A Chitinase from Tex6 Maize Kernels Inhibits Growth of Aspergillus flavus

Characterization of Uromyces appendiculatus First Races in Sri Lanka and Identification of Genes for the Development of Rust-Resistant Snap Beans

Bean rust is a devastating disease of snap beans in Sri Lanka. Our study endeavored to develop snap beans with resistance to Uromyces appendiculatus, the bean rust pathogen reported to have numerous virulent races. Accordingly, we needed a detailed understanding of the virulence of U. appendiculatus and to identify genes conferring effective rust resistance. To investigate the virulence, we collected leaves of snap bean with rust symptoms from four snap bean producing districts of Sri Lanka. From these samples we established 57 single pustule isolates. Each isolate was inoculated on a set of 12 differential cultivars, six from the Andean and six from the Middle American gene pools of common bean. The virulence of the isolates segregated into four distinct groups that comprised the first four reported races of U. appendiculatus in Sri Lanka, 23-5, 31-1, 31-11, and 63-21. All races were significantly more virulent on the Andean than on the Middle American differential cultivars. PI 1819966, a Middle American differential cultivar containing the Ur-11 gene, conferred resistance to all four races. Three other Middle American cultivars were resistant to three races. Conversely, five Andean differential cultivars were susceptible to all four races. PI 160418 was the only Andean differential cultivar with resistance to three races. The results suggested that rust resistance genes from the Middle American differential cultivars could be combined in different ways with the resistance of Andean PI 260418 to develop snap bean varieties with broad resistance to all races of U. appendiculatus in Sri Lanka.

A novel microbial-derived family 19 endochitinase with exochitinase activity and its immobilization

A novel chitinase gene of 888 bp from Streptomyces bacillaris was cloned and expressed in Escherichia coli BL21. The purified recombinant enzyme (SbChiAJ103) was identified as the first microbial-derived family 19 endochitinase that showed exochitinase activity. SbChiAJ103 exhibited the substrate preference for N-acetylchitooligosaccharides with even degrees of polymerization and the capability to specifically hydrolyze colloidal chitin into (GlcNAc)₂. Mono-methyl adipate was employed as a novel linker for the efficient covalent immobilization of chitinase on magnetic nanoparticles (MNPs). The immobilized SbChiAJ103, SbChiAJ103@MNPs, exhibited superior pH tolerance, temperature stability, and storage stability than free SbChiAJ103. Even after incubation at 45 °C for 24 h, SbChiAJ103@MNPs could retain more than 60.0% initial activity. As a result, the enzymatic hydrolysis yield of SbChiAJ103@MNPs increased to 1.58 times that of free SbChiAJ103. Moreover, SbChiAJ103@MNPs could be reused by convenient magnetic separation. After 10 recycles, SbChiAJ103@MNPs could retain almost 80.0% of its initial activity. The immobilization of the novel chitinase SbChiAJ103 paves the way to the efficient and eco-friendly commercial production of (GlcNAc)₂. KEY POINTS: • The first microbial GH19 endochitinase with exochitinase activity was reported. • Mono-methyl adipate was first employed to immobilize chitinase. • SbChiAJ103@MNPs showed excellent pH stability, thermal stability, and reusability.

Development of solid-state fermentation process of spent coffee grounds for the differentiated obtaining of chlorogenic, quinic, and caffeic acids

BACKGROUND

Spent coffee grounds (SCGs) are a good source of chlorogenic acid (CGA), which can be hydrolyzed to quinic acid (QA) and caffeic acid (CA). These molecules have antioxidant and neuroprotective capacities, benefiting human health. The hydrolysis of CGA can be done by biotechnological processes, such as solid-state fermentation (SSF). This work evaluated the use of SSF with Aspergillus sp. for the joint release of the three molecules from SCGs.

RESULTS

Hydroalcoholic extraction of the total phenolic compounds (TPCs) from SCGs was optimized, obtaining 28.9 ± 1.97 g gallic acid equivalent (GAE) kg^-1 SCGs using 0.67 L ethanol per 1 L, a 1:9 solid/liquid ratio, and a 63 min extraction time. Subsequently, SSF was performed for 30 days, achieving the maximum yields for CGA, QA, and TPCs on the 16th day: 7.12 ± 0.01 g kg^-1 , 4.68 ± 0.11 g kg^-1 , and 54.96 ± 0.49 g GAE kg^-1 respectively. CA reached its maximum value on the 23rd day, at 4.94 ± 0.04 g kg^-1 . The maximum antioxidant capacity was 635.7 mmol Trolox equivalents kg^-1 on the 14th day. Compared with unfermented SCGs extracts, TPCs and CGA increase their maximum values 2.3-fold, 18.6-fold for CA, 14.2 for QA, and 6.4-fold for antioxidant capacity. Additionally, different extracts' profiles were obtained throughout the SSF process, allowing us to adjust the type of enriched extract to be produced based on the SSF time.

CONCLUSION

SSF represents an alternative to produce extracts with different compositions and, consequently, different antioxidant capacities, which is a potentially attractive fermentation process for different applications. © 2022 Society of Chemical Industry.

