The Wall-associated Kinase gene family in rice genomes

ZmWAK02 encoding an RD-WAK protein confers maize resistance against gray leaf spot

Gray leaf spot (GLS) caused by Cercospora zeina or C. zeae-maydis is a major maize disease throughout the world. Although more than 100 QTLs resistant against GLS have been identified, very few of them have been cloned. Here, we identified a major resistance QTL against GLS, qRglsSB, explaining 58.42% phenotypic variation in SB12×SA101 BC1 F1 population. By fine-mapping, it was narrowed down into a 928 kb region. By using transgenic lines, mutants and complementation lines, it was confirmed that the ZmWAK02 gene, encoding an RD wall-associated kinase, is the responsible gene in qRglsSB resistant against GLS. The introgression of the ZmWAK02 gene into hybrid lines significantly improves their grain yield in the presence of GLS pressure and does not reduce their grain yield in the absence of GLS. In summary, we cloned a gene, ZmWAK02, conferring large effect of GLS resistance and confirmed its great value in maize breeding.

Genome-wide identification and characterization of wall-associated kinases, molecular docking and polysaccharide elicitation of monoterpenoid indole alkaloids in micro-propagated Catharanthus roseus

Wall-associated kinases (WAKs) are a unique family of proteins that are predominantly localized on the plasma membrane and simultaneously bound to the cell wall. WAKs play a pivotal role in signal transduction to regulate growth, defense, and response to environmental stimuli in plants. These kinases have been identified and characterized in various plant species, however, similar information for Catharanthus roseus is scarce. C. roseus is an evergreen ornamental plant that produces a repertoire of biologically active compounds. The plant is best characterized for the production of antineoplastic monoterpenoid indole alkaloids (MIAs) namely vinblastine and vincristine. Owing to the diverse composition of phytochemicals, C. roseus is known as a "model non-model" plant for secondary metabolite research. Genome analyses showed 37 putative CrWAK genes present in C. roseus, largely localized on the plasma membrane. Phylogenetic analysis revealed six clusters of CrWAKs. Diverse cis-acting elements, including those involved in defense responses, were identified on the promotor regions of CrWAK genes. The highest binding affinity (- 12.6 kcal/mol) was noted for CrWAK-22 against tri-galacturonic acid. Tri-galacturonic acid stimulated 2.5-fold higher production of vinblastine, sixfold upregulation of the expression of ORCA3 transcription factor, and 6.14-fold upregulation of CrWAK-22 expression. Based on these results it was concluded that the expression of CrWAK genes induced by biotic elicitors may have an important role in the production of MIAs. The current findings may serve as a basis for functional characterization and mechanistic explanation of the role of CrWAK genes in the biosynthesis of MIAs upon elicitation.

Genome-wide characterization of the wall-associated kinase-like (WAKL) family in sesame (Sesamum indicum) identifies a SiWAKL6 gene involved in resistance to Macrophomina Phaseolina

BACKGROUND

Sesame charcoal rot caused by Macrophomina phaseolina is one of the most serious fungal diseases in sesame production, and threatens the yield and quality of sesame. WAKL genes are important in the plant response to biotic stresses by sensing and transmitting external signals to the intracellular receptor. However, there is still a lack about the WAKL gene family and its function in sesame resistance to M. phaseolina. The aim of this study was to interpret the roles of WAKL genes in sesame resistance to M. phaseolina.

RESULTS

In this study, a comprehensive study of the WAKL gene family was conducted and 31 WAKL genes were identified in the sesame genome. Tandem duplication events were the main factor in expansion of the SiWAKL gene family. Phylogenetic analysis showed that the sesame SiWAKL gene family was divided into 4 groups. SiWAKL genes exhibited different expression patterns in diverse tissues. Under M. phaseolina stress, most SiWAKL genes were significantly induced. Notably, SiWAKL6 was strongly induced in the resistant variety "Zhengzhi 13". Functional analysis showed that SiWAKL6 was induced by salicylic acid but not methyl jasmonate in sesame. Overexpression of SiWAKL6 in transgenic Arabidopsis thaliana plants enhanced their resistance to M. phaseolina by inducing the expression of genes involved in the salicylic acid signaling pathway and reconstructing reactive oxygen species homeostasis.

CONCLUSIONS

Taken together, the results provide a better understanding of functions about SiWAKL gene family and suggest that manipulation of these SiWAKL genes can improve plant resistance to M. phaseolina. The findings contributed to further understanding of functions of SiWAKL genes in plant immunity.

Origin, evolution, and diversification of the wall-associated kinase gene family in plants

KEY MESSAGE

The study of the origin, evolution, and diversification of the wall-associated kinase gene family in plants facilitates their functional investigations in the future. Wall-associated kinases (WAKs) make up one subfamily of receptor-like kinases (RLKs), and function directly in plant cell elongation and responses to biotic and abiotic stresses. The biological functions of WAKs have been extensively characterized in angiosperms; however, the origin and evolutionary history of the WAK family in green plants remain unclear. Here, we performed a comprehensive analysis of the WAK family to reveal its origin, evolution, and diversification in green plants. In total, 1061 WAK genes were identified in 37 species from unicellular algae to multicellular plants, and the results showed that WAK genes probably originated before bryophyte differentiation and were widely distributed in land plants, especially angiosperms. The phylogeny indicated that the land plant WAKs gave rise to five clades and underwent lineage-specific expansion after species differentiation. Cis-acting elements and expression patterns analyses of WAK genes in Arabidopsis and rice demonstrated the functional diversity of WAK genes in these two species. Many gene gains and losses have occurred in angiosperms, leading to an increase in the number of gene copies. The evolutionary trajectory of the WAK family during polyploidization was uncovered using Gossypium species. Our results provide insights into the evolution of WAK genes in green plants, facilitating their functional investigations in the future.

Analysis of WAK Genes in Nine Cruciferous Species with a Focus on Brassica napus L

The wall-associated kinase family contributes to plant cell elongation and pathogen recognition. Nine Cruciferous species were studied for identification and molecular evolution of the WAK gene family. Firstly, 178 WAK genes were identified. A phylogenetic tree was constructed of the Cruciferous WAK proteins into four categories, of which the Brassica rapa, Brassica oleracea and Brassica napus genes in the U's triangle were more closely related. The WAK gene family was unevenly distributed in B. napus chromosomal imaging, with the largest number of BnWAK genes located on chromosome C08. In the expression analysis, the expression patterns of the WAK gene family varied under different stress treatments, and some members of BnWAKs were significantly different under stress treatments. This study lays a foundation for further revealing the functional mechanisms of the WAK gene family in Brassica napus.

