Adaptive evolution of Xa21 homologs in Gramineae

Comparative genomics analysis of WAK/WAKL family in Rosaceae identify candidate WAKs involved in the resistance to Botrytis cinerea

BACKGROUND

Wall associated kinase (WAK) and WAK-like (WAKL) are typical pattern recognition receptors act as the first sentry of plant defense. But little of WAK/WAKL family is known in Rosaceae.

RESULTS

In this study, 131 WAK/WAKL genes from apple, peach and strawberry were identified using a bioinformatics approach. Together with 68 RcWAK/RcWAKL in rose, we performed a comparative analysis of 199 WAK/WAKL in four Rosaceae crops. The phylogenetic analysis divided all the WAK/WAKL into five clades. Among them, the cis-elements of Clade II and Clade V promoters were enriched in jasmonic acid (JA) signaling and abiotic stress, respectively. And this can also be verified by the rose transcriptome responding to different hormone treatments. WAK/WAKL families have experienced a considerable proportion of purifying selection during evolution, but still 26 amino acid sites evolved under positive selection, which focused on extracellular conserved domains. WAK/WAKL genes presented collinearity relationship within and between crops, throughout four crops we mined four orthologous groups (OGs). The WAK/WAKL genes in OG1 and OG4 were speculated to involve in plant-Botrytis cinerea interaction, which were validated in rose via VIGS as well as strawberry by qRT-PCR.

CONCLUSIONS

These results not only provide genetic resources and valuable information for the evolutionary relationship of WAK/WAKL gene family, but also offer a reference for future in-depth studies of Rosaceae WAK/WAKL genes.

Adaptive evolution driving the young duplications in six Rosaceae species

Background

In plant genomes, high proportions of duplicate copies reveals that gene duplications play an important role in the evolutionary processes of plant species. A series of gene families under positive selection after recent duplication events in plant genomes indicated the evolution of duplicates driven by adaptive evolution. However, the genome-wide evolutionary features of young duplicate genes among closely related species are rarely reported.

Results

In this study, we conducted a systematic survey of young duplicate genes at genome-wide levels among six Rosaceae species, whose whole-genome sequencing data were successively released in recent years. A total of 35,936 gene families were detected among the six species, in which 60.25% were generated by young duplications. The 21,650 young duplicate gene families could be divided into two expansion types based on their duplication patterns, species-specific and lineage-specific expansions. Our results showed the species-specific expansions advantaging over the lineage-specific expansions. In the two types of expansions, high-frequency duplicate domains exhibited functional preference in response to environmental stresses.

Conclusions

The functional preference of the young duplicate genes in both the expansion types showed that they were inclined to respond to abiotic or biotic stimuli. Moreover, young duplicate genes under positive selection in both species-specific and lineage-specific expansions suggested that they were generated to adapt to the environmental factors in Rosaceae species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07422-7.

Origin and diversification of leucine-rich repeat receptor-like protein kinase (LRR-RLK) genes in plants

Background

Leucine-rich repeat receptor-like protein kinases (LRR-RLKs) are the largest group of receptor-like kinases in plants and play crucial roles in development and stress responses. The evolutionary relationships among LRR-RLK genes have been investigated in flowering plants; however, no comprehensive studies have been performed for these genes in more ancestral groups. The subfamily classification of LRR-RLK genes in plants, the evolutionary history and driving force for the evolution of each LRR-RLK subfamily remain to be understood.

Results

We identified 119 LRR-RLK genes in the Physcomitrella patens moss genome, 67 LRR-RLK genes in the Selaginella moellendorffii lycophyte genome, and no LRR-RLK genes in five green algae genomes. Furthermore, these LRR-RLK sequences, along with previously reported LRR-RLK sequences from Arabidopsis thaliana and Oryza sativa, were subjected to evolutionary analyses. Phylogenetic analyses revealed that plant LRR-RLKs belong to 19 subfamilies, eighteen of which were established in early land plants, and one of which evolved in flowering plants. More importantly, we found that the basic structures of LRR-RLK genes for most subfamilies are established in early land plants and conserved within subfamilies and across different plant lineages, but divergent among subfamilies. In addition, most members of the same subfamily had common protein motif compositions, whereas members of different subfamilies showed variations in protein motif compositions. The unique gene structure and protein motif compositions of each subfamily differentiate the subfamily classifications and, more importantly, provide evidence for functional divergence among LRR-RLK subfamilies. Maximum likelihood analyses showed that some sites within four subfamilies were under positive selection.

Conclusions

Much of the diversity of plant LRR-RLK genes was established in early land plants. Positive selection contributed to the evolution of a few LRR-RLK subfamilies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0891-5) contains supplementary material, which is available to authorized users.

