Characterization of genetic diversity and linkage disequilibrium of ZmLOX4 and ZmLOX5 loci in maize

Identification and Expression Analysis of Lipoxygenase Gene in Bitter Gourd (Momordica charantia)

BACKGROUND

Lipoxygenases (LOXs) are key enzymes in the unsaturated fatty acid oxidation reaction pathway and play an important regulatory role in the synthesis of fruit aroma volatiles.

METHODS

LOX gene family members were identified in the whole genome database of bitter gourd and analyzed bioinformatically. An RT-qPCR was used to analyze the expression differences in different tissues. Monoterpenes were determined by gas chromatography-mass spectrometry (GC-MS) technique.

RESULTS

A total of 12 LOX gene family members were identified in the genome. The expression of LOX genes varied significantly among the tissues of roots, stems, leaves, flowers, fruits, seeds and tendrils. A total of 29 monoterpenes were detected in the fruits of five different fruit colors of bitter gourd, mainly containing six types of alcohols, aldehydes, terpenes, ketones, esters and alkynes, with the highest relative content of alcohols.

CONCLUSIONS

The present study provides a reference for further elucidation of the biological functions of the LOX gene in the synthesis pathway of aroma volatiles in bitter gourd.

Duplicated Copy Number Variant of the Maize 9-Lipoxygenase ZmLOX5 Improves 9,10-KODA-Mediated Resistance to Fall Armyworms

Extensive genome structure variations, such as copy number variations (CNVs) and presence/absence variations, are the basis for the remarkable genetic diversity of maize; however, the effect of CNVs on maize herbivory defense remains largely underexplored. Here, we report that the naturally occurring duplication of the maize 9-lipoxygenase gene ZmLOX5 leads to increased resistance of maize to herbivory by fall armyworms (FAWs). Previously, we showed that ZmLOX5-derived oxylipins are required for defense against chewing insect herbivores and identified several inbred lines, including Yu796, that contained duplicated CNVs of ZmLOX5, referred to as Yu796-2×LOX5. To test whether introgression of the Yu796-2×LOX5 locus into a herbivore-susceptible B73 background that contains a single ZmLOX5 gene is a feasible approach to increase resistance, we generated a series of near-isogenic lines that contained either two, one, or zero copies of the Yu796-2×LOX5 locus in the B73 background via six backcrosses (BC6). Droplet digital PCR (ddPCR) confirmed the successful introgression of the Yu796-2×LOX5 locus in B73. The resulting B73-2×LOX5 inbred line displayed increased resistance against FAW, associated with increased expression of ZmLOX5, increased wound-induced production of its primary oxylipin product, the α-ketol, 9-hydroxy-10-oxo-12(Z),15(Z)-octadecadienoic acid (9,10-KODA), and the downstream defense hormones regulated by this molecule, 12-oxo-phytodienoic acid (12-OPDA) and abscisic acid (ABA). Surprisingly, wound-induced JA-Ile production was not increased in B73-2×LOX5, resulting from the increased JA catabolism. Furthermore, B73-2×LOX5 displayed reduced water loss in response to drought stress, likely due to increased ABA and 12-OPDA content. Taken together, this study revealed that the duplicated CNV of ZmLOX5 quantitively contributes to maize antiherbivore defense and presents proof-of-concept evidence that the introgression of naturally occurring duplicated CNVs of a defensive gene into productive but susceptible crop varieties is a feasible breeding approach for enhancing plant resistance to herbivory and tolerance to abiotic stress.