The wall-associated kinase gene family in pea (Pisum sativum) and its function in response to B deficiency and Al toxicity

Plant cell walls are embedded in a pectin matrix which is physically linked with the wall-associated kinases (WAKs), a subfamily of receptor-like kinases that participate in the cell wall integrity (CWI) sensing. Since cell walls are also the main binding sites for boron (B) and aluminum (Al), WAK may be potentially associated with the regulation of plant responses to Al toxicity and B deficiency. Using pea as a model species, we have identified a total of 28 WAK genes in the genome and named them according to its chromosomal location. All the PsWAKs were phylogenetically grouped into three clades. Phylogenetic relationship and synteny analysis showed that the PsWAKs in pea and Glycine max or Medicago truncatula shared a relatively conserved evolutionary history. Protein domain, motif, and transmembrane analysis indicated that all PsWAK proteins were predicted to be localized to the plasma membrane, and most PsWAKs shared a similar structure to their homologs. The RNA-seq data showed that the expression pattern of WAK genes in response to B deficiency was similar to that of Al toxicity, with most of PsWAKs being up-regulated. The qRT-PCR results further confirmed that PsWAK5, PsWAK9 and PsWAK14 were more specific for both B-deficiency and Al toxicity, and the expression levels of PsWAK5, PsWAK9 and PsWAK14 were significantly higher in the Al-sensitive cultivar Hyogo than in the Al-resistant cultivar Alaska under Al toxicity. This study provided an important basis for the functional and evolutionary analysis of PsWAKs and linked them to responses to cell wall damage induced by B-deficiency and Al toxicity, suggesting that PsWAKs may play a key role in the perception of cell wall integrity under Al toxicity or B-deficiency, as well as in the regulation of Al tolerance in pea.

Comparative transcriptomics and co-expression networks reveal cultivar-specific molecular signatures associated with reproductive-stage cold stress in rice

The resistance of Huaidao5 results from the high constitutive expression of tolerance genes, while that of Huaidao9 is due to the cold-induced resistance in flag leaves and panicles. The regulation mechanism of rice seedlings' cold tolerance is relatively clear, and knowledge of its underlying mechanisms at the reproductive stage is limited. We performed differential expression and co-expression network analyses to transcriptomes from panicle and flag leaf tissues of a cold-tolerant cultivar (Huaidao5), and a sensitive cultivar (Huaidao9), under reproductive-stage cold stress. The results revealed that the expression levels of genes in stress-related pathways such as MAPK signaling pathway, diterpenoid biosynthesis, glutathione metabolism, plant-pathogen interaction and plant hormone signal transduction were constitutively highly expressed in Huaidao5, especially in panicles. Moreover, the Hudaidao5's panicle sample-specific (under cold) module contained some genes related to rice yield, such as GW5L, GGC2, SG1 and CTPS1. However, the resistance of Huaidao9 was derived from the induced resistance to cold in flag leaves and panicles. In the flag leaves, the responses included a series of stress response and signal transduction, while in the panicles nitrogen metabolism was severely affected, especially 66 endosperm-specific genes. Through integrating differential expression with co-expression networks, we predicted 161 candidate genes (79 cold-responsive genes common to both cultivars and 82 cold-tolerance genes associated with differences in cold tolerance between cultivars) potentially affecting cold response/tolerance, among which 85 (52.80%) were known to be cold-related genes. Moreover, 52 (65.82%) cold-responsive genes (e.g., TIFY11C, LSK1 and LPA) could be confirmed by previous transcriptome studies and 72 (87.80%) cold-tolerance genes (e.g., APX5, OsFbox17 and OsSTA109) were located within QTLs associated with cold tolerance. This study provides an efficient strategy for further discovery of mechanisms of cold tolerance in rice.

A Novel Wall-Associated Kinase TaWAK-5D600 Positively Participates in Defense against Sharp Eyespot and Fusarium Crown Rot in Wheat

Sharp eyespot and Fusarium crown rot, mainly caused by soil-borne fungi Rhizoctonia cerealis and Fusarium pseudograminearum, are destructive diseases of major cereal crops including wheat (Triticum aestivum). However, the mechanisms underlying wheat-resistant responses to the two pathogens are largely elusive. In this study, we performed a genome-wide analysis of wall-associated kinase (WAK) family in wheat. As a result, a total of 140 TaWAK (not TaWAKL) candidate genes were identified from the wheat genome, each of which contains an N-terminal signal peptide, a galacturonan binding domain, an EGF-like domain, a calcium binding EGF domain (EGF-Ca), a transmembrane domain, and an intracellular Serine/Threonine protein kinase domain. By analyzing the RNA-sequencing data of wheat inoculated with R. cerealis and F. pseudograminearum, we found that transcript abundance of TaWAK-5D600 (TraesCS5D02G268600) on chromosome 5D was significantly upregulated, and that its upregulated transcript levels in response to both pathogens were higher compared with other TaWAK genes. Importantly, knock-down of TaWAK-5D600 transcript impaired wheat resistance against the fungal pathogens R. cerealis and F. pseudograminearum, and significantly repressed expression of defense-related genes in wheat, TaSERK1, TaMPK3, TaPR1, TaChitinase3, and TaChitinase4. Thus, this study proposes TaWAK-5D600 as a promising gene for improving wheat broad resistance to sharp eyespot and Fusarium crown rot (FCR) in wheat.

Identification and characterization of WAK gene family in Saccharum and the negative roles of ScWAK1 under the pathogen stress

Wall-associated kinase (WAK) is widely involved in signal transduction, reproductive growth, responses to pathogen infection and metal ion stress in plants. In this study, 19, 12, and 37 SsWAK genes were identified in Saccharum spontaneum, Saccharum hybrid and Sorghum bicolor, respectively. Phylogenetic tree showed that they could be divided into three groups. These WAK genes contained multiple cis-acting elements related to stress, growth and hormone response. RNA-seq analysis demonstrated that SsWAK genes were constitutively expressed in different sugarcane tissues and involved in response to smut pathogen (Sporisorium scitamineum) stress. Additionally, ScWAK1 (GenBank Accession No. OP479864), was then isolated from sugarcane cultivar ROC22. It was highly expressed in leaves and roots and its expression could be induced under SA and MeJA stress. Besides, ScWAK1 was significantly downregulated in both smut-resistant and susceptible sugarcane cultivars in response to S. scitamineum infection. ScWAK1 was a membrane protein without self-activating activity. Furthermore, transient expression of ScWAK1 in Nicotiana benthamiana enhanced the susceptibility of tobacco to the inoculation of Ralstonia solanacearum and Fusarium solani var. coeruleum, suggesting its negative role in disease resistance. The present study reveals the origin, distribution and evolution of WAK gene family and provides potential gene resources for sugarcane molecular breeding.

