MLO Differentially Regulates Barley Root Colonization by Beneficial Endophytic and Mycorrhizal Fungi

The development of pleiotropic phenotypes in powdery mildew-resistant barley and Arabidopsis thaliana mlo mutants is linked to nitrogen availability

Powdery mildew-resistant barley (Hordeum vulgare) and Arabidopsis thaliana mlo mutant plants exhibit pleiotropic phenotypes such as the spontaneous formation of callose-rich cell wall appositions and early leaf chlorosis and necrosis, indicative of premature leaf senescence. The exogenous factors governing the occurrence of these undesired side effects remain poorly understood. Here, we characterised the formation of these symptoms in detail. Ultrastructural analysis revealed that the callose-rich cell wall depositions spontaneously formed in A. thaliana mlo mutants are indistinguishable from those induced by the bacterial pattern epitope, flagellin 22 (flg22). We further found that increased plant densities during culturing enhance the extent of the leaf senescence syndrome in A. thaliana mlo mutants. Application of a liquid fertiliser rescued the occurrence of leaf chlorosis and necrosis in both A. thaliana and barley mlo mutant plants. Controlled fertilisation experiments uncovered nitrogen as the macronutrient whose deficiency promotes the extent of pleiotropic phenotypes in A. thaliana mlo mutants. Light intensity and temperature had a modulatory impact on the incidence of leaf necrosis in the case of barley mlo mutant plants. Collectively, our data indicate that the development of pleiotropic phenotypes associated with mlo mutants is governed by various exogenous factors.

Temperature-dependent action of pepper mildew resistance locus O 1 in inducing pathogen immunity and thermotolerance

Plant diseases tend to be more serious under conditions of high-temperature/high-humidity (HTHH) than under moderate conditions, and hence disease resistance under HTHH is an important determinant for plant survival. However, how plants cope with diseases under HTHH remains poorly understood. In this study, we used the pathosystem consisting of pepper (Capsicum annuum) and Ralstonia solanacearum (bacterial wilt) as a model to examine the functions of the protein mildew resistance locus O 1 (CaMLO1) and U-box domain-containing protein 21 (CaPUB21) under conditions of 80% humidity and either 28 °C or 37 °C. Expression profiling, loss- and gain-of-function assays involving virus-induced gene-silencing and overexpression in pepper plants, and protein-protein interaction assays were conducted, and the results showed that CaMLO1 acted negatively in pepper immunity against R. solanacearum at 28 °C but positively at 37 °C. In contrast, CaPUB21 acted positively in immunity at 28 °C but negatively at 37 °C. Importantly, CaPUB21 interacted with CaMLO1 under all of the tested conditions, but only the interaction in response to R. solanacearum at 37 °C or to exposure to 37 °C alone led to CaMLO1 degradation, thereby turning off defence responses against R. solanacearum at 37 °C and under high-temperature stress to conserve resources. Thus, we show that CaMLO1 and CaPUB21 interact with each other and function distinctly in pepper immunity against R. solanacearum in an environment-dependent manner.

A GH81-type β-glucan-binding protein enhances colonization by mutualistic fungi in barley

Cell walls are important interfaces of plant-fungal interactions, acting as robust physical and chemical barriers against invaders. Upon fungal colonization, plants deposit phenolics and callose at the sites of fungal penetration to prevent further fungal progression. Alterations in the composition of plant cell walls significantly impact host susceptibility. Furthermore, plants and fungi secrete glycan hydrolases acting on each other's cell walls. These enzymes release various sugar oligomers into the apoplast, some of which activate host immunity via surface receptors. Recent characterization of cell walls from plant-colonizing fungi has emphasized the abundance of β-glucans in different cell wall layers, which makes them suitable targets for recognition. To characterize host components involved in immunity against fungi, we performed a protein pull-down with the biotinylated β-glucan laminarin. Thereby, we identified a plant glycoside hydrolase family 81-type glucan-binding protein (GBP) as a β-glucan interactor. Mutation of GBP1 and its only paralog, GBP2, in barley led to decreased colonization by the beneficial root endophytes Serendipita indica and S. vermifera, as well as the arbuscular mycorrhizal fungus Rhizophagus irregularis. The reduction of colonization was accompanied by enhanced responses at the host cell wall, including an extension of callose-containing cell wall appositions. Moreover, GBP mutation in barley also reduced fungal biomass in roots by the hemibiotrophic pathogen Bipolaris sorokiniana and inhibited the penetration success of the obligate biotrophic leaf pathogen Blumeria hordei. These results indicate that GBP1 is involved in the establishment of symbiotic associations with beneficial fungi-a role that has potentially been appropriated by barley-adapted pathogens.