Functional Study of Lipoxygenase-Mediated Resistance against Fusarium verticillioides and Aspergillus flavus Infection in Maize

Mycotoxin contamination of maize kernels by fungal pathogens like Fusarium verticillioides and Aspergillus flavus is a chronic global challenge impacting food and feed security, health, and trade. Maize lipoxygenase genes (ZmLOXs) synthetize oxylipins that play defense roles and govern host-fungal interactions. The current study investigated the involvement of ZmLOXs in maize resistance against these two fungi. A considerable intraspecific genetic and transcript variability of the ZmLOX family was highlighted by in silico analysis comparing publicly available maize pan-genomes and pan-transcriptomes, respectively. Then, phenotyping and expression analysis of ZmLOX genes along with key genes involved in oxylipin biosynthesis were carried out in a maize mutant carrying a Mu transposon insertion in the ZmLOX4 gene (named UFMulox4) together with Tzi18, Mo17, and W22 inbred lines at 3- and 7-days post-inoculation with F. verticillioides and A. flavus. Tzi18 showed the highest resistance to the pathogens coupled with the lowest mycotoxin accumulation, while UFMulox4 was highly susceptible to both pathogens with the most elevated mycotoxin content. F. verticillioides inoculation determined a stronger induction of ZmLOXs and maize allene oxide synthase genes as compared to A. flavus. Additionally, oxylipin analysis revealed prevalent linoleic (18:2) peroxidation by 9-LOXs, the accumulation of 10-oxo-11-phytoenoic acid (10-OPEA), and triglyceride peroxidation only in F. verticillioides inoculated kernels of resistant genotypes.

Characterization of the maize lipoxygenase gene family in relation to aflatoxin accumulation resistance

Maize (Zea mays L.) is a globally important staple food crop prone to contamination by aflatoxin, a carcinogenic secondary metabolite produced by the fungus Aspergillus flavus. An efficient approach to reduce accumulation of aflatoxin is the development of germplasm resistant to colonization and toxin production by A. flavus. Lipoxygenases (LOXs) are a group of non-heme iron containing dioxygenase enzymes that catalyze oxygenation of polyunsaturated fatty acids (PUFAs). LOX derived oxylipins play critical roles in plant defense against pathogens including A. flavus. The objectives of this study were to summarize sequence diversity and expression patterns for all LOX genes in the maize genome, and map their effect on aflatoxin accumulation via linkage and association mapping. In total, 13 LOX genes were identified, characterized, and mapped. The sequence of one gene, ZmLOX10, is reported from 5 inbred lines. Genes ZmLOX1/2, 5, 8, 9, 10 and 12 (GRMZM2G156861, or V4 numbers ZM00001D042541 and Zm00001D042540, GRMZM2G102760, GRMZM2G104843, GRMZM2G017616, GRMZM2G015419, and GRMZM2G106748, respectively) fell under previously published QTL in one or more mapping populations and are linked to a measurable reduction of aflatoxin in maize grains. Association mapping results found 28 of the 726 SNPs tested were associated with reduced aflatoxin levels at p ≤ 9.71 x 10-4 according to association statistics. These fell within or near nine of the ZmLOX genes. This work confirms the importance of some lipoxygenases for resistance to aflatoxin accumulation and may be used to direct future genetic selection in maize.

Synthesis and Functions of Jasmonates in Maize

Of the over 600 oxylipins present in all plants, the phytohormone jasmonic acid (JA) remains the best understood in terms of its biosynthesis, function and signaling. Much like their eicosanoid analogues in mammalian system, evidence is growing for the role of the other oxylipins in diverse physiological processes. JA serves as the model plant oxylipin species and regulates defense and development. For several decades, the biology of JA has been characterized in a few dicot species, yet the function of JA in monocots has only recently begun to be elucidated. In this work, the synthesis and function of JA in maize is presented from the perspective of oxylipin biology. The maize genes responsible for catalyzing the reactions in the JA biosynthesis are clarified and described. Recent studies into the function of JA in maize defense against insect herbivory, pathogens and its role in growth and development are highlighted. Additionally, a list of JA-responsive genes is presented for use as biological markers for improving future investigations into JA signaling in maize.

Genetic variation, population structure and linkage disequilibrium in Switchgrass with ISSR, SCoT and EST-SSR markers

Background

To evaluate genetic variation, population structure, and the extent of linkage disequilibrium (LD), 134 switchgrass (Panicum virgatum L.) samples were analyzed with 51 markers, including 16 ISSRs, 20 SCoTs, and 15 EST-SSRs.