Antiviral modes of action of the novel compound GLY-15 containing pyrimidine heterocycle and moroxydine skeleton against tobacco mosaic virus

BACKGROUND

Plant virus diseases are difficult to prevent and control, causing serious economic losses to the agricultural production world. To develop new pesticides with antiviral activity, a serial of compounds containing the structure of pyrimidine and moroxydine were synthesized, among which GLY-15 exhibited good antiviral activity against tobacco mosaic virus (TMV), while the mechanism of antiviral activity remains to be clarified.

RESULTS

GLY-15 treatment significantly inhibited the formation of necrotic spots caused by TMV in Nicotiana glutinosa, and effectively suppressed the systemic transportation of TMV expressing a reporter gene (p35S-30B:GFP) in N. benthamiana and markedly reduced the accumulation of a movement deficient TMV in plants as well as viral RNA accumulation in tobacco protoplasts. The results of RNA sequencing showed that GLY-15 induced significant differential expression of genes or pathways involved in the stress response, defense response and signal transduction, phytohormone response and metabolism. Among them, real-time quantitative PCR validated that the expression of 12 critical genes such as heat shock protein, receptor kinase, cell-wall-related protein, disease-related protein and glucan endo-1,3-β-glucosidase were significantly up-regulated. In addition, GLY-15 triggered reactive oxygen species (ROS) production and induced the activity of several crucial defense related enzymes in plants. The results of molecular docking showed potential binding ability of GLY-15 with TMV helicase and the coat protein.

CONCLUSION

This study provide valuable insights into antiviral mechanism of action for GLY-15, which is expected to be applied as a pesticide for the management of plant viruses. © 2022 Society of Chemical Industry.

Genome-Wide Identification and Expression Analysis of Wall-Associated Kinase (WAK) Gene Family in Cannabis sativa L

Wall-associated kinases (WAKs) are receptors that bind pectin or small pectic fragments in the cell wall and play roles in cell elongation and pathogen response. In the Cannabis sativa (Cs) genome, 53 CsWAK/CsWAKL (WAK-like) protein family members were identified and characterized; their amino acid lengths and molecular weights varied from 582 to 983, and from 65.6 to 108.8 kDa, respectively. They were classified into four main groups by a phylogenetic tree. Out of the 53 identified CsWAK/CsWAKL genes, 23 CsWAK/CsWAKL genes were unevenly distributed among six chromosomes. Two pairs of genes on chromosomes 4 and 7 have undergone duplication. The number of introns and exons among CsWAK/CsWAKL genes ranged from 1 to 6 and from 2 to 7, respectively. The promoter regions of 23 CsWAKs/CsWAKLs possessed diverse cis-regulatory elements that are involved in light, development, environmental stress, and hormone responsiveness. The expression profiles indicated that our candidate genes (CsWAK1, CsWAK4, CsWAK7, CsWAKL1, and CsWAKL7) are expressed in leaf tissue. These genes exhibit different expression patterns than their homologs in other plant species. These initial findings are useful resources for further research work on the potential roles of CsWAK/CsWAKL in cellular signalling during development, environmental stress conditions, and hormone treatments.

Characterization of the WAK Gene Family Reveals Genes for FHB Resistance in Bread Wheat (Triticum aestivum L.)

Wall-associated kinases (WAKs) are important receptor-like proteins that play major roles in plant defense against pathogens. Fusarium head blight (FHB), one of the most widespread and devastating crop diseases, reduces wheat yield and leads to quality deterioration. Although WAK gene families have been studied in many plants, systematic research on bread wheat (Triticum aestivum) and its role in FHB resistance, in particular, is lacking. In this study, we identified and characterized 320 genes of the TaWAK family in wheat distributed across all chromosomes except 4B and divided them into three phylogenetic groups. Duplication and synteny analyses provided valuable information on the evolutionary characteristics of the TaWAK genes. The gene expression pattern analysis suggested that TaWAK genes play diverse roles in plant biological processes and that at least 30 genes may be involved in the response to Fusarium infection in wheat spikes, with most of the genes contributing to pectin- and chitin-induced defense pathways. Furthermore, 45 TaWAK genes were identified within 17 hcmQTLs that are related to wheat FHB resistance. Our findings provide potential candidate genes for improving FHB resistance and insights into the future functional analysis of TaWAK genes in wheat.

Identification and Characterization of Wall-Associated Kinase (WAK) and WAK-like (WAKL) Gene Family in Juglans regia and Its Wild Related Species Juglans mandshurica

Wall-associated kinase (WAK) and WAK-like kinase (WAKL) are receptor-like kinases (RLKs), which play important roles in signal transduction between the cell wall and the cytoplasm in plants. WAK/WAKLs have been studied in many plants, but were rarely studied in the important economic walnut tree. In this study, 27 and 14 WAK/WAKL genes were identified in Juglans regia and its wild related species Juglans mandshurica, respectively. We found tandem duplication might play a critical role in the expansion of WAK/WAKL gene family in J. regia, and most of the WAK/WAKL homologous pairs underwent purified selection during evolution. All WAK/WAKL proteins have the extracellular WAK domain and the cytoplasmic protein kinase domain, and the latter was more conserved than the former. Cis-acting elements analysis showed that WAK/WAKL might be involved in plant growth and development, plant response to abiotic stress and hormones. Gene expression pattern analysis further indicated that most WAK/WAKL genes in J. regia might play a role in the development of leaves and be involved in plant response to biotic stress. Our study provides a new perspective for the evolutionary analysis of gene families in tree species and also provides potential candidate genes for studying WAK/WAKL gene function in walnuts.

Perception and First Defense Responses Against Pseudomonas syringae pv. phaseolicola in Phaseolus vulgaris: Identification of Wall-Associated Kinase Receptors

Common bean (Phaseolus vulgaris) is attacked by several pathogens such as the biotrophic gamma-proteobacterium Pseudomonas syringae pv. phaseolicola. To study the P. syringae pv. phaseolicola-bean interaction during the first stages of infection, leaf discs of a susceptible bean cultivar Riñón were infected with pathogenic P. syringae pv. phaseolicola. Using this experimental system, we tested six new putative wall-associated kinase (WAK) receptors, previously identified in silico. These six P. vulgaris WAKs (PvWAKs) showed high protein sequence homology to the well-described Arabidopsis thaliana WAK1 (AtWAK1) receptor and, by phylogenetic analysis, clustered together with AtWAKs. The expression of PvWAK1 increased at very early stages after the P. syringae pv. phaseolicola infection. Time course experiments were performed to evaluate the accumulation of apoplastic H₂O₂, Ca²⁺ influx, total H₂O₂, antioxidant enzymatic activities, lipid peroxidation, and the concentrations of abscisic acid and salicylic acid (SA), as well as the expression of six defense-related genes: MEKK-1, MAPKK, WRKY33, RIN4, PR1, and NPR1. The results showed that overexpression of PR1 occurred 2 h after P. syringae pv. phaseolicola infection without a concomitant increase in SA levels. Although apoplastic H₂O₂ increased after infection, the oxidative burst was neither intense nor rapid, and an efficient antioxidant response did not occur, suggesting that the observed cellular damage was caused by the initial increase in total H₂O₂ early after infection. In conclusion, Riñón can perceive the presence of P. syringae pv. phaseolicola, but this recognition results in only a modest and slow activation of host defenses, leading to high susceptibility to P. syringae pv. phaseolicola.