Transcriptome Profiling of Buffalograss Challenged with the Leaf Spot Pathogen Curvularia inaequalis

Buffalograss (Bouteloua dactyloides) is a low maintenance U. S. native turfgrass species with exceptional drought, heat, and cold tolerance. Leaf spot caused by Curvularia inaequalis negatively impacts buffalograss visual quality. Two leaf spot susceptible and two resistant buffalograss lines were challenged with C. inaequalis. Samples were collected from treated and untreated leaves when susceptible lines showed symptoms. Transcriptome sequencing was done and differentially expressed genes were identified. Approximately 27 million raw sequencing reads were produced per sample. More than 86% of the sequencing reads mapped to an existing buffalograss reference transcriptome. De novo assembly of unmapped reads was merged with the existing reference to produce a more complete transcriptome. There were 461 differentially expressed transcripts between the resistant and susceptible lines when challenged with the pathogen and 1552 in its absence. Previously characterized defense-related genes were identified among the differentially expressed transcripts. Twenty one resistant line transcripts were similar to genes regulating pattern triggered immunity and 20 transcripts were similar to genes regulating effector triggered immunity. There were also nine up-regulated transcripts in resistance lines which showed potential to initiate systemic acquired resistance (SAR) and three transcripts encoding pathogenesis-related proteins which are downstream products of SAR. This is the first study characterizing changes in the buffalograss transcriptome when challenged with C. inaequalis.

Arabidopsis EF-Tu receptor enhances bacterial disease resistance in transgenic wheat

Perception of pathogen (or microbe)-associated molecular patterns (PAMPs/MAMPs) by pattern recognition receptors (PRRs) is a key component of plant innate immunity. The Arabidopsis PRR EF-Tu receptor (EFR) recognizes the bacterial PAMP elongation factor Tu (EF-Tu) and its derived peptide elf18. Previous work revealed that transgenic expression of AtEFR in Solanaceae confers elf18 responsiveness and broad-spectrum bacterial disease resistance. In this study, we developed a set of bioassays to study the activation of PAMP-triggered immunity (PTI) in wheat. We generated transgenic wheat (Triticum aestivum) plants expressing AtEFR driven by the constitutive rice actin promoter and tested their response to elf18. We show that transgenic expression of AtEFR in wheat confers recognition of elf18, as measured by the induction of immune marker genes and callose deposition. When challenged with the cereal bacterial pathogen Pseudomonas syringae pv. oryzae, transgenic EFR wheat lines had reduced lesion size and bacterial multiplication. These results demonstrate that AtEFR can be transferred successfully from dicot to monocot species, further revealing that immune signalling pathways are conserved across these distant phyla. As novel PRRs are identified, their transfer between plant families represents a useful strategy for enhancing resistance to pathogens in crops.

Transgenic expression of the dicotyledonous pattern recognition receptor EFR in rice leads to ligand-dependent activation of defense responses

Plant plasma membrane localized pattern recognition receptors (PRRs) detect extracellular pathogen-associated molecules. PRRs such as Arabidopsis EFR and rice XA21 are taxonomically restricted and are absent from most plant genomes. Here we show that rice plants expressing EFR or the chimeric receptor EFR::XA21, containing the EFR ectodomain and the XA21 intracellular domain, sense both Escherichia coli- and Xanthomonas oryzae pv. oryzae (Xoo)-derived elf18 peptides at sub-nanomolar concentrations. Treatment of EFR and EFR::XA21 rice leaf tissue with elf18 leads to MAP kinase activation, reactive oxygen production and defense gene expression. Although expression of EFR does not lead to robust enhanced resistance to fully virulent Xoo isolates, it does lead to quantitatively enhanced resistance to weakly virulent Xoo isolates. EFR interacts with OsSERK2 and the XA21 binding protein 24 (XB24), two key components of the rice XA21-mediated immune response. Rice-EFR plants silenced for OsSERK2, or overexpressing rice XB24 are compromised in elf18-induced reactive oxygen production and defense gene expression indicating that these proteins are also important for EFR-mediated signaling in transgenic rice. Taken together, our results demonstrate the potential feasibility of enhancing disease resistance in rice and possibly other monocotyledonous crop species by expression of dicotyledonous PRRs. Our results also suggest that Arabidopsis EFR utilizes at least a subset of the known endogenous rice XA21 signaling components.

Plants possess multi-layered immune recognition systems. Early in the infection process, plants use receptor proteins to recognize pathogen molecules. Some of these receptors are present in only in a subset of plant species. Transfer of these taxonomically restricted immune receptors between plant species by genetic engineering is a promising approach for boosting the plant immune system. Here we show the successful transfer of an immune receptor from a species in the mustard family, called EFR, to rice. Rice plants expressing EFR are able to sense the bacterial ligand of EFR and elicit an immune response. We show that the EFR receptor is able to use components of the rice immune signaling pathway for its function. Under laboratory conditions, this leads to an enhanced resistance response to two weakly virulent isolates of an economically important bacterial disease of rice.