Digging for Stress-Responsive Cell Wall Proteins for Developing Stress-Resistant Maize

As a vital component of plant cell walls, proteins play important roles in stress response by modifying the structure of cell walls and involving in the wall integrity signaling pathway. Recently, we have critically reviewed the predictors, databases, and cross-referencing of the subcellular locations of possible cell wall proteins (CWPs) in plants (Briefings in Bioinformatics 2018;19:1130-1140). Here, we briefly introduce strategies for isolating CWPs during proteomic analysis. Taking maize (Zea mays) as an example, we retrieved 1873 probable maize CWPs recorded in the UniProt KnowledgeBase (UniProtKB). After curation, 863 maize CWPs were identified and classified into 59 kinds of protein families. By referring to gene ontology (GO) annotations and gene differential expression in the Expression Atlas, we have highlighted the potential of CWPs acting in the front line of defense against biotic and abiotic stresses. Moreover, the analysis results of cis-acting elements revealed the responsiveness of the genes encoding CWPs toward phytohormones and various stresses. We suggest that the stress-responsive CWPs could be promising candidates for applications in developing varieties of stress-resistant maize.

Influence of Wounding and Temperature on Resistance of Maize Landraces From Mexico to Aflatoxin Contamination

Maize is a staple for billions across the globe. However, in tropical and sub-tropical regions, maize is frequently contaminated with aflatoxins by Aspergillus section Flavi fungi. There is an ongoing search for sources of aflatoxin resistance in maize to reduce continuous exposures of human populations to those dangerous mycotoxins. Large variability in susceptibility to aflatoxin contamination exists within maize germplasm. In Mexico, several maize landrace (MLR) accessions possess superior resistance to both Aspergillus infection and aflatoxin contamination but their mechanisms of resistance have not been reported. Influences of kernel integrity on resistance of four resistant and four susceptible MLR accessions were evaluated in laboratory assays. Wounds significantly (P < 0.05) increased susceptibility to aflatoxin contamination even when kernel viability was unaffected. Treatments supporting greater A. flavus reproduction did not (P > 0.05) proportionally support higher aflatoxin accumulation suggesting differential influences by some resistance factors between sporulation and aflatoxin biosynthesis. Physical barriers (i.e., wax and cuticle) prevented both aflatoxin accumulation and A. flavus sporulation in a highly resistant MLR accession. In addition, influence of temperature on aflatoxin contamination was evaluated in both viable and non-viable kernels of a resistant and a susceptible MLR accession, and a commercial hybrid. Both temperature and living embryo status influenced (P < 0.05) resistance to both aflatoxin accumulation and A. flavus sporulation. Lower sporulation on MLR accessions suggests their utilization would result in reduced speed of propagation and associated epidemic increases in disease both in the field and throughout storage. Results from the current study should encourage researchers across the globe to exploit the large potential that MLRs offer to breed for aflatoxin resistant maize. Furthermore, the studies provide support to the importance of resistance based on the living host and maintaining living status to reducing episodes of post-harvest contamination.

Functional Biology and Molecular Mechanisms of Host-Pathogen Interactions for Aflatoxin Contamination in Groundnut (Arachis hypogaea L.) and Maize (Zea mays L.)

Aflatoxins are secondary metabolites produced by soilborne saprophytic fungus Aspergillus flavus and closely related species that infect several agricultural commodities including groundnut and maize. The consumption of contaminated commodities adversely affects the health of humans and livestock. Aflatoxin contamination also causes significant economic and financial losses to producers. Research efforts and significant progress have been made in the past three decades to understand the genetic behavior, molecular mechanisms, as well as the detailed biology of host-pathogen interactions. A range of omics approaches have facilitated better understanding of the resistance mechanisms and identified pathways involved during host-pathogen interactions. Most of such studies were however undertaken in groundnut and maize. Current efforts are geared toward harnessing knowledge on host-pathogen interactions and crop resistant factors that control aflatoxin contamination. This study provides a summary of the recent progress made in enhancing the understanding of the functional biology and molecular mechanisms associated with host-pathogen interactions during aflatoxin contamination in groundnut and maize.

A biophysical and structural study of two chitinases from Agave tequilana and their potential role as defense proteins

Plant chitinases are enzymes that have several functions, including providing protection against pathogens. Agave tequilana is an economically important plant that is poorly studied. Here, we identified a chitinase from short reads of the A. tequilana transcriptome (AtChi1). A second chitinase, differing by only six residues from the first, was isolated from total RNA of plants infected with Fusarium oxysporum (AtChi2). Both enzymes were overexpressed in Escherichia coli and analysis of their sequences indicated that they belong to the class I glycoside hydrolase family19, whose members exhibit two domains: a carbohydrate-binding module and a catalytic domain, connected by a flexible linker. Activity assays and thermal shift experiments demonstrated that the recombinant Agave enzymes are highly thermostable acidic endochitinases with Tm values of 75 °C and 71 °C. Both exhibit a molecular mass close to 32 kDa, as determined by MALDI-TOF, and experimental pIs of 3.7 and 3.9. Coupling small-angle x-ray scattering information with homology modeling and docking simulations allowed us to structurally characterize both chitinases, which notably show different interactions in the binding groove. Even when the six different amino acids are all exposed to solvent in the loops located near the linker and opposite to the binding site, they confer distinct kinetic parameters against colloidal chitin and similar affinity for (GlnNAc)_6, as shown by isothermal titration calorimetry. Interestingly, binding is more enthalpy-driven for AtChi2. Whereas the physiological role of these chitinases remains unknown, we demonstrate that they exhibit important antifungal activity against chitin-rich fungi such as Aspergillus sp. DATABASE: SAXS structural data are available in the SASBDB database with accession numbers SASDDE7 and SASDDA6. ENZYMES: Chitinases (EC3.2.1.14).