Role of protein phosphatases in the regulation of nitrogen nutrition in plants

The reversible protein phosphorylation and dephosphorylation mediated by protein kinases and phosphatases regulate different biological processes and their response to environmental cues, including nitrogen (N) availability. Nitrate assimilation is under the strict control of phosphorylation-dephosphorylation mediated post-translational regulation. The protein phosphatase family with approximately 150 members in Arabidopsis and around 130 members in rice is a promising player in N uptake and assimilation pathways. Protein phosphatase 2A (PP2A) enhances the activation of nitrate reductase (NR) by deactivating SnRK1 and reduces the binding of inhibitory 14-3-3 proteins on NR. The functioning of nitrate transporter NPF6.3 is regulated by phosphorylation of CBL9 (Calcineurin B like protein 9) and CIPK23 (CBL interacting protein kinase 23) module. Phosphorylation by CIPK23 inhibits the activity of NPF6.3, whereas protein phosphatases (PP2C) enhance the NPF6.3-dependent nitrate sensing. PP2Cs and CIPK23 also regulate ammonium transporters (AMTs). Under either moderate ammonium supply or high N demand, CIPK23 is bound and inactivated by PP2Cs. Ammonium uptake is mediated by nonphosphorylated and active AMT1s. Whereas, under high ammonium availability, CIPK23 gets activated and phosphorylate AMT1;1 and AMT1;2 rendering them inactive. Recent reports suggest the critical role of protein phosphatases in regulating N use efficiency (NUE). In rice, PP2C9 regulates NUE by improving N uptake and assimilation. Comparative leaf proteome of wild type and PP2C9 over-expressing transgenic rice lines showed 30 differentially expressed proteins under low N level. These proteins are involved in photosynthesis, N metabolism, signalling, and defence.

Identification and Analysis of Genes Involved in Double Fertilization in Rice

Double fertilization is a key determinant of grain yield, and the failure of fertilization during hybridization is one important reason for reproductive isolation. Therefore, fertilization has a very important role in the production of high-yield and well-quality hybrid of rice. Here, we used RNA sequencing technology to study the change of the transcriptome during double fertilization with the help of the mutant fertilization barrier (feb) that failed to finish fertilization process and led to seed abortion. The results showed that 1669 genes were related to the guided growth of pollen tubes, 332 genes were involved in the recognition and fusion of the male–female gametes, and 430 genes were associated with zygote formation and early free endosperm nuclear division. Among them, the genes related to carbohydrate metabolism; signal transduction pathways were enriched in the guided growth of pollen tubes, the genes involved in the photosynthesis; fatty acid synthesis pathways were activated by the recognition and fusion of the male–female gametes; and the cell cycle-related genes might play an essential role in zygote formation and early endosperm nuclear division. Furthermore, among the 1669 pollen tube-related genes, it was found that 7 arabinogalactan proteins (AGPs), 1 cysteine-rich peptide (CRP), and 15 receptor-like kinases (RLKs) were specifically expressed in anther, while 2 AGPs, 7 CRPs, and 5 RLKs in pistil, showing obvious unequal distribution which implied they might play different roles in anther and pistil during fertilization. These studies laid a solid foundation for revealing double fertilization mechanism of rice and for the follow-up investigation.

Characterization of wall-associated kinase/wall-associated kinase-like (WAK/WAKL) family in rose (Rosa chinensis) reveals the role of RcWAK4 in Botrytis resistance

BACKGROUND

Wall-associated kinase (WAK)/WAK-like (WAKL) is one of the subfamily of receptor like kinases (RLK). Although previous studies reported that WAK/WAKL played an important role in plant cell elongation, response to biotic and abiotic stresses, there are no systematic studies on RcWAK/RcWAKL in rose.

RESULTS

In this study, we identified a total of 68 RcWAK/RcWAKL gene family members within rose (Rosa chinensis) genome. The RcWAKs contained the extracellular galacturonan-binding domain and calcium-binding epidermal growth factor (EGF)-like domain, as well as an intracellular kinase domains. The RcWAKLs are missing either calcium-binding EGF-like domain or the galacturonan-binding domain in their extracellular region. The phylogenetic analysis showed the RcWAK/RcWAKL gene family has been divided into five groups, and these RcWAK/RcWAKL genes were unevenly distributed on the 7 chromosomes of rose. 12 of RcWAK/RcWAKL genes were significantly up-regulated by Botrytis cinerea-inoculated rose petals, where RcWAK4 was the most strongly expressed. Virus induced gene silencing of RcWAK4 increased the rose petal sensitivity to B. cinerea. The results indicated RcWAK4 is involved in the resistance of rose petal against B. cinerea.

CONCLUSION

Our study provides useful information to further investigate the function of the RcWAK/RcWAKL gene family and breeding research for resistance to B. cinerea in rose.

OsWAK112, A Wall-Associated Kinase, Negatively Regulates Salt Stress Responses by Inhibiting Ethylene Production

The wall-associated kinase (WAK) multigene family plays critical roles in various cellular processes and stress responses in plants, however, whether WAKs are involved in salt tolerance is obscure. Herein, we report the functional characterization of a rice WAK, WAK112, whose expression is suppressed by salt. Overexpression of OsWAK112 in rice and heterologous expression of OsWAK112 in Arabidopsis significantly decreased plant survival under conditions of salt stress, while knocking down the OsWAK112 in rice increased plant survival under salt stress. OsWAK112 is universally expressed in plant and associated with cell wall. Meanwhile, in vitro kinase assays and salt tolerance analyses showed that OsWAK112 possesses kinase activity and that it plays a negative role in the response of plants to salt stress. In addition, OsWAK112 interacts with S-adenosyl-L-methionine synthetase (SAMS) 1/2/3, which catalyzes SAM synthesis from ATP and L-methionine, and promotes OsSAMS1 degradation under salt stress. Furthermore, in OsWAK112-overexpressing plants, there is a decreased SAMS content and a decreased ethylene content under salt stress. These results indicate that OsWAK112 negatively regulates plant salt responses by inhibiting ethylene production, possibly via direct binding with OsSAMS1/2/3.

A cysteine-rich receptor-like protein kinase CaCKR5 modulates immune response against Ralstonia solanacearum infection in pepper

BACKGROUND

Cysteine-rich receptor-like kinases (CRKs) represent a large subfamily of receptor-like kinases and play vital roles in diverse physiological processes in regulating plant growth and development.

