Ayed M. Al-Abdallat

The Highly Similar Arabidopsis Homologs of Trithorax ATX1 and ATX2 Encode Proteins with Divergent Biochemical Functions

Gene duplication followed by functional specialization is a potent force in the evolution of biological diversity. A comparative study of two highly conserved duplicated genes, ARABIDOPSIS TRITHORAX-LIKE PROTEIN1 (ATX1) and ATX2, revealed features of both partial redundancy and of functional divergence. Although structurally similar, their regulatory sequences have diverged, resulting in distinct temporal and spatial patterns of expression of the ATX1 and ATX2 genes. We found that ATX2 methylates only a limited fraction of nucleosomes and that ATX1 and ATX2 influence the expression of largely nonoverlapping gene sets. Even when coregulating shared targets, ATX1 and ATX2 may employ different mechanisms. Most remarkable is the divergence of their biochemical activities: both proteins methylate K4 of histone H3, but while ATX1 trimethylates it, ATX2 dimethylates it. ATX2 and ATX1 provide an example of separated K4 di from K4 trimethyltransferase activity.

The Arabidopsis homologs of trithorax (ATX1) and enhancer of zeste (CLF) establish ‘bivalent chromatin marks’ at the silent AGAMOUS locus

Tightly balanced antagonism between the Polycomb group (PcG) and the Trithorax group (TrxG) complexes maintain Hox expression patterns in Drosophila and murine model systems. Factors belonging to the PcG/TrxG complexes control various processes in plants as well but whether they participate in mechanisms that antagonize, balance or maintain each others effects at a particular gene locus is unknown. CURLY LEAF (CLF), an Arabidopsis homolog of enhancer of zeste (EZ) and the ARABIDOPSIS HOMOLOG OF TRITHORAX (ATX1) control the expression of the flower homeotic gene AGAMOUS (AG). Disrupted ATX1 or CLF function results in misexpression of AG, recognizable phenotypes and loss of H3K4me3 or H3K27me3 histone H3-tail marks, respectively. A novel idea suggested by our results here, is that PcG and TrxG complexes function as a specific pair generating bivalent chromatin marks at the silent AG locus. Simultaneous loss of ATX1 and CLF restored AG repression and normalized leaf phenotypes. At the molecular level, disrupted ATX1 and CLF functions did not lead to erasure of the CLF- and ATX1-generated epigenetic marks, as expected: instead, in the double mutants, H3K27me3 and H3K4me3 tags were partially restored. We demonstrate that ATX1 and CLF physically interact linking mechanistically the observed effects.

Over-expression of SlSHN1 gene improves drought tolerance by increasing cuticular wax accumulation in tomato.

Increasing cuticular wax accumulation in plants has been associated with improving drought tolerance in plants. In this study, a cDNA clone encoding the SlSHN1 transcription factor, the closest ortholog to WIN/SHN1 gene in Arabidopsis, was isolated from tomato plant. Expression analysis of SlSHN1 indicated that it is induced in response to drought conditions. The over-expression of SlSHN1 in tomato under the control of the constitutive CaMV 35S promoter produced plants that showed mild growth retardation phenotype with shiny and dark green leaves. Scanning electron microscopy showed that the over-expression of SlSHN1 in tomato resulted in higher cuticular wax deposition on leaf epidermial tissue when compared to non-transformed plants. Expression analysis in transgenic lines over-expressing SlSHN1 indicated that several wax-related synthesis genes were induced. Transgenic tomato plants over-expressing SlSHN1 showed higher drought tolerance when compared with wild type plants; this was reflected in delayed wilting of transgenic lines, improved water status and reduced water loss rate when compared with wild type plants. In conclusion, the SlSHN1 gene can modulate wax accumulation and could be utilized to enhance drought tolerance in tomato plant.