Results

In this study, a high level of genetic variation was observed in the switchgrass samples and they had an average Nei’s gene diversity index (H) of 0.311. A total of 793 bands were obtained, of which 708 (89.28 %) were polymorphic. Using a parameter marker index (MI), the efficiency of the three types of markers (ISSR, SCoT, and EST-SSR) in the study were compared and we found that SCoT had a higher marker efficiency than the other two markers. The 134 switchgrass samples could be divided into two sub-populations based on STRUCTURE, UPGMA clustering, and principal coordinate analyses (PCA), and upland and lowland ecotypes could be separated by UPGMA clustering and PCA analyses. Linkage disequilibrium analysis revealed an average r² of 0.035 across all 51 markers, indicating a trend of higher LD in sub-population 2 than that in sub-population 1 (P < 0.01).

Conclusions

The population structure revealed in this study will guide the design of future association studies using these switchgrass samples.

Resistance to Fusarium verticillioides and fumonisin accumulation in maize inbred lines involves an earlier and enhanced expression of lipoxygenase (LOX) genes

Fusarium verticillioides causes ear rot in maize and contaminates the kernels with the fumonisin mycotoxins. It is known that plant lipoxygenase (LOX)-derived oxylipins regulate defence against pathogens and that the host-pathogen lipid cross-talk influences the pathogenesis. The expression profiles of fifteen genes of the LOX pathway were studied in kernels of resistant and susceptible maize lines, grown in field condition, at 3, 7 and 14 days post inoculation (dpi) with F. verticillioides. Plant defence responses were correlated with the pathogen growth, the expression profiles of fungal FUM genes for fumonisin biosynthesis and fumonisin content in the kernels. The resistant genotype limited fungal growth and fumonisin accumulation between 7 and 14 dpi. Pathogen growth became exponential in the susceptible line after 7 dpi, in correspondence with massive transcription of FUM genes and fumonisins augmented exponentially at 14 dpi. LOX pathway genes resulted strongly induced after pathogen inoculation in the resistant line at 3 and 7 dpi, whilst in the susceptible line the induction was reduced or delayed at 14 dpi. In addition, all genes resulted overexpressed before infection in kernels of the resistant genotype already at 3 dpi. The results suggest that resistance in maize may depend on an earlier activation of LOX genes and genes for jasmonic acid biosynthesis.

Genome wide association study for drought, aflatoxin resistance, and important agronomic traits of maize hybrids in the sub-tropics

The primary maize (Zea mays L.) production areas are in temperate regions throughout the world and this is where most maize breeding is focused. Important but lower yielding maize growing regions such as the sub-tropics experience unique challenges, the greatest of which are drought stress and aflatoxin contamination. Here we used a diversity panel consisting of 346 maize inbred lines originating in temperate, sub-tropical and tropical areas testcrossed to stiff-stalk line Tx714 to investigate these traits. Testcross hybrids were evaluated under irrigated and non-irrigated trials for yield, plant height, ear height, days to anthesis, days to silking and other agronomic traits. Irrigated trials were also inoculated with Aspergillus flavus and evaluated for aflatoxin content. Diverse maize testcrosses out-yielded commercial checks in most trials, which indicated the potential for genetic diversity to improve sub-tropical breeding programs. To identify genomic regions associated with yield, aflatoxin resistance and other important agronomic traits, a genome wide association analysis was performed. Using 60,000 SNPs, this study found 10 quantitative trait variants for grain yield, plant and ear height, and flowering time after stringent multiple test corrections, and after fitting different models. Three of these variants explained 5-10% of the variation in grain yield under both water conditions. Multiple identified SNPs co-localized with previously reported QTL, which narrows the possible location of causal polymorphisms. Novel significant SNPs were also identified. This study demonstrated the potential to use genome wide association studies to identify major variants of quantitative and complex traits such as yield under drought that are still segregating between elite inbred lines.