Cultural and Genetic Approaches to Manage Aflatoxin Contamination: Recent Insights Provide Opportunities for Improved Control

Aspergillus flavus is a morphologically complex species that can produce the group of polyketide derived carcinogenic and mutagenic secondary metabolites, aflatoxins, as well as other secondary metabolites such as cyclopiazonic acid and aflatrem. Aflatoxin causes aflatoxicosis when aflatoxins are ingested through contaminated food and feed. In addition, aflatoxin contamination is a major problem, from both an economic and health aspect, in developing countries, especially Asia and Africa, where cereals and peanuts are important food crops. Earlier measures for control of A. flavus infection and consequent aflatoxin contamination centered on creating unfavorable environments for the pathogen and destroying contaminated products. While development of atoxigenic (nonaflatoxin producing) strains of A. flavus as viable commercial biocontrol agents has marked a unique advance for control of aflatoxin contamination, particularly in Africa, new insights into the biology and sexuality of A. flavus are now providing opportunities to design improved atoxigenic strains for sustainable biological control of aflatoxin. Further, progress in the use of molecular technologies such as incorporation of antifungal genes in the host and host-induced gene silencing, is providing knowledge that could be harnessed to develop germplasm that is resistant to infection by A. flavus and aflatoxin contamination. This review summarizes the substantial progress that has been made to understand the biology of A. flavus and mitigate aflatoxin contamination with emphasis on maize. Concepts developed to date can provide a basis for future research efforts on the sustainable management of aflatoxin contamination.

Chitin and chitin-related compounds in plant-fungal interactions

Chitin is the second abundant polysaccharide in the world after cellulose. It is a vital structural component of the fungal cell wall but not for plants. In plants, fungi are recognised through the perception of conserved microbe-associated molecular patterns (MAMPs) to induce MAMP-triggered immunity (MTI). Chitin polymers and their modified form, chitosan, induce host defence responses in both monocotyledons and dicotyledons. The plants' response to chitin, chitosan, and derived oligosaccharides depends on the acetylation degree of these compounds which indicates possible biocontrol regulation of plant immune system. There has also been a considerable amount of recent research aimed at elucidating the roles of chitin hydrolases in fungi and plants as chitinase production in plants is not considered solely as an antifungal resistance mechanism. We discuss the importance of chitin forms and chitinases in the plant-fungal interactions and their role in persistent and possible biocontrol. Abbreviations ET, ethylene; GAP, GTPase-activating protein; GEF, GDP/GTP exchange factor; JA, jasmonic acid; LysM, lysin motif; MAMP, microbe-associated molecular pattern; MTI, MAMP-triggered immunity; NBS, nucleotide-binding site; NBS-LRR, nucleotide-binding site leucine-rich repeats; PM, powdery mildew; PR, pathogenesis-related; RBOH, respiratory burst oxidase homolog; RLK, receptor-like kinase; RLP, receptor-like protein; SA, salicylic acid; TF, transcription factor.

Dissection of mechanisms of resistance to Aspergillus flavus and aflatoxin using tropical maize germplasm

Aspergillus flavus induced ear rots and subsequent contamination of maize (Zea mays L.) by aflatoxin is a serious food safety issue, especially in developing countries where the crop is mostly cultivated by smallholder famers for own consumption and income generation. A better understanding of the mechanisms of resistance could help breeders to develop resistant maize varieties. In this study, a set of six tropical maize inbred lines previously identified as resistant or susceptible under natural field conditions were evaluated for response to A. flavus colonisation and aflatoxin contamination. Fungal biomass was significantly higher (P<0.05) in susceptible than resistant maize inbred lines, and this was highly correlated (P=0.001) to aflatoxin levels. Maize inbred lines MRI, MR2 and MR3 had low fungal biomass and low aflatoxin levels, suggesting that resistance in these lines was mediated through restricted fungal colonisation and establishment. Among the three putatively resistant inbred lines mentioned above, MR2 had a relatively high colonisation compared to the other two lines, revealing that A. flavus could establish and colonise kernels that were injured during inoculation, but did not contain high levels of aflatoxin. This could signify the presence of host genes that interfere with the aflatoxin biosynthetic pathway.

Advances in molecular and genomic research to safeguard food and feed supply from aflatoxin contamination

Worldwide recognition that aflatoxin contamination of agricultural commodities by the fungus Aspergillus flavus is a global problem has significantly benefitted from global collaboration for understanding the contaminating fungus, as well as for developing and implementing solutions against the contamination. The effort to address this serious food and feed safety issue has led to a detailed understanding of the taxonomy, ecology, physiology, genomics and evolution of A. flavus, as well as strategies to reduce or control pre-harvest aflatoxin contamination, including (1) biological control, using atoxigenic aspergilli, (2) proteomic and genomic analyses for identifying resistance factors in maize as potential breeding markers to enable development of resistant maize lines, and (3) enhancing host-resistance by bioengineering of susceptible crops, such as cotton, maize, peanut and tree nuts. A post-harvest measure to prevent the occurrence of aflatoxin contamination in storage is also an important component for reducing exposure of populations worldwide to aflatoxins in food and feed supplies. The effect of environmental changes on aflatoxin contamination levels has recently become an important aspect for study to anticipate future contamination levels. The ability of A. flavus to produce dozens of secondary metabolites, in addition to aflatoxins, has created a new avenue of research for understanding the role these metabolites play in the survival and biodiversity of this fungus. The understanding of A. flavus, the aflatoxin contamination problem, and control measures to prevent the contamination has become a unique example for an integrated approach to safeguard global food and feed safety.