RESULTS

CaCRK5 transcripts were induced in pepper upon the infection of Ralstonia solanacearum and treatment with salicylic acid. The fusions between CaCRK5 and green fluorescence protein were targeted to the plasma membrane. Suppression of CaCRK5 via virus-induced gene silencing (VIGS) made pepper plants significantly susceptible to R. solanacearum infection, which was accompanied with decreased expression of defense related genes CaPR1, CaSAR8.2, CaDEF1 and CaACO1. Overexpression of CaCRK5 increased resistance against R. solanacearum in Nicotiana benthamiana. Furthermore, electrophoretic mobility shift assay and chromatin immunoprecipitation coupled with quantitative real-time PCR analysis revealed that a homeodomain zipper I protein CaHDZ27 can active the expression of CaCRK5 through directly binding to its promoter. Yeast two-hybrid and bimolecular fluorescence complementation (BiFC) analyses suggested that CaCRK5 heterodimerized with the homologous member CaCRK6 on the plasma membrane.

CONCLUSIONS

Our data revealed that CaCRK5 played a positive role in regulating immune responses against R. solanacearum infection in pepper.

Characterization and expression analysis of wall-associated kinase (WAK) and WAK-like family in cotton

The wall-associated kinases (WAKs) and WAK-like kinases (WAKLs) form a group of receptor-like kinases (RLKs) with extracellular domains tightly linked to the cell wall. The WAKs/WAKLs have been known to be involved in plant growth, development, and stress responses. However, the functions of WAKs/WAKLs are less well known in cotton. In this study, 58, 66, and 99 WAK/WAKL genes were identified in Gossypium arboreum, G. raimondii, and G. hirsutum, respectively. Phylogenetic analysis showed they were classified into five groups, with two groups specific to cotton. Collinearity analysis revealed that segmental and tandem duplications resulted in expansion of the WAK/WAKL gene family in cotton. Moreover, the Ka/Ks ratios indicated this family was exposed to purifying selection pressure during evolution. The structures of the GhWAK/WAKL genes and encoded proteins suggested the functions of WAKs/WAKLs in cotton were conserved. Transient expression of four WAK/WAKL-GFP fusion constructs in Arabidopsis protoplasts indicated that they were localized on the plasma membrane. The cis-elements in the GhWAK/WAKL promoters were responsive to multiple phytohormones and abiotic stresses. Expression profiling showed that GhWAK/WAKL genes were induced by various abiotic stresses. This study provides insights into the evolution of WAK/WAKL genes and presents fundamental information for further analysis in cotton.

Identification of cell wall-associated kinases as important regulators involved in Gossypium hirsutum resistance to Verticillium dahliae

BACKGROUND

Verticillium wilt, caused by the soil borne fungus Verticillium dahliae, is a major threat to cotton production worldwide. An increasing number of findings indicate that WAK genes participate in plant-pathogen interactions, but their roles in cotton resistance to V. dahliae remain largely unclear.

RESULTS

Here, we carried out a genome-wide analysis of WAK gene family in Gossypium hirsutum that resulted in the identification of 81 putative GhWAKs, which were all predicated to be localized on plasma membrane. In which, GhWAK77 as a representative was further located in tobacco epidermal cells using transient expression of fluorescent fusion proteins. All GhWAKs could be classified into seven groups according to their diverse protein domains, indicating that they might sense different outside signals to trigger intracellular signaling pathways that were response to various environmental stresses. A lot of cis-regulatory elements were predicted in the upstream region of GhWAKs and classified into four main groups including hormones, biotic, abiotic and light. As many as 28 GhWAKs, playing a potential role in the interaction between cotton and V. dahliae, were screened out by RNA-seq and qRT-PCR. To further study the function of GhWAKs in cotton resistance to V. dahliae, VIGS technology was used to silence GhWAKs. At 20 dpi, VIGSed plants exhibited more chlorosis and wilting than the control plants. The disease indices of VIGSed plants were also significantly higher than those of the control. Furthermore, silencing of GhWAKs significantly affected the expression of JA- and SA-related marker genes, increased the spread of V. dahliae in the cotton stems, dramatically compromised V. dahliae-induced accumulation of lignin, H2O2 and NO, but enhanced POD activity.

CONCLUSION

Our study presents a comprehensive analysis on cotton WAK gene family for the first time. Expression analysis and VIGS assay provided direct evidences on GhWAKs participation in the cotton resistance to V. dahliae.

The Craterostigma plantagineum protein kinase CpWAK1 interacts with pectin and integrates different environmental signals in the cell wall

The cell wall protein CpWAK1 interacts with pectin, participates in decoding cell wall signals, and induces different downstream responses. Cell wall-associated protein kinases (WAKs) are transmembrane receptor kinases. In the desiccation-tolerant resurrection plant Craterostigma plantagineum, CpWAK1 has been shown to be involved in stress responses and cell expansion by forming a complex with the C. plantagineum glycine-rich protein1 (CpGRP1). This prompted us to extend the studies of WAK genes in C. plantagineum. The phylogenetic analyses of WAKs from C. plantagineum and from other species suggest that these genes have been duplicated after species divergence. Expression profiles indicate that CpWAKs are involved in various biological processes, including dehydration-induced responses and SA- and JA-related reactions to pathogens and wounding. CpWAK1 shows a high affinity for "egg-box" pectin structures. ELISA assays revealed that the binding of CpWAKs to pectins is modulated by CpGRP1 and it depends on the apoplastic pH. The formation of CpWAK multimers is the prerequisite for the CpWAK-pectin binding. Different pectin extracts lead to opposite trends of CpWAK-pectin binding in the presence of Ca²⁺ at pH 8. These observations demonstrate that CpWAKs can potentially discriminate and integrate cell wall signals generated by diverse stimuli, in concert with other elements, such as CpGRP1, pH_apo, Ca²⁺_[apo], and via the formation of CpWAK multimers.

Comparative transcriptome and metabolome profiling reveal molecular mechanisms underlying OsDRAP1-mediated salt tolerance in rice

Integration of transcriptomics and metabolomics data can provide detailed information for better understanding the molecular mechanisms underlying salt tolerance in rice. In the present study, we report a comprehensive analysis of the transcriptome and metabolome of rice overexpressing the OsDRAP1 gene, which encodes an ERF transcription factor and was previously identified to be conferring drought tolerance. Phenotypic analysis showed that OsDRAP1 overexpression (OE) improved salt tolerance by increasing the survival rate under salt stress. OsDRAP1 affected the physiological indices such as superoxide dismutase (SOD), catalase (CAT) and malondialdehyde (MDA) to enhance redox homeostasis and membrane stability in response to salt stress. Higher basal expression of OsDRAP1 resulted in differential expression of genes that potentially function in intrinsic salt tolerance. A core set of genes with distinct functions in transcriptional regulation, organelle gene expression and ion transport were substantially up-regulated in the OE line in response to salt stress, implying their important role in OsDRAP1-mediated salt tolerance. Correspondingly, metabolome profiling detected a number of differentially metabolites in the OE line relative to the wild type under salt stress. These metabolites, including amino acids (proline, valine), organic acids (glyceric acid, phosphoenolpyruvic acid and ascorbic acid) and many secondary metabolites, accumulated to higher levels in the OE line, demonstrating their role in salt tolerance. Integration of transcriptome and metabolome analysis highlights the crucial role of amino acids and carbohydrate metabolism pathways in OsDRAP1-mediated salt tolerance.