Co-Application of Silver Nanoparticles and Symbiotic Fungus Piriformospora indica Improves Secondary Metabolite Production in Black Rice

In the current research, unique Nano-Embedded Fungus (NEF), made by the synergic association of silver nanoparticles (AgNPs) and endophytic fungus (Piriformospora indica), is studied, and the impact of NEF on black rice secondary metabolites is reported. AgNPs were synthesized by chemical reduction process using the temperature-dependent method and characterized for morphological and structural features through UV visible absorption spectroscopy, zeta potential, XRD, SEM-EDX, and FTIR spectroscopy. The NEF, prepared by optimizing the AgNPs concentration (300 ppm) in agar and broth media, showed better fungal biomass, colony diameter, spore count, and spore size than the control P. indica. Treatment with AgNPs, P. indica, and NEF resulted in growth enhancement in black rice. NEF and AgNPs stimulated the production of secondary metabolites in its leaves. The concentrations of chlorophyll, carotenoids, flavonoids, and terpenoids were increased in plants inoculated with P. indica and AgNPs. The findings of the study highlight the synergistic effect of AgNPs and the fungal symbionts in augmenting the secondary metabolites in leaves of black rice.

Barley Ror1 encodes a class XI myosin required for mlo-based broad-spectrum resistance to the fungal powdery mildew pathogen

Loss-of-function alleles of plant MLO genes confer broad-spectrum resistance to powdery mildews in many eudicot and monocot species. Although barley (Hordeum vulgare) mlo mutants have been used in agriculture for more than 40 years, understanding of the molecular principles underlying this type of disease resistance remains fragmentary. Forward genetic screens in barley have revealed mutations in two Required for mlo resistance (Ror) genes that partially impair immunity conferred by mlo mutants. While Ror2 encodes a soluble N-ethylmaleimide-sensitive factor-attached protein receptor (SNARE), the identity of Ror1, located at the pericentromeric region of barley chromosome 1H, remained elusive. We report the identification of Ror1 based on combined barley genomic sequence information and transcriptomic data from ror1 mutant plants. Ror1 encodes the barley class XI myosin Myo11A (HORVU.MOREX.r3.1HG0046420). Single amino acid substitutions of this myosin, deduced from non-functional ror1 mutant alleles, map to the nucleotide-binding region and the interface between the relay-helix and the converter domain of the motor protein. Ror1 myosin accumulates transiently in the course of powdery mildew infection. Functional fluorophore-labeled Ror1 variants associate with mobile intracellular compartments that partially colocalize with peroxisomes. Single-cell expression of the Ror1 tail region causes a dominant-negative effect that phenocopies ror1 loss-of-function mutants. We define a myosin motor for the establishment of mlo-mediated resistance, suggesting that motor protein-driven intracellular transport processes are critical for extracellular immunity, possibly through the targeted transfer of antifungal and/or cell wall cargoes to pathogen contact sites.

Unmasking Mildew Resistance Locus O

Loss of Mildew Resistance Locus O (MLO) in barley confers durable resistance to powdery mildew fungi, which has led to its wide deployment in agriculture. Although MLO is a susceptibility factor, it has become nearly synonymous with powdery mildew resistance. However, MLO has been recently implicated in colonization by arbuscular mycorrhizal fungi and a fungal endophyte, confirming its importance for biotrophic interactions and in promoting symbiosis. Other MLO proteins are involved in essential sensory processes, particularly fertilization and thigmotropism. We propose external stimulus perception as a common theme in these interactions and consider a unified biochemical role, potentially relating to reactive oxygen species (ROS) and calcium regulation, for MLOs across tissues and processes.