Genetics and mapping of a new anthracnose resistance locus in Andean common bean Paloma

BACKGROUND

The Andean cultivar Paloma is resistant to Mesoamerican and Andean races of Colletotrichum lindemuthianum, the fungal pathogen that causes the destructive anthracnose disease in common bean. Remarkably, Paloma is resistant to Mesoamerican races 2047 and 3481, which are among the most virulent races of the anthracnose pathogen. Most genes conferring anthracnose resistance in common bean are overcome by these races. The genetic mapping and the relationship between the resistant Co-Pa gene of Paloma and previously characterized anthracnose resistance genes can be a great contribution for breeding programs.

RESULTS

The inheritance of resistance studies for Paloma was performed in F2 population from the cross Paloma (resistant) × Cornell 49-242 (susceptible) inoculated with race 2047, and in F2 and F2:3 generations from the cross Paloma (resistant) × PI 207262 (susceptible) inoculated with race 3481. The results of these studies demonstrated that a single dominant gene confers the resistance in Paloma. Allelism tests performed with multiple races of C. lindemuthianum showed that the resistance gene in Paloma, provisionally named Co-Pa, is independent from the anthracnose resistance genes Co-1, Co-2, Co-3, Co-4, Co-5, Co-6, Co-12, Co-13, Co-14, Co-15 and Co-16. Bulk segregant analysis using the SNP chip BARCBean6K_3 positioned the approximate location of Co-Pa in the lower arm of chromosome Pv01. Further mapping analysis located the Co-Pa gene at a 390 kb region of Pv01 flanked by SNP markers SS82 and SS83 at a distance of 1.3 and 2.1 cM, respectively.

CONCLUSIONS

The results presented here showed that Paloma cultivar has a new dominant gene conferring resistance to anthracnose, which is independent from those genes previously described. The linkage between the Co-Pa gene and the SS82 and SS83 SNP markers will be extremely important for marker-assisted introgression of the gene into elite cultivars in order to enhance resistance.

Comparative Histological and Transcriptional Analysis of Maize Kernels Infected with Aspergillus flavus and Fusarium verticillioides

Aspergillus flavus and Fusarium verticillioides infect maize kernels and contaminate them with the mycotoxins aflatoxin, and fumonisin, respectively. Genetic resistance in maize to these fungi and to mycotoxin contamination has been difficult to achieve due to lack of identified resistance genes. The objective of this study was to identify new candidate resistance genes by characterizing their temporal expression in response to infection and comparing expression of these genes with genes known to be associated with plant defense. Fungal colonization and transcriptional changes in kernels inoculated with each fungus were monitored at 4, 12, 24, 48, and 72 h post inoculation (hpi). Maize kernels responded by differential gene expression to each fungus within 4 hpi, before the fungi could be observed visually, but more genes were differentially expressed between 48 and 72 hpi, when fungal colonization was more extensive. Two-way hierarchal clustering analysis grouped the temporal expression profiles of the 5,863 differentially expressed maize genes over all time points into 12 clusters. Many clusters were enriched for genes previously associated with defense responses to either A. flavus or F. verticillioides. Also within these expression clusters were genes that lacked either annotation or assignment to functional categories. This study provided a comprehensive analysis of gene expression of each A. flavus and F. verticillioides during infection of maize kernels, it identified genes expressed early and late in the infection process, and it provided a grouping of genes of unknown function with similarly expressed defense related genes that could inform selection of new genes as targets in breeding strategies.

Maize peroxidase Px5 has a highly conserved sequence in inbreds resistant to mycotoxin producing fungi which enhances fungal and insect resistance

Mycotoxin presence in maize causes health and economic issues for humans and animals. Although many studies have investigated expression differences of genes putatively governing resistance to producing fungi, few have confirmed a resistance role, or examined putative resistance gene structure in more than a couple of inbreds. The pericarp expression of maize Px5 has previously been associated with resistance to Aspergillus flavus growth and insects in a set of inbreds. Genes from 14 different inbreds that included ones with resistance and susceptibility to A. flavus, Fusarium proliferatum, F. verticillioides and F. graminearum and/or mycotoxin production were cloned using high fidelity enzymes, and sequenced. The sequence of Px5 from all resistant inbreds was identical, except for a single base change in two inbreds, only one of which affected the amino acid sequence. Conversely, the Px5 sequence from several susceptible inbreds had several base variations, some of which affected amino acid sequence that would potentially alter secondary structure, and thus enzyme function. The sequence of the maize peroxidase Px5 common to inbreds resistant to mycotoxigenic fungi was overexpressed in maize callus. Callus transformants overexpressing the gene caused significant reductions in growth for fall armyworms, corn earworms, and F. graminearum compared to transformant callus with a β-glucuronidase gene. This study demonstrates rarer transcripts of potential resistance genes overlooked by expression screens can be identified by sequence comparisons. A role in pest resistance can be verified by callus expression of the candidate genes, which can thereby justify larger scale transformation and regeneration of transgenic plants expressing the resistance gene for further evaluation.