Root responses of contrasting rice genotypes to low temperature stress

Rice (Oryza sativa L.) ssp. indica is the most cultivated species in the South of Brazil. However, these plants face low temperature stress from September to November, which is the period of early sowing, affecting plant development during the initial stages of growth, and reducing rice productivity. This study aimed to characterize the root response to low temperature stress during the early vegetative stage of two rice genotypes contrasting in their cold tolerance (CT, cold-tolerant; and CS, cold-sensitive). Root dry weight and length, as well as the number of root hairs, were higher in CT than CS when exposed to cold treatment. Histochemical analyses indicated that roots of CS genotype present higher levels of lipid peroxidation and H₂O₂ accumulation, along with lower levels of plasma membrane integrity than CT under low temperature stress. RNAseq analyses revealed that the contrasting genotypes present completely different molecular responses to cold stress. The number of over-represented functional categories was lower in CT than CS under cold condition, suggesting that CS genotype is more impacted by low temperature stress than CT. Several genes might contribute to rice cold tolerance, including the ones related with cell wall remodeling, cytoskeleton and growth, signaling, antioxidant system, lipid metabolism, and stress response. On the other hand, high expression of the genes SRC2 (defense), root architecture associated 1 (growth), ACC oxidase, ethylene-responsive transcription factor, and cytokinin-O-glucosyltransferase 2 (hormone-related) seems to be related with cold sensibility. Since these two genotypes have a similar genetic background (sister lines), the differentially expressed genes found here can be considered candidate genes for cold tolerance and could be used in future biotechnological approaches aiming to increase rice tolerance to low temperature.

Genome-Wide Identification, Classification, Characterization, and Expression Analysis of the Wall-Associated Kinase Family during Fruit Development and under Wound Stress in Tomato (Solanum lycopersicum L.)

The wall-associated kinase (WAK) and wall-associated kinase like (WAKL) is a subfamily of receptor-like kinases associated with the cell wall, which have been suggested as sensors of the extracellular environment and triggers of intracellular signals. However, these proteins have not yet been comprehensively analyzed in tomato (Solanum lycopersicum L.). In this study, 11 SlWAK and 18 SlWAKL genes were identified in an uneven distribution in 9 of 12 chromosomes. GUB-WAK-bind (wall-associated receptor kinase galacturonan-binding) and epidermal growth factor (EGF) domains appear more often in SlWAK proteins. However, more SlWAKLs (wall-associated kinase like) have a WAK-assoc (wall-associated receptor kinase C-terminal) domain. Based on their phylogenetic relationships, 29 SlWAK-RLKs (wall associated kinase-receptor like kinases) were clustered into three distinct categories analogous to those in Arabidopsis thaliana. High similarities were found in conserved motifs of the genes within each group. Cis-elements in the promoter region of these 29 genes were found mainly in response to methyl jasmonate (MeJA), abscisic acid (ABA), salicylic acid (SA), anaerobic, light, wound, and MYB transcription factors. Public tomato genome RNA-seq data indicates that multiple SlWAK-RLKs showed different expression patterns under developmental and ripening stages of fruits, such as SlWAK4, SlWAKL11, SlWAKL9, SlWAKL15, SlWAKL14, and SlWAKL1, their RPKM (Reads Per Kilo bases per Million reads) value constantly increases during the fruit expansion period, and decreases as the fruit matures. In tomato leaves, our RNA-seq data showed that nine SlWAK-RLKs transcripts (SlWAK3, SlWAK4, SlWAK10,SlWAKL1, SlWAKL2, SlWAKL3, SlWAKL5, SlWAKL14, and SlWAKL18) were significantly induced (p < 0.001), and three transcripts (SlWAK2, SlWAK5, and SlWAKL15) were significantly inhibited (p < 0.001) under mechanical wounding. The qRT-PCR (Quantitative reverse transcription polymerase chain reaction) of SlWAKL1 and SlWAKL6 verify these results.

Genome-wide analysis of wall associated kinase (WAK) gene family in barley

In plants, wall associated kinases (WAKs) form a unique subfamily of receptor like-kinases (RLKs). In Arabidopsis thaliana, WAK-RLKs are known to regulate biotic stress, cell expansion, and metal tolerance, but their detailed characterization in barley is lacking. In this study, we identified a total of 91 WAK genes in the barley genome and classified them into five groups. Evolutionary analysis of HvWAKs with AtWAKs revealed their species-specific expansion. The maximum number (19 to 20) of WAK genes were located on chromosomes 3, 5 and 6. WAK proteins exhibited similar types of motif distribution in their group. Characterization of a Ds transposon insertion mutant of the wak1 revealed differences in the root length. Further, HvSPL23 transcription factor was identified as a positive co-expressing gene with HvWAK1, suggesting its possible upstream regulator. Taken together, our study provides a base for the functional characterization of WAK family members in the future.

CsWAKL08, a pathogen-induced wall-associated receptor-like kinase in sweet orange, confers resistance to citrus bacterial canker via ROS control and JA signaling

Citrus bacterial canker (CBC) is a disease resulting from Xanthomonas citri subsp. citri (Xcc) infection and poses a grave threat to citrus production worldwide. Wall-associated receptor-like kinases (WAKLs) are proteins with a central role in resisting a range of fungal and bacterial diseases. The roles of WAKLs in the context of CBC resistance, however, remain unclear. Here, we explored the role of CsWAKL08, which confers resistance to CBC, and we additionally analyzed the molecular mechanisms of CsWAKL08-mediated CBC resistance. Based on systematic annotation and induced expression analysis of the CsWAKL family in Citrus sinensis, CsWAKL08 was identified as a candidate that can be upregulated by Xcc infection in the CBC-resistant variety. CsWAKL08 can also be induced by the phytohormones salicylic acid (SA) and methyl jasmonic acid (MeJA) and spans the plasma membrane. Overexpression of CsWAKL08 resulted in strong CBC resistance in transgenic sweet oranges, whereas silencing of CsWAKL08 resulted in susceptibility to CBC. The peroxidase (POD) and superoxide dismutase (SOD) activities were significantly enhanced in the CsWAKL08-overexpressing plants compared to the control plants, thereby mediating reactive oxygen species (ROS) homeostasis in the transgenic plants. Moreover, the JA levels and the expression of JA biosynthesis and JA responsive genes were substantially elevated in the CsWAKL08 overexpression plants relative to the controls upon Xcc infection. Based on these findings, we conclude that the wall-associated receptor-like kinase CsWAKL08 positively regulates CBC resistance through a mechanism involving ROS control and JA signaling. These results further highlight the importance of this kinase family in plant pathogen resistance.