Genome-Wide Characterization of the MLO Gene Family in Cannabis sativa Reveals Two Genes as Strong Candidates for Powdery Mildew Susceptibility

Cannabis sativa is increasingly being grown around the world for medicinal, industrial, and recreational purposes. As in all cultivated plants, cannabis is exposed to a wide range of pathogens, including powdery mildew (PM). This fungal disease stresses cannabis plants and reduces flower bud quality, resulting in significant economic losses for licensed producers. The Mildew Locus O (MLO) gene family encodes plant-specific proteins distributed among conserved clades, of which clades IV and V are known to be involved in susceptibility to PM in monocots and dicots, respectively. In several studies, the inactivation of those genes resulted in durable resistance to the disease. In this study, we identified and characterized the MLO gene family members in five different cannabis genomes. Fifteen Cannabis sativa MLO (CsMLO) genes were manually curated in cannabis, with numbers varying between 14, 17, 19, 18, and 18 for CBDRx, Jamaican Lion female, Jamaican Lion male, Purple Kush, and Finola, respectively (when considering paralogs and incomplete genes). Further analysis of the CsMLO genes and their deduced protein sequences revealed that many characteristics of the gene family, such as the presence of seven transmembrane domains, the MLO functional domain, and particular amino acid positions, were present and well conserved. Phylogenetic analysis of the MLO protein sequences from all five cannabis genomes and other plant species indicated seven distinct clades (I through VII), as reported in other crops. Expression analysis revealed that the CsMLOs from clade V, CsMLO1 and CsMLO4, were significantly upregulated following Golovinomyces ambrosiae infection, providing preliminary evidence that they could be involved in PM susceptibility. Finally, the examination of variation within CsMLO1 and CsMLO4 in 32 cannabis cultivars revealed several amino acid changes, which could affect their function. Altogether, cannabis MLO genes were identified and characterized, among which candidates potentially involved in PM susceptibility were noted. The results of this study will lay the foundation for further investigations, such as the functional characterization of clade V MLOs as well as the potential impact of the amino acid changes reported. Those will be useful for breeding purposes in order to develop resistant cultivars.

Stop helping pathogens: engineering plant susceptibility genes for durable resistance

Alternatives to protect crops against diseases are desperately needed to secure world food production and make agriculture more sustainable. Genetic resistance to pathogens utilized so far is mostly based on single dominant resistance genes that mediate specific recognition of invaders and that is often rapidly broken by pathogen variants. Perturbation of plant susceptibility (S) genes offers an alternative providing plants with recessive resistance that is proposed to be more durable. S genes enable the establishment of plant disease, and their inactivation provides opportunities for resistance breeding of crops. However, loss of S gene function can have pleiotropic effects. Developments in genome editing technology promise to provide powerful methods to precisely interfere with crop S gene functions and reduce tradeoffs.

Plant evolution driven by interactions with symbiotic and pathogenic microbes

During 450 million years of diversification on land, plants and microbes have evolved together. This is reflected in today's continuum of associations, ranging from parasitism to mutualism. Through phylogenetics, cell biology, and reverse genetics extending beyond flowering plants into bryophytes, scientists have started to unravel the genetic basis and evolutionary trajectories of plant-microbe associations. Protection against pathogens and support of beneficial, symbiotic, microorganisms are sustained by a blend of conserved and clade-specific plant mechanisms evolving at different speeds. We propose that symbiosis consistently emerges from the co-option of protection mechanisms and general cell biology principles. Exploring and harnessing the diversity of molecular mechanisms used in nonflowering plant-microbe interactions may extend the possibilities for engineering symbiosis-competent and pathogen-resilient crops.