Tissue-specific gene expression in maize seeds during colonization by Aspergillus flavus and Fusarium verticillioides

Aspergillus flavus and Fusarium verticillioides are fungal pathogens that colonize maize kernels and produce the harmful mycotoxins aflatoxin and fumonisin, respectively. Management practice based on potential host resistance to reduce contamination by these mycotoxins has proven difficult, resulting in the need for a better understanding of the infection process by these fungi and the response of maize seeds to infection. In this study, we followed the colonization of seeds by histological methods and the transcriptional changes of two maize defence-related genes in specific seed tissues by RNA in situ hybridization. Maize kernels were inoculated with either A. flavus or F. verticillioides 21-22 days after pollination, and harvested at 4, 12, 24, 48, 72, 96 and 120 h post-inoculation. The fungi colonized all tissues of maize seed, but differed in their interactions with aleurone and germ tissues. RNA in situ hybridization showed the induction of the maize pathogenesis-related protein, maize seed (PRms) gene in the aleurone and scutellum on infection by either fungus. Transcripts of the maize sucrose synthase-encoding gene, shrunken-1 (Sh1), were observed in the embryo of non-infected kernels, but were induced on infection by each fungus in the aleurone and scutellum. By comparing histological and RNA in situ hybridization results from adjacent serial sections, we found that the transcripts of these two genes accumulated in tissue prior to the arrival of the advancing pathogens in the seeds. A knowledge of the patterns of colonization and tissue-specific gene expression in response to these fungi will be helpful in the development of resistance.

Characterization of the Maize Chitinase Genes and Their Effect on Aspergillus flavus and Aflatoxin Accumulation Resistance

Maize (Zea mays L.) is a crop of global importance, but prone to contamination by aflatoxins produced by fungi in the genus Aspergillus. The development of resistant germplasm and the identification of genes contributing to resistance would aid in the reduction of the problem with a minimal need for intervention by farmers. Chitinolytic enzymes respond to attack by potential pathogens and have been demonstrated to increase insect and fungal resistance in plants. Here, all chitinase genes in the maize genome were characterized via sequence diversity and expression patterns. Recent evolution within this gene family was noted. Markers from within each gene were developed and used to map the phenotypic effect on resistance of each gene in up to four QTL mapping populations and one association panel. Seven chitinase genes were identified that had alleles associated with increased resistance to aflatoxin accumulation and A. flavus infection in field grown maize. The chitinase in bin 1.05 identified a new and highly significant QTL, while chitinase genes in bins 2.04 and 5.03 fell directly beneath the peaks of previously published QTL. The expression patterns of these genes corroborate possible grain resistance mechanisms. Markers from within the gene sequences or very closely linked to them are presented to aid in the use of marker assisted selection to improve this trait.

Biotechnological advances for combating Aspergillus flavus and aflatoxin contamination in crops

Aflatoxins are toxic, carcinogenic, mutagenic, teratogenic and immunosuppressive byproducts of Aspergillus spp. that contaminate a wide range of crops such as maize, peanut, and cotton. Aflatoxin not only affects crop production but renders the produce unfit for consumption and harmful to human and livestock health, with stringent threshold limits of acceptability. In many crops, breeding for resistance is not a reliable option because of the limited availability of genotypes with durable resistance to Aspergillus. Understanding the fungal/crop/environment interactions involved in aflatoxin contamination is therefore essential in designing measures for its prevention and control. For a sustainable solution to aflatoxin contamination, research must be focused on identifying and improving knowledge of host-plant resistance factors to aflatoxin accumulation. Current advances in genetic transformation, proteomics, RNAi technology, and marker-assisted selection offer great potential in minimizing pre-harvest aflatoxin contamination in cultivated crop species. Moreover, developing effective phenotyping strategies for transgenic as well as precision breeding of resistance genes into commercial varieties is critical. While appropriate storage practices can generally minimize post-harvest aflatoxin contamination in crops, the use of biotechnology to interrupt the probability of pre-harvest infection and contamination has the potential to provide sustainable solution.

Discovery and confirmation of genes/proteins associated with maize aflatoxin resistance

Maize (Zea mays L.) is one of the major crops susceptible to Aspergillus flavus infection and subsequent aflatoxin contamination. Many earlier studies indicated the roles of kernel proteins, especially constitutively expressed proteins, in maize resistance to A. flavus infection and aflatoxin production. In this review, we examined the past and current efforts in identifying maize genes and proteins from kernel, rachis, and silk tissues that may play an important role in resistance to A. flavus infection and aflatoxin contamination, as well as the efforts in determining the importance or involvement of them in maize resistance through biochemical, molecular and genetics studies. Through these studies, we gained a better understanding of host resistance mechanism: resistant lines appear to either express some stress-related and antifungal proteins at higher levels in endosperm, embryo, rachis and silk tissues before A. flavus infection or induce the expression of these proteins much faster compared to susceptible maize lines. In addition, we summarised several recent efforts in enhancing maize resistance to aflatoxin contamination using native genes from maize or heterologous and synthetic genes from other sources as well as from A. flavus. These efforts to either suppress A. flavus growth or aflatoxin production, have all shown some promising preliminary success. For example, maize plants transformed with an ?-amylase inhibitor protein from Lablab purpurea showed reduced aflatoxin levels by 56% in kernel screening assays. The antifungal potentials of transgenic maize plants expressing synthetic lytic peptides, such as cecropin-based D4E1 or tachyplesin-based AGM peptides with demonstrated anti-flavus activity (IC50 = 2.5 to 10 ?M), are yet to be assayed. Further investigation in these areas may provide a more cost-effective alternative to biocontrol in managing aflatoxin contamination in maize and other susceptible crops.