A comparative approach expands the protein-protein interaction node of the immune receptor XA21 in wheat and rice

The rice (Oryza sativa) OsXA21 receptor kinase is a well-studied immune receptor that initiates a signal transduction pathway leading to resistance to Xanthomonas oryzae pv. oryzae. Two homologs of OsXA21 were identified in wheat (Triticum aestivum): TaXA21-like1 located in a syntenic region with OsXA21, and TaXA21-like2 located in a nonsyntenic region. Proteins encoded by these two wheat genes interact with four wheat orthologs of known OsXA21 interactors. In this study, we screened a wheat yeast-two-hybrid (Y2H) library using the cytosolic portion of TaXA21-like1 as bait to identify additional interactors. Using full-length T. aestivum and T. monococcum proteins and Y2H assays we identified three novel TaXA21-like1 interactors (TaARG, TaPR2, TmSKL1) plus one previously known in rice (TaSGT1). An additional full-length wheat protein (TaCIPK14) interacted with TaXA21-like2 and OsXA21 but not with TaXA21-like1. The interactions of TaXA21-like1 with TmSKL1 and TaSGT1 were also observed in rice protoplasts using bimolecular fluorescence complementation assays. We then cloned the rice homologs of the novel wheat interactors and confirmed that they all interact with OsXA21. This last result suggests that interspecific comparative interactome analyses can be used not only to transfer known interactions from rice to wheat, but also to identify novel interactions in rice.

Comparative analysis of protein-protein interactions in the defense response of rice and wheat

Background

Despite the importance of wheat as a major staple crop and the negative impact of diseases on its production worldwide, the genetic mechanisms and gene interactions involved in the resistance response in wheat are still poorly understood. The complete sequence of the rice genome has provided an extremely useful parallel road map for genetic and genomics studies in wheat. The recent construction of a defense response interactome in rice has the potential to further enhance the translation of advances in rice to wheat and other grasses. The objective of this study was to determine the degree of conservation in the protein-protein interactions in the rice and wheat defense response interactomes. As entry points we selected proteins that serve as key regulators of the rice defense response: the RAR1/SGT1/HSP90 protein complex, NPR1, XA21, and XB12 (XA21 interacting protein 12).

Results

Using available wheat sequence databases and phylogenetic analyses we identified and cloned the wheat orthologs of these four rice proteins, including recently duplicated paralogs, and their known direct interactors and tested 86 binary protein interactions using yeast-two-hybrid (Y2H) assays. All interactions between wheat proteins were further tested using in planta bimolecular fluorescence complementation (BiFC). Eighty three percent of the known rice interactions were confirmed when wheat proteins were tested with rice interactors and 76% were confirmed using wheat protein pairs. All interactions in the RAR1/SGT1/ HSP90, NPR1 and XB12 nodes were confirmed for the identified orthologous wheat proteins, whereas only forty four percent of the interactions were confirmed in the interactome node centered on XA21. We hypothesize that this reduction may be associated with a different sub-functionalization history of the multiple duplications that occurred in this gene family after the divergence of the wheat and rice lineages.

Conclusions

The observed high conservation of interactions between proteins that serve as key regulators of the rice defense response suggests that the existing rice interactome can be used to predict interactions in wheat. Such predictions are less reliable for nodes that have undergone a different history of duplications and sub-functionalization in the two lineages.

On the move: induced resistance in monocots

Although plants possess an arsenal of constitutive defences such as structural barriers and preformed antimicrobial defences, many attackers are able to overcome the pre-existing defence layers. In response, a range of inducible plant defences is set up to battle these pathogens. These mechanisms, commonly integrated as induced resistance (IR), control pathogens and pests by the activation of specific defence pathways. IR mechanisms have been extensively studied in the Dicotyledoneae, whereas knowledge of IR in monocotyledonous plants, including the globally important graminaceous crop plants, is elusive. Considering the potential of IR for sustainable agriculture and the recent advances in monocot genomics and biotechnology, IR in monocots is an emerging research field. In the following, current facts and trends concerning basal immunity, and systemic acquired/induced systemic resistance in the defence of monocots against pathogens and herbivores will be summarized.

Impacts of resistance gene genetics, function, and evolution on a durable future

Studies on resistance gene function and evolution lie at the confluence of structural and molecular biology, genetics, and plant breeding. However, knowledge from these disparate fields has yet to be extensively integrated. This review draws on ideas and information from these different fields to elucidate the influences driving the evolution of different types of resistance genes in plants and the concurrent evolution of virulence in pathogens. It provides an overview of the factors shaping the evolution of recognition, signaling, and response genes in the context of emerging functional information along with a consideration of the new opportunities for durable resistance enabled by high-throughput DNA sequencing technologies.