Genetic Diversity within a Global Panel of Durum Wheat (Triticum durum) Landraces and Modern Germplasm Reveals the History of Alleles Exchange

Durum wheat is the 10th most important crop in the world, and its use traces back to the origin of agriculture. Unfortunately, in the last century only part of the genetic diversity available for this species has been captured in modern varieties through breeding. Here, the population structure and genetic diversity shared among elites and landraces collected from 32 countries was investigated. A total of 370 entries were genotyped with Axiom 35K array to identify 8,173 segregating single nucleotide polymorphisms (SNPs). Of these, 500 were selected as highly informative with a PIC value above 0.32 and used to test population structure via DAPC, STRUCTURE, and neighbor joining tree. A total of 10 sub-populations could be identified, six constituted by modern germplasm and four by landraces of different geographical origin. Interestingly, genomic comparison among groups indicated that Middle East and Ethiopia had the lowest level of allelic diversity, while breeding programs and landraces collected outside these regions were the richest in rare alleles. Further, phylogenetic analysis among landraces indicated that Ethiopia might represent a second center of origin of durum wheat, rather than a second domestication site as previously believed. Together, the analyses carried here provide a global picture of the available genetic diversity for this crop and shall guide its targeted use by breeders.

Molecular and Field Comparison of Selected Barley Cultivars for Drought Tolerance

A field experiment was conducted using five most-planted barley (Hordeum vulgare L.) cultivars in Jordan — a country with 90% of its total land area receiving an annual rainfall of less than 200 mm. The objective of this study was to determine the response of these five cultivars evaluated under drought-stressed (DS) and non-stressed (NS) environments and to investigate polymorphism of 10 candidate genes for drought response. Drought susceptibility index (DSI) was used to rank cultivars according to their performance. Biological and grain yield for all cultivars was lower in DS compared with NS. Reduction in biological yield caused by drought stress ranged from 19% for ‘Mutah’ to 45% for ‘Rum’ cultivar. The least affected by drought stress was ‘Mutah’ with 13% percent reduction (PR) in grain yield. DSI for grain yield ranged from 0.33 for ‘Mutah’ to 1.41 for ‘Rum’. Depending on PR and DSI for biological and grain yield, ‘Mutah’ was the most tolerant cultivar. However, no clear differences were detected among cultivars for the tested genes. The use of this tolerant and improved genotype should be maximized in breeding and genetic studies to identify, map, and pyramid new drought-tolerance genes and to enhance sustainable farming systems.

In vitro response of two citrus rootstocks to salt stress.

The in vitro response of two citrus rootstocks (sour orange, Citrus aurantium L. and Volkamer lemon, Citrus volkameriana Ten. & Pasq.) to two types of salts (NaCl and CaCl2) of different concentrations (0, 50, 100, 150, 200, or 300 mM) was investigated. Results showed that increasing NaCl level in the growth medium led to increased Na and Cl accumulation and decreased Ca concentrations in plant tissue. Increasing CaCl2 levels in the growth medium led to increased Ca accumulation and decreased Na concentrations in plant tissue. Plant K concentrations decreased with increased salinity level in the media. In general, increased salinity level in growth medium, using NaCl, CaCl2, or a combination of the two, led to reduced plant growth (leaf number, plant length, fresh weight, dry weight, and increased plant leaf damage) in Volkamer lemon and sour orange after two months in culture.

Genetic and Phenotypic Variation Among Faba Bean Landraces and Cultivars

Landraces of crops can contain phenotypic and genotypic variability that can be exploited so that new lines can be developed. The phenotypic and genotypic variability among 11 Jordanian faba bean (Vicia faba L.) landraces and five imported cultivars were studied. Yield and 12 yield components and yield-related traits were evaluated at two locations. Analysis indicated significant variability among germplasm for all characters and among locations for 10 characters, in which final plant height, pods/plant, and seeds/pod were not significant. Genetic variability and relationships among germplasm were established using 12 single sequence repeat (SSR) primer pairs. Eleven primers were particularly efficient in amplification of banding patterns among germplasm. The overall polymorphic percentage for the 31 loci generated by 11 SSR markers was 84%. This was sufficient to estimate genetic variation among faba bean germplasm. The unweighted pair group method with arithmetic average ordered germplasm into three groups based on a Dice similarity coefficient of 0.71, range 0.7 to 0.89. Clustering based on phenotypical traits indicated five smaller groups that were consistent with the three larger SSR-based groups. These landraces can be used for future faba bean breeding programs and derivation of new plant lines.

