Unravelling the genetic diversity and population structure of common walnut in the Iranian Plateau

Transcriptome analysis of lipid biosynthesis during kernel development in two walnut (Juglans regia L.) varieties of 'Xilin 3' and 'Xiangling'

BACKGROUND

Walnut is an oilseed tree species and an ecologically important woody tree species that is rich in oil and nutrients. In light of differences in the lipid content, fatty acid composition and key genes expression patterns in different walnut varieties, the key gene regulatory networks for lipid biosynthesis in different varieties of walnuts were intensively investigated.

RESULTS

The kernels of two walnut varieties, 'Xilin 3' (X3) and 'Xiangling' (XL) were sampled at 60, 90, and 120 days post-anthesis (DPA) to construct 18 cDNA libraries, and the candidate genes related to oil synthesis were identified via sequencing and expression analysis. A total of 106 differentially expressed genes associated with fatty acid biosynthesis, fatty acid elongation, unsaturated fatty acid biosynthesis, triglyceride assembly, and oil body storage were selected from the transcriptomes. Weighted gene co-expression network analysis (WGCNA), correlation analysis and quantitative validation confirmed the key role of the FAD3 (109002248) gene in lipid synthesis in different varieties.

CONCLUSIONS

These results provide valuable resources for future investigations and new insights into genes related to oil accumulation and lipid metabolism in walnut seed kernels. The findings will also aid future molecular studies and ongoing efforts to genetically improve walnut.

Population genetic insights into the conservation of common walnut (Juglans regia) in Central Asia

The common walnut (Juglans regia) is one of the most economically important nut trees cultivated worldwide. Despite its importance, no comprehensive evaluation of walnut tree population genetics has been undertaken across the range where it originated, Central Asia. In this study, we investigated the genetic diversity and population structure of 1082 individuals from 46 populations across Central Asia. We found moderate genetic diversity of J. regia across Central Asia, with 46 populations clustered into three groups with a weak relationship between genetic and geographic distance. Our findings reveal that the western Himalaya might be the core region of common walnut genetic diversity in Central Asia and that, except for two populations in Gongliu Wild Walnut Valley, humans might have introduced walnut populations to Xinjiang, China. The observed distribution of the genetic landscape has probably been affected by historical climate fluctuation, breeding system, and prolonged anthropogenic activity. We propose the conservation of the core genetic diversity resources in the western Himalaya and pay special attention to populations from Gongliu in Xinjiang. These findings enhance our understanding of the genetic variation throughout the distribution range of J. regia in Central Asia, which will provide a key prerequisite for evidence-based conservation and management.

Molecular Characterization of Local Walnut (Juglans regia) Genotypes in the North-East Parnon Mountain Region of Greece

Walnut is one of the most important nuts regarding their production and consumption. The available but uncharacterized genetic resources of walnut are important for the development and breeding of local varieties. Greece holds an important number of genetically uncharacterized walnut landraces, especially within the area of Parnon, which is considered to play a significant role as an in situ gene bank, due to its unique location traits. However, the genetic characterization and further use of these resources has been insufficient, due to the absence of genetic studies. In this study, we implemented SSR molecular markers, both to genetically characterize the walnut tree genetic diversity of the Parnon area and to identify its unique genetic structure, which will form the starting material for subsequent breeding programs. Overall, high levels of genetic variation were found among the individual walnut accessions that were collected in the Parnon mountain region.

A Multiplex PCR System of Novel Microsatellite Loci for Population Genetic Application in Walnuts

Multiplex polymerase chain reaction (PCR) of microsatellite loci allows for simultaneous amplification of two or more pairs of primers in a single PCR reaction; hence, it is cost and time effective. However, very few attempts have been reported in non-model species. In this study, by combining a genome-based de novo development and cross-species application approach, a multiplex PCR system comprising 5 PCR reactions of 33 microsatellites consisting of 26 novel genomic and 7 literature-sourced loci was tested for polymorphisms, cross-species transferability, and the ability to assess genetic diversity and population structure of three walnut species (Juglans spp.). We found that the genome-based approach is more efficient than other methods. An allelic ladder was developed for each locus to enhance consistent genotyping among laboratories. The population genetic analysis results showed that all 33 loci were successfully transferred across the three species, showing high polymorphism and a strong genetic structure. Hence, the multiplex PCR system is highly applicable in walnut species. Furthermore, we propose an efficient pipeline to characterize and genotype polymorphic microsatellite loci. The novel toolbox developed here will aid future ecology and evolution studies in walnut and could serve as a model for other plant species.

Exploring the phenolic profile, antibacterial, and antioxidant properties of walnut leaves (Juglans regia L.)

The aim of this study was to identify phenolic compounds in walnut leaves from northern Iraq and evaluate their ability to act as antibacterial and antioxidant agents. Phenolic compounds were determined by reversed-phase HPLC. Antibacterial activity was tested against various bacteria. Antioxidant properties were evaluated by various assays, including reducing power and DPPH radical scavenging activity. The HPLC profiles of walnut leaf fractions revealed quercetin, hydroquinone, 4-hydroxybenzoic acid, and caffeic acid in three fractions. The inhibitory activity of DPPH was determined as 47.66, 32.41, and 51.90 μg/mL for fractions I, II, and III, respectively. For ferric reducing power activity, fraction II > fraction III > fraction I and the FRAP activity was observed as 64.43, 73.19, and 68.18 μg/mL for fractions I, II, and III, respectively. All extracted fractions had antibacterial properties against all bacterial strains tested. Observations showed that fraction I was able to produce similar zones of inhibition as streptomycin in most cases. These results suggest that the fractions of this plant extract are plausible natural antioxidants that could be used as prime candidates for the synthesis of antioxidant drugs that can be used for the treatment of many oxidative stress-related diseases.

Genetic imprints of grafting in wild iron walnut populations in southwestern China

BACKGROUND

Anthropogenic activities are causing unprecedented loss of genetic diversity in many species. However, the effects on genetic diversity from large-scale