Aflatoxin Resistance in Maize: What Have We Learned Lately?

Aflatoxin contamination of maize grain is a huge economic and health problem, causing death and increased disease burden in much of the developing world and income loss in the developed world. Despite the gravity of the problem, deployable solutions are still being sought. In the past 15 years, much progress has been made in creating resistant maize inbred lines; mapping of genetic factors associated with resistance; and identifying possible resistance mechanisms. This review highlights this progress, most of which has occurred since the last time a review was published on this topic. Many of the needs highlighted in the last reviews have been addressed, and several solutions, taken together, can now greatly reduce the aflatoxin problem in maize grain. Continued research will soon lead to further solutions, which promise to further reduce and even eliminate the problem completely.

UBL1 of Fusarium verticillioides links the N-end rule pathway to extracellular sensing and plant pathogenesis

Fusarium verticillioides produces fumonisin mycotoxins during colonization of maize. Currently, molecular mechanisms underlying responsiveness of F.verticillioides to extracellular cues during pathogenesis are poorly understood. In this study, insertional mutants were created and screened to identify genes involved in responses to extracellular starch. In one mutant, the restriction enzyme-mediated integration cassette disrupted a gene (UBL1) encoding a UBR-Box/RING domain E3 ubiquitin ligase involved in the N-end rule pathway. Disruption of UBL1 in F.verticillioides (Δubl1) influenced conidiation, hyphal morphology, pigmentation and amylolysis. Disruption of UBL1 also impaired kernel colonization, but the ratio of fumonisin B1 per unit growth was not significantly reduced. The inability of a Δubl1 mutant to recognize an N-end rule degron confirmed involvement of UBL1 in the N-end rule pathway. Additionally, Ubl1 physically interacted with two G protein α subunits of F.verticillioides, thus implicating UBL1 in G protein-mediated sensing of the external environment. Furthermore, deletion of the UBL1 orthologue in F.graminearum reduced virulence on wheat and maize, thus indicating that UBL1 has a broader role in virulence among Fusarium species. This study provides the first linkage between the N-end rule pathway and fungal pathogenesis, and illustrates a new mechanism through which fungi respond to the external environment.

Two cold-induced family 19 glycosyl hydrolases from cherimoya (Annona cherimola) fruit: an antifungal chitinase and a cold-adapted chitinase

Two cold-induced chitinases were isolated and purified from the mesocarp cherimoyas (Annona cherimola Mill.) and they were characterised as acidic endochitinases with a Mr of 24.79 and 47.77kDa (AChi24 and AChi48, respectively), both family 19 glycosyl hydrolases. These purified chitinases differed significantly in their biochemical and biophysical properties. While both enzymes had similar optimal acidic pH values, AChi24 was enzymatically active and stable at alkaline pH values, as well as displaying an optimal temperature of 45°C and moderate thermostability. Kinetic studies revealed a great catalytic efficiency of AChi24 for oligomeric and polymeric substrates. Conversely, AChi48 hydrolysis showed positive co-operativity that was associated to a mixture of different functional oligomeric states through weak transient protein interactions. The rise in the AChi48 kcat at increasing enzyme concentrations provided evidence of its oligomerisation. AChi48 chitinase was active and stable in a broad acidic pH range, and while it was relatively labile as temperatures increased, with an optimal temperature of 35°C, it retained about 50% of its maximal activity from 5 to 50°C. Thermodynamic characterisation reflected the high kcat of AChi48 and the remarkably lower ΔH(‡), ΔS(‡) and ΔG(‡) values at 5°C compared to AChi24, indicating that the hydrolytic activity of AChi48 was less thermodependent. In vitro functional studies revealed that AChi24 had a strong antifungal defence potential against Botrytis cinerea, whereas they displayed no cryoprotective or antifreeze activity. Hence, based on biochemical, thermodynamic and functional data, this study demonstrates that two acidic endochitinases are induced at low temperatures in a subtropical fruit, and that one of them acts in an oligomeric cold-adapted manner.

Relationship between aflatoxin contamination and physiological responses of corn plants under drought and heat stress

Increased aflatoxin contamination in corn by the fungus Aspergillus flavus is associated with frequent periods of drought and heat stress during the reproductive stages of the plants. The objective of this study was to evaluate the relationship between aflatoxin contamination and physiological responses of corn plants under drought and heat stress. The study was conducted in Stoneville, MS, USA under irrigated and non-irrigated conditions. Five commercial hybrids, P31G70, P33F87, P32B34, P31B13 and DKC63-42 and two inbred germplasm lines, PI 639055 and PI 489361, were evaluated. The plants were inoculated with Aspergillus flavus (K-54) at mid-silk stage, and aflatoxin contamination was determined on the kernels at harvest. Several physiological measurements which are indicators of stress response were determined. The results suggested that PI 639055, PI 489361 and hybrid DKC63-42 were more sensitive to drought and high temperature stress in the non-irrigated plots and P31G70 was the most tolerant among all the genotypes. Aflatoxin contamination was the highest in DKC63-42 and PI 489361 but significantly lower in P31G70. However, PI 639055, which is an aflatoxin resistant germplasm, had the lowest aflatoxin contamination, even though it was one of the most stressed genotypes. Possible reasons for these differences are discussed. These results suggested that the physiological responses were associated with the level of aflatoxin contamination in all the genotypes, except PI 639055. These and other physiological responses related to stress may help examine differences among corn genotypes in aflatoxin contamination.