Assessment of Genetic Diversity among Pleurotus spp. Isolates from Jordan

Pleurotus is considered an important genus that belongs to the family Pleurotaceae and includes the edible King Oyster mushroom (Pleurotus eryngii). In the present study, 19 Pleurotus isolates were collected from two locations in the north of Jordan (Tell ar-Rumman and Um-Qais). The morphological characteristics among collected isolates revealed that there was a morphological similarity among the collected isolates. Nucleotide sequence analysis of the internal transcribed spacer (ITS1–5.8S rDNA–ITS4 region) and 28S nuclear large subunit (nLSU) in the ribosomal DNA gene of the isolated stains showed that all of them share over 98% sequence similarity with P. eryngii. Genetic diversity among the collected strains was assessed using inter simple sequence repeat (ISSR) analysis using 18 different primer pairs. Using this approach, 141 out of 196 bands obtained were considered polymorphic and the highest percentage of polymorphism was observed using primer UBC827 (92.3%) with an overall Polymorphism Information Content (PIC) value of 70.56%. Cluster analysis showed that the Jordanian Pleurotus isolates fall into two main clades with a coefficient of similarity values ranging from 0.59 to 0.74 with a clear clustering based on collection sites. The results of the present study reveal that molecular techniques of ISSR and rDNA sequencing can greatly aid in classification and identification of Pleurotus spp. in Jordan.

In Vitro Preservation of Transgenic Tomato (Solanum lycopersicum L.) Plants Overexpressing the Stress-Related SlAREB1 Transcription Factor

In vitro preservation of transgenic tomato lines overexpressing the stress-responsive transcription factor SlAREB1 was studied by using slow growth and cryopreservation techniques. Slow growth preservation was performed by using different concentrations of sucrose (0, 100, 200, 300 mm) and abscisic acid (0, 4, 8, 12 μm) in Murashige and Skoog (MS) media, while cryopreservation was conducted by using encapsulation dehydration, V-cryoplates and seeds. Significant differences were observed between tested lines grown on MS media supplemented with 200 mm sucrose where transgenic lines overexpressing SlAREB1 showed improved growth when compared with negative control. The addition of abscisic acid (ABA) to the preservation media affected negatively transgenic lines growth and development when compared with ABA-free media. In encapsulation dehydration, non-cryopreserved transgenic lines overexpressing SlAREB1 pretreated in 0.8 M sucrose for 1 day and subjected to different dehydration periods showed significantly higher survival percentages when compared with negative control. For V-cryoplates technique, cryopreserved transgenic lines overexpressing SlAREB1 treated in 0.3 M sucrose for 3 days with or without cold acclimatization showed significantly higher survival percentages when compared with the negative control. Seed cryopreservation was performed successfully with a clear reduction in germination percentage in transgenic lines overexpressing high levels of SlAREB1. In conclusion, transgenic tomato lines overexpressing SlAREB1 were found to improve tolerance against different abiotic stresses associated with different in vitro preservation protocols.

Genome Wide Association Mapping of Seedling and Adult Plant Resistance to Barley Stripe Rust (Puccinia striiformis f. sp. hordei) in India

Barley stripe rust is caused by Puccinia striiformis f.sp. hordei, (Psh), occurs worldwide, and is a major disease in South Asia. The aim of this work was to identify and estimate effects of loci underlying quantitative resistance to rust at seedling and adult plant stages. HI-AM panel of 261 barley genotypes consisting of released cultivars from North and South America, Europe, Australia, advanced breeding lines, and local landraces from ICARDA barley program were screened at seedling and adult plant stages for resistance to Psh. Seedling resistance was evaluated with the five prevalent Psh races in India. Screening for the adult plant stage resistance was also performed in two different locations by inoculating with a mixture of the five races used for seedling screeing. The panel was genotyped using DaRT-Seq high-throughput genotyping platform. The genome-wide association mapping (GWAM) showed a total of 45 QTL located across the seven barley chromosomes for seedling resistance to the five races and 18 QTL for adult plant stage resistance. Common QTL for different races at seedling stage were found on all chromosomes except on chromosome 1H. Four common QTL associated with seedling and adult plant stage resistance were found on chromosomes 2, 5, and 6H. Moreover, one of the QTL located on the long arm of chromosome 5H showed stable effects across environments for adult plant stage resistance. Several QTL identified in this study were also reported before in bi-parental and association mapping populations studies validating current GWAM. However 15 new QTL were found at adult plant stage on all chromosomes except the 4H, explaining up to 36.79% of the variance. The promising QTL detected at both stages, once validated, can be used for MAS in Psh resistance breeding program globally.