A secreted fungal histidine- and alanine-rich protein regulates metal ion homeostasis and oxidative stress

Like pathogens, beneficial endophytic fungi secrete effector proteins to promote plant colonization, for example, through perturbation of host immunity. The genome of the root endophyte Serendipita indica encodes a novel family of highly similar, small alanine- and histidine-rich proteins, whose functions remain unknown. Members of this protein family carry an N-terminal signal peptide and a conserved C-terminal DELD motif. Here we report on the functional characterization of the plant-responsive DELD family protein Dld1 using a combination of structural, biochemical, biophysical and cytological analyses. The crystal structure of Dld1 shows an unusual, monomeric histidine zipper consisting of two antiparallel coiled-coil helices. Similar to other histidine-rich proteins, Dld1 displays varying affinity to different transition metal ions and undergoes metal ion- and pH-dependent unfolding. Transient expression of mCherry-tagged Dld1 in barley leaf and root tissue suggests that Dld1 localizes to the plant cell wall and accumulates at cell wall appositions during fungal penetration. Moreover, recombinant Dld1 enhances barley root colonization by S. indica, and inhibits H₂ O₂ -mediated radical polymerization of 3,3'-diaminobenzidine. Our data suggest that Dld1 has the potential to enhance micronutrient accessibility for the fungus and to interfere with oxidative stress and reactive oxygen species homeostasis to facilitate host colonization.

Mildew Locus O facilitates colonization by arbuscular mycorrhizal fungi in angiosperms

Loss of barley Mildew Resistance Locus O (MLO) is known to confer durable and robust resistance to powdery mildew (Blumeria graminis), a biotrophic fungal leaf pathogen. Based on the increased expression of MLO in mycorrhizal roots and its presence in a clade of the MLO family that is specific to mycorrhizal-host species, we investigated the potential role of MLO in arbuscular mycorrhizal interactions. Using mutants from barley (Hordeum vulgare), wheat (Triticum aestivum), and Medicago truncatula, we demonstrate a role for MLO in colonization by the arbuscular mycorrhizal fungus Rhizophagus irregularis. Early mycorrhizal colonization was reduced in mlo mutants of barley, wheat, and M. truncatula, and this was accompanied by a pronounced decrease in the expression of many of the key genes required for intracellular accommodation of arbuscular mycorrhizal fungi. These findings show that clade IV MLOs are involved in the establishment of symbiotic associations with beneficial fungi, a role that has been appropriated by powdery mildew.

Evidence for Allele-Specific Levels of Enhanced Susceptibility of Wheat mlo Mutants to the Hemibiotrophic Fungal Pathogen Magnaporthe oryzae pv. Triticum

Barley mlo mutants are well known for their profound resistance against powdery mildew disease. Recently, mlo mutant plants were generated in hexaploid bread wheat (Triticum aestivum) with the help of transgenic (transcription-activator-like nuclease, TALEN) and non-transgenic (targeted induced local lesions in genomes, TILLING) biotechnological approaches. While full-gene knockouts in the three wheat Mlo (TaMlo) homoeologs, created via TALEN, confer full resistance to the wheat powdery mildew pathogen (Blumeria graminis f.sp. tritici), the currently available TILLING-derived Tamlo missense mutants provide only partial protection against powdery mildew attack. Here, we studied the infection phenotypes of TALEN- and TILLING-derived Tamlo plants to the two hemibiotrophic pathogens Zymoseptoria tritici, causing Septoria leaf blotch in wheat, and Magnaporthe oryzae pv. Triticum (MoT), the causal agent of wheat blast disease. While Tamlo plants showed unaltered outcomes upon challenge with Z. tritici, we found evidence for allele-specific levels of enhanced susceptibility to MoT, with stronger powdery mildew resistance correlated with more invasive growth by the blast pathogen. Surprisingly, unlike barley mlo mutants, young wheat mlo mutant plants do not show undesired pleiotropic phenotypes such as spontaneous callose deposits in leaf mesophyll cells or signs of early leaf senescence. In conclusion, our study provides evidence for allele-specific levels of enhanced susceptibility of Tamlo plants to the hemibiotrophic wheat pathogen MoT.