Breeding maize for resistance to ear rot caused by Fusarium moniliforme

Maize ear rots are among the most important impediments to increased maize production in Egypt. The present research was conducted to estimate combining abilities, heterosis and correlation coefficients for resistance to ear rot disease in seven corn inbred lines and their 21 crosses under field conditions. Results demonstrated that both additive and non-additive gene actions were responsible for the genetic expression of all characters with the preponderance of non-additive actions for days to 50% silking. The parental line L51 was the best combiner for earliness, low infection severity %, high phenols content, short plants and reasonable grain yield, while L101 was good combiner for low ear rot infection only. The cross: L122 x L84, L122 x L101, L51 x L101, L76 x L36, L76 x L84, L36 x L84, L36 x L81 and L36 x L101 which involved one or both parents with good General Combining Ability (GCA) effects expressed useful significant heterosis and Specific Combining Ability (SCA) effects for low infection severity %, high phenol contents, early silking, tall plants and high grain yield. Phenotypic and genotypic correlation coefficients suggest that selection for resistance to ear rot should identify lines with high yielding ability, early silking, tall plants, high phenols content and chitinase activity.

Developing resistance to aflatoxin in maize and cottonseed

At this time, no "magic bullet" for solving the aflatoxin contamination problem in maize and cottonseed has been identified, so several strategies must be utilized simultaneously to ensure a healthy crop, free of aflatoxins. The most widely explored strategy for the control of aflatoxin contamination is the development of preharvest host resistance. This is because A. flavus infects and produces aflatoxins in susceptible crops prior to harvest. In maize production, the host resistance strategy has gained prominence because of advances in the identification of natural resistance traits. However, native resistance in maize to aflatoxin contamination is polygenic and complex and, therefore, markers need to be identified to facilitate the transfer of resistance traits into agronomically viable genetic backgrounds while limiting the transfer of undesirable traits. Unlike maize, there are no known cotton varieties that demonstrate enhanced resistance to A. flavus infection and aflatoxin contamination. For this reason, transgenic approaches are being undertaken in cotton that utilize genes encoding antifungal/anti-aflatoxin factors from maize and other sources to counter fungal infection and toxin production. This review will present information on preharvest control strategies that utilize both breeding and native resistance identification approaches in maize as well as transgenic approaches in cotton.

Monitoring the expression of maize genes in developing kernels under drought stress using oligo-microarray

Preharvest aflatoxin contamination of grain grown on the US southeastern Coast Plain is provoked and aggravated by abiotic stress. The primary abiotic stress is drought along with high temperatures. The objectives of the present study were to monitor gene expression in developing kernels in response to drought stress and to identify drought-responsive genes for possible use in germplasm assessment. The maize breeding line Tex6 was used, and gene expression profiles were analyzed in developing kernels under drought stress verses well-watered conditions at the stages of 25, 30, 35, 40, 45 d after pollination (DAP) using the 70 mer maize oligo-arrays. A total of 9 573 positive array spots were detected with unique gene IDs, and 7 988 were common in both stressed and well-watered samples. Expression patterns of some genes in several stress response-associated pathways, including abscisic acid, jasmonic acid and phenylalanine ammonia-lyase, were examined, and these specific genes were responsive to drought stress positively. Real-time quantitative polymerase chain reaction validated microarray expression data. The comparison between Tex6 and B73 revealed that there were significant differences in specific gene expression, patterns and levels. Several defense-related genes had been downregulated, even though some defense-related or drought responsive genes were upregulated at the later stages.

Proteomic analysis of the maize rachis: potential roles of constitutive and induced proteins in resistance to Aspergillus flavus infection and aflatoxin accumulation

Infection of the maize (Zea mays L.) with aflatoxigenic fungus Aspergillus flavus and consequent contamination with carcinogenic aflatoxin is a persistent and serious agricultural problem causing disease and significant crop losses worldwide. The rachis (cob) is an important structure of maize ear that delivers essential nutrients to the developing kernels and A. flavus spreads through the rachis to infect kernels within the ear. Therefore, rachis plays an important role in fungal proliferation and subsequent kernel contamination. We used proteomic approaches and investigated the rachis tissue from aflatoxin accumulation resistant (Mp313E and Mp420) and susceptible (B73 and SC212m) maize inbred lines. First, we compared rachis proteins from resistant and susceptible inbred lines, which revealed that the young resistant rachis contains higher levels of abiotic stress-related proteins and proteins from phenylpropanoid metabolism, whereas susceptible young rachis contains pathogenesis-related proteins, which are generally inducible upon biotic stress. Second, we identified A. flavus-responsive proteins in rachis of both resistant and susceptible genotypes after 10- and 35-day infection. Differential expression of many stress/defense proteins during rachis juvenility, maturation and after A. flavus challenge demonstrates that resistant rachis relies on constitutive defenses, while susceptible rachis is more dependent on inducible defenses.