Effects of elevated [CO2 ] on maize defence against mycotoxigenic Fusarium verticillioides

Maize is by quantity the most important C4 cereal crop; however, future climate changes are expected to increase maize susceptibility to mycotoxigenic fungal pathogens and reduce productivity. While rising atmospheric [CO2 ] is a driving force behind the warmer temperatures and drought, which aggravate fungal disease and mycotoxin accumulation, our understanding of how elevated [CO2 ] will effect maize defences against such pathogens is limited. Here we report that elevated [CO2 ] increases maize susceptibility to Fusarium verticillioides proliferation, while mycotoxin levels are unaltered. Fumonisin production is not proportional to the increase in F. verticillioides biomass, and the amount of fumonisin produced per unit pathogen is reduced at elevated [CO2 ]. Following F. verticillioides stalk inoculation, the accumulation of sugars, free fatty acids, lipoxygenase (LOX) transcripts, phytohormones and downstream phytoalexins is dampened in maize grown at elevated [CO2 ]. The attenuation of maize 13-LOXs and jasmonic acid production correlates with reduced terpenoid phytoalexins and increased susceptibility. Furthermore, the attenuated induction of 9-LOXs, which have been suggested to stimulate mycotoxin biosynthesis, is consistent with reduced fumonisin per unit fungal biomass at elevated [CO2 ]. Our findings suggest that elevated [CO2 ] will compromise maize LOX-dependent signalling, which will influence the interactions between maize and mycotoxigenic fungi.

The novel monocot-specific 9-lipoxygenase ZmLOX12 is required to mount an effective jasmonate-mediated defense against Fusarium verticillioides in maize

Fusarium verticillioides is a major limiting factor for maize production due to ear and stalk rot and the contamination of seed with the carcinogenic mycotoxin fumonisin. While lipoxygenase (LOX)-derived oxylipins have been implicated in defense against diverse pathogens, their function in maize resistance against F. verticillioides is poorly understood. Here, we functionally characterized a novel maize 9-LOX gene, ZmLOX12. This gene is distantly related to known dicot LOX genes, with closest homologs found exclusively in other monocot species. ZmLOX12 is predominantly expressed in mesocotyls in which it is strongly induced in response to F. verticillioides infection. The Mutator transposon-insertional lox12-1 mutant is more susceptible to F. verticillioides colonization of mesocotyls, stalks, and kernels. The infected mutant kernels accumulate a significantly greater amount of the mycotoxin fumonisin. Reduced resistance to the pathogen is accompanied by diminished levels of the jasmonic acid (JA) precursor 12-oxo phytodienoic acid, JA-isoleucine, and expression of jasmonate-biosynthetic genes. Supporting the strong defense role of jasmonates, the JA-deficient opr7 opr8 double mutant displayed complete lack of immunity to F. verticillioides. Unexpectedly, the more susceptible lox12 mutant accumulated higher levels of kauralexins, suggesting that F. verticillioides is tolerant to this group of antimicrobial phytoalexins. This study demonstrates that this unique monocot-specific 9-LOX plays a key role in defense against F. verticillioides in diverse maize tissues and provides genetic evidence that JA is the major defense hormone against this pathogen.

Aspergillus flavus infection induces transcriptional and physical changes in developing maize kernels

Maize kernels are susceptible to infection by the opportunistic pathogen Aspergillus flavus. Infection results in reduction of grain quality and contamination of kernels with the highly carcinogenic mycotoxin, aflatoxin. To understanding host response to infection by the fungus, transcription of approximately 9000 maize genes were monitored during the host-pathogen interaction with a custom designed Affymetrix GeneChip® DNA array. More than 4000 maize genes were found differentially expressed at a FDR of 0.05. This included the up regulation of defense related genes and signaling pathways. Transcriptional changes also were observed in primary metabolism genes. Starch biosynthetic genes were down regulated during infection, while genes encoding maize hydrolytic enzymes, presumably involved in the degradation of host reserves, were up regulated. These data indicate that infection of the maize kernel by A. flavus induced metabolic changes in the kernel, including the production of a defense response, as well as a disruption in kernel development.