Cell surface immune receptors: the guardians of the plant's extracellular spaces

Since the original 'Zigzag model', several iterations have been proposed to reconcile both the Pattern Triggered Immunity (PTI) and the Effector Triggered Immunity (ETI) branches of the plant immune system. The recent cloning of new disease resistance genes, functioning in gene-for-gene interactions, which structurally resemble cell surface broad spectrum Pattern Recognition Receptors, have further blurred the distinctions between PTI and ETI in plant immunity. In an attempt to simplify further the existing conceptual models, we, herein, propose a scheme based on the spatial localization of the key proteins (receptors) which function to induce plant immune responses. We believe this 'Spatial Invasion model' will prove useful for understanding how immune receptors interact with different pathogen types which peripherally or totally invade plant cells, colonize solely extracellularly or switch locations during a successful infection.

Transcriptional Reprogramming of Rice Cells by Xanthomonas oryzae TALEs

Rice-pathogenic Xanthomonas oryzae bacteria cause severe harvest loss and challenge a stable food supply. The pathogen virulence relies strongly on bacterial TALE (transcription activator-like effector) proteins that function as transcriptional activators inside the plant cell. To understand the plant targets of TALEs, we determined the genome sequences of the Indian X. oryzae pv. oryzae (Xoo) type strain ICMP 3125T and the strain PXO142 from the Philippines. Their complete TALE repertoire was analyzed and genome-wide TALE targets in rice were characterized. Integrating computational target predictions and rice transcriptomics data, we were able to verify 12 specifically induced target rice genes. The TALEs of the Xoo strains were reconstructed and expressed in a TALE-free Xoo strain to attribute specific induced genes to individual TALEs. Using reporter assays, we could show that individual TALEs act directly on their target promoters. In particular, we show that TALE classes assigned by AnnoTALE reflect common target genes, and that TALE classes of Xoo and the related pathogen X. oryzae pv. oryzicola share more common target genes than previously believed. Taken together, we establish a detailed picture of TALE-induced plant processes that significantly expands our understanding of X. oryzae virulence strategies and will facilitate the development of novel resistances to overcome this important rice disease.

Fine mapping and candidate gene screening of the downy mildew resistance gene RPF1 in Spinach

A SLAF-BSA approach was used to locate the RPF1 locus. The three most likely candidate genes were identified which provide a basic for cloning the resistance gene at the RPF1 locus. Spinach downy mildew is a globally devastating oomycete disease. The use of downy mildew resistance genes constitutes the most effective approach for disease management. Hence, the objective of the present study was to fine map the first-reported resistance locus RPF1. The resistance allele at this resistance locus was effective against races 1-7, 9, 11, 13, and 15 of Peronospora farinosa f. sp. spinaciae (P. effusa). The approach fine mapped RPF1 using specific-locus amplified fragment sequencing (SLAF-Seq) technology combined with bulked segregant analysis. A 1.72 Mb region localized on chromosome 3 was found to contain RPF1 based on association analysis. After screening recombinants with the SLAF markers within the region, the region was narrowed down to 0.89 Mb. Within this region, 14 R genes were identified based on the annotation information. To identify the genes involved in resistance, resequencing of two resistant inbred lines (12S2 and 12S3) and three susceptible inbred lines (12S1, 12S4, and 10S2) was performed. The three most likely candidate genes were identified via amino acid sequence analysis and conserved domain analysis between resistant and susceptible inbred lines. These included Spo12729, encoding a receptor-like protein, and Spo12784 and Spo12903, encoding a nucleotide-binding site and leucine-rich repeat domains. Additionally, based on the sequence variation in the three genes between the resistant and susceptible lines, molecular markers were developed for marker-assisted selection. The results could be valuable in cloning the RPF1 alleles and improving our understanding of the interaction between the host and pathogen.

Early molecular events associated with nitrogen deficiency in rice seedling roots

Nitrogen (N) deficiency is one of the most common problems in rice. The symptoms of N deficiency are well documented, but the underlying molecular mechanisms are largely unknown in rice. Here, we studied the early molecular events associated with N starvation (−N, 1 h), focusing on amino acid analysis and identification of −N-regulated genes in rice roots. Interestingly, levels of glutamine rapidly decreased within 15 min of −N treatment, indicating that part of the N-deficient signals could be mediated by glutamine. Transcriptome analysis revealed that genes involved in metabolism, plant hormone signal transduction (e.g. abscisic acid, auxin, and jasmonate), transporter activity, and oxidative stress responses were rapidly regulated by −N. Some of the −N-regulated genes encode transcription factors, protein kinases and protein phosphatases, which may be involved in the regulation of early −N responses in rice roots. Previously, we used similar approaches to identify glutamine-, glutamate-, and ammonium nitrate-responsive genes. Comparisons of the genes induced by different forms of N with the −N-regulated genes identified here have provided a catalog of potential N regulatory genes for further dissection of the N signaling pathwys in rice.

OsMPH1 regulates plant height and improves grain yield in rice

Plant height is a major trait affecting yield potential in rice. Using a large-scale hybrid transcription factor approach, we identified the novel MYB-like transcription factor OsMPH1 (MYB-like gene of Plant Height 1), which is involved in the regulation of plant height in rice. Overexpression of OsMPH1 leads to increases of plant height and grain yield in rice, while knockdown of OsMPH1 leads to the opposite phenotypes. Microscopy of longitudinal stem sections indicated that a change in internode cell length resulted in the change in plant height. RNA sequencing (RNA-seq) analysis of transgenic rice lines showed that multiple genes related to cell elongation and cell wall synthesis, which are associated with plant height and yield phenotypes, exhibited an altered expression profile. These results imply that OsMPH1 might be involved in specific recognition and signal transduction processes related to plant height and yield formation, providing further insights into the mechanisms underlying the regulation of plant height and providing a candidate gene for the efficient improvement of rice yield.

Genetic mapping with testcrossing associations and F2:3 populations reveals the importance of heterosis in chilling tolerance at maize seedling stage

Maize seedlings are sensitive to low temperatures, and genetic mapping for chilling tolerance at the seedling stage with genetically diverse populations would facilitate the genetic improvement of this important trait. In this study, quantitative trait loci (QTL) mapping for four chilling tolerance-related traits at the seedling stage was conducted via a genome-wide association study (GWAS) with 338 testcrosses. A total of 32 significant loci and 36 stress tolerance-related candidate genes were identified, though none of them have been revealed by QTL mapping using maize inbred lines in previous reports. Moreover, expression of ten of the candidate genes was induced by chilling stress in a maize hybrid, though only a few of these genes were upregulated in its tolerant parent. These implied that heterosis might be involved in maize chilling tolerance. To further evaluate the importance of heterosis in chilling tolerance at the seedling stage, genetic mapping for chilling tolerance was conducted using an F_2:3 population derived from the two inbred lines used for the gene expression assay. Of the seven QTL revealed, six loci showed partial dominance or over-dominance effects. Results from this study demonstrate that heterosis plays an important role in chilling tolerance in maize seedlings.

Cytological and Molecular Characterization of ZmWAK-Mediated Head-Smut Resistance in Maize

Head smut, caused by the fungal pathogen Sporisorium reilianum, poses a threat to maize production worldwide. ZmWAK, a cell wall-associated receptor kinase, confers quantitative resistance to head smut disease. Here, two near-isogenic lines (NILs), susceptible line Huangzao4 and its ZmWAK-converted resistant line Huangzao4R, were used to decipher the role of ZmWAK in head smut resistance. Cytological and molecular characterization in response to S. reilianum infection was compared between two NILs. Upon S. reilianum infection, the growth of pathogen hyphae was severely arrested in the ZmWAK-converted resistant line Huangzao4R, relative to its susceptible parental line Huangzao4. Infected cells exhibited apoptosis-like features in Huangzao4R and hyphae were sequestered within dead cells, whereas pathogen invasion caused autophagy in Huangzao4, which failed to prevent hyphal spreading. Integrated transcriptomic and metabolomic analysis indicated that ZmWAK functions as a hub in the trade-off between growth and defense, whereby ZmWAK promotes cell growth in the absence of the pathogen and switches to a defense response upon S. reilianum attack. These findings shed light on an elegant regulatory mechanism governed by ZmWAK in the trade-off between growth and head smut defense.

Genome-Wide Differences in DNA Methylation Changes in Two Contrasting Rice Genotypes in Response to Drought Conditions

Differences in drought stress tolerance within diverse rice genotypes have been attributed to genetic diversity and epigenetic alterations. DNA methylation is an important epigenetic modification that influences diverse biological processes, but its effects on rice drought stress tolerance are poorly understood. In this study, methylated DNA immunoprecipitation sequencing and an Affymetrix GeneChip rice genome array were used to profile the DNA methylation patterns and transcriptomes of the drought-tolerant introgression line DK151 and its drought-sensitive recurrent parent IR64 under drought and control conditions. The introgression of donor genomic DNA induced genome-wide DNA methylation changes in DK151 plants. A total of 1190 differentially methylated regions (DMRs) were detected between the two genotypes under normal growth conditions, and the DMR-associated genes in DK151 plants were mainly related to stress response, programmed cell death, and nutrient reservoir activity, which are implicated to constitutive drought stress tolerance. A comparison of the DNA methylation changes in the two genotypes under drought conditions indicated that DK151 plants have a more stable methylome, with only 92 drought-induced DMRs, than IR64 plants with 506 DMRs. Gene ontology analyses of the DMR-associated genes in drought-stressed plants revealed that changes to the DNA methylation status of genotype-specific genes are associated with the epigenetic regulation of drought stress responses. Transcriptome analysis further helped to identify a set of 12 and 23 DMR-associated genes that were differentially expressed in DK151 and IR64, respectively, under drought stress compared with respective controls. Correlation analysis indicated that DNA methylation has various effects on gene expression, implying that it affects gene expression directly or indirectly through diverse regulatory pathways. Our results indicate that drought-induced alterations to DNA methylation may influence an epigenetic mechanism that regulates the expression of unique genes responsible for drought stress tolerance.

Molecular genetics and evolution of disease resistance in cereals

Contents 320 I. 320 II. 321 III. 321 IV. 322 V. 324 VI. 328 VII. 329 330 References 330 SUMMARY: Cereal crops produce a large part of the globally consumed food and feed. Because of the constant presence of devastating pathogens, the molecular characterization of disease resistance is a major research area and highly relevant for breeding. There has been recent and accelerating progress in the understanding of three distinct resistance mechanisms in cereals: resistance conferred by plasma membrane-localized receptor proteins; race-specific resistance conferred by intracellular immune receptors; and quantitative disease resistance. Intracellular immune receptors provide a particularly rich source for evolutionary studies, and have, for example, resulted in the recent discovery of a novel detection mechanism based on integrated decoy domains. Evolutionary studies have also revealed the origins of active resistance genes in both wild progenitors of today's cereals as well as in cultivated forms. In addition, independent evolution of orthologous genes in related cereals has resulted in resistance to different pathogen species. Quantitative resistance genes have been best characterized in wheat. The quantitative resistance genes identified so far in wheat encode transporter proteins or unusual kinase proteins. The recent discoveries in these three different resistance mechanisms have contributed to the basic molecular understanding of cereal immunity against pathogens and have suggested novel applications for resistance breeding.

The maize disease resistance gene Htn1 against northern corn leaf blight encodes a wall-associated receptor-like kinase

Northern corn leaf blight (NCLB) caused by the hemibiotrophic fungus Exserohilum turcicum is an important foliar disease of maize that is mainly controlled by growing resistant maize cultivars. The Htn1 locus confers quantitative and partial NCLB resistance by delaying the onset of lesion formation. Htn1 represents an important source of genetic resistance that was originally introduced from a Mexican landrace into modern maize breeding lines in the 1970s. Using a high-resolution map-based cloning approach, we delimited Htn1 to a 131.7-kb physical interval on chromosome 8 that contained three candidate genes encoding two wall-associated receptor-like kinases (ZmWAK-RLK1 and ZmWAK-RLK2) and one wall-associated receptor-like protein (ZmWAK-RLP1). TILLING (targeting induced local lesions in genomes) mutants in ZmWAK-RLK1 were more susceptible to NCLB than wild-type plants, both in greenhouse experiments and in the field. ZmWAK-RLK1 contains a nonarginine-aspartate (non-RD) kinase domain, typically found in plant innate immune receptors. Sequence comparison showed that the extracellular domain of ZmWAK-RLK1 is highly diverse between different maize genotypes. Furthermore, an alternative splice variant resulting in a truncated protein was present at higher frequency in the susceptible parents of the mapping populations compared with in the resistant parents. Hence, the quantitative Htn1 disease resistance in maize is encoded by an unusual innate immune receptor with an extracellular wall-associated kinase domain. These results further highlight the importance of this protein family in resistance to adapted pathogens.