Microbial symbionts in insects influence down-regulation of defense genes in maize

Diabrotica virgifera virgifera larvae are root-feeding insects and significant pests to maize in North America and Europe. Little is known regarding how plants respond to insect attack of roots, thus complicating the selection for plant defense targets. Diabrotica virgifera virgifera is the most successful species in its genus and is the only Diabrotica beetle harboring an almost species-wide Wolbachia infection. Diabrotica virgifera virgifera are infected with Wolbachia and the typical gut flora found in soil-living, phytophagous insects. Diabrotica virgifera virgifera larvae cannot be reared aseptically and thus, it is not possible to observe the response of maize to effects of insect gut flora or other transient microbes. Because Wolbachia are heritable, it is possible to investigate whether Wolbachia infection affects the regulation of maize defenses. To answer if the success of Diabrotica virgifera virgifera is the result of microbial infection, Diabrotica virgifera virgifera were treated with antibiotics to eliminate Wolbachia and a microarray experiment was performed. Direct comparisons made between the response of maize root tissue to the feeding of antibiotic treated and untreated Diabrotica virgifera virgifera show down-regulation of plant defenses in the untreated insects compared to the antibiotic treated and control treatments. Results were confirmed via QRT-PCR. Biological and behavioral assays indicate that microbes have integrated into Diabrotica virgifera virgifera physiology without inducing negative effects and that antibiotic treatment did not affect the behavior or biology of the insect. The expression data and suggest that the pressure of microbes, which are most likely Wolbachia, mediate the down-regulation of many maize defenses via their insect hosts. This is the first report of a potential link between a microbial symbiont of an insect and a silencing effect in the insect host plant. This is also the first expression profile for a plant attacked by a root-feeding insect.

A novel small antifungal peptide from Bacillus strain B-TL2 isolated from tobacco stems

A novel small antifungal peptide produced by a Bacillus strain B-TL2 isolated from tobacco stems was purified. The purification procedure consisted of ammonium sulfate precipitation, cation exchange chromatography on CM-Sepharose Fast Flow column and reverse-phase HPLC on SOURCE 5RPC column. After the final isolation step, one peptide with antifungal activity, designated as BTL, was obtained. The molecular mass of the purified BTL was determined as 2500 Da and 2237.7 Da by SDS-PAGE and matrix-assisted laser desorption/ionization time of flight mass spectrometry, respectively. The N-amino acid sequence of BTL was determined to be NH(2)-KQQLATEAESAGPIL, which shows relatively low identity to other antimicrobial peptides from bacteria. The peptide exhibited strong inhibitory activity against mycelial growth of Bipolaris maydis, Alternaria brassicae, Aspergillus niger, Cercospora personata. The purified BTL displayed thermostability, almost retaining 100% activity at 100 degrees C for 15 min.

Isolation and characterization of a novel thermostable non-specific lipid transfer protein-like antimicrobial protein from motherwort (Leonurus japonicus Houtt) seeds

In screening for potent antimicrobial proteins from plant seeds, a novel heat-stable antimicrobial protein, designated LJAMP2, was purified from seeds of the motherwort (Leonurus japonicus Houtt), a medicine herb, with a procedure involving cation exchange chromatography on a CM FF column, and reverse phase HPLCs on C8 column and C18 column. LJAMP2 exhibited a molecular mass of 6.2 kDa determined. Automated Edman degradation determined the partial N-terminal sequence of LJAMP2 to be NH2-AIGCNTVASKMAPCLPYVTGKGPLGGCCGGVKGLIDAARTTPDRQAVCNCLKTLAKSYSG, which displays homology with plant non-specific lipid transfer proteins (nsLTPs). In vitro bioassays showed that LJAMP2 inhibits the growth of a variety of microbes, including filamentous fungi, bacteria and yeast. The growth of three phytopathogenic fungi, Alternaria brassicae, Botrytis maydis, and Rhizoctonia cerealis, are inhibited at 7.5 microM of LJAMP2, whereas Bacillus subtilis is about 15 microM. The IC(50) of LJAMP2 for Aspergillus niger, B. maydis, Fusarium oxysporum, Penicillium digitatum and Saccharomyces cerevisiae are 5.5, 6.1, 9.3, 40.0, and 76.0 microM, respectively.

Identification of a Maize Kernel Stress-Related Protein and Its Effect on Aflatoxin Accumulation

Aflatoxins are carcinogens produced mainly by Aspergillus flavus during infection of susceptible crops such as maize. Through proteomic comparisons of maize kernel embryo proteins of resistant and susceptible genotypes, several protein spots previously were found to be unique or upregulated in resistant embryos. In the present study, one of these protein spots was sequenced and identified as glyoxalase I (GLX-I; EC 4.4.1.5). The full-length cDNA of the glyoxalase I gene (glx-I) was cloned. GLX-I constitutive activity was found to be significantly higher in the resistant maize lines compared with susceptible ones. After kernel infection by A. flavus, GLX-I activity remained lower in susceptible genotypes than in resistant genotypes. However, fungal infection significantly increased methylglyoxal (MG) levels in two of three susceptible genotypes. Further, MG was found to induce aflatoxin production in A. flavus culture at a concentration as low as 5.0 μM. The mode of action of MG may be to stimulate the expression of aflR, an aflatoxin biosynthesis regulatory gene, which was found to be significantly upregulated in the presence of 5 to 20 μM MG. These data suggest that GLX-I may play an important role in controlling MG levels inside kernels, thereby contributing to the lower levels of aflatoxins found in resistant maize genotypes.