metabolizing enzymes were essential for SAR development. In contrast, when infection was done in the morning followed by 3.5 h or more of exposure to light, SAR developed in the absence of MeSA. However, MeSA was generally required for optimal SAR development. In addition to resolving the conflicting results concerning MeSA and SAR, this study underscores the importance of environmental factors on the plant’s response to infection.

Gymnosporangium juniperi-virginianae is a plant pathogen that causes cedar-apple rust. In virtually any location where apples or crabapples (Malus) and Eastern red cedar (Juniperus virginiana) coexist, cedar apple rust can be a destructive or disfiguring disease on both the apples and cedars. Quince and hawthorn can substitute for the apples as hosts and many species of juniper can substitute for the Eastern red cedars.

See also http://www.kuriositas.com/2011/06/attack-of-many-tentacled-carrot.html

The wood planted by Sir Paul McCartney in tribute to his wife, Linda, has been ravaged by sudden oak death disease.

When hundreds of trees were planted in remembrance of Linda McCartney, it was hoped that they would grow to be a living and lasting memorial to the first wife of Sir Paul McCartney.
   Now, however, all the larch trees in Linda’s Wood have had to be chopped down because of a deadly disease sweeping the country.
  The trees were the victim of Phytophthora ramorum, or sudden oak death, which has jumped species and is infecting other trees including larch and the Douglas fir.
   Linda’s Wood borders the 100-acre nature sanctuary St John’s Wood, which the couple bought in Somerset in 1990.
  The trees were planted after the wife of the former Beatle died of breast cancer in 1998, aged 56. They included some of the New York-born photographer’s favourite species, such as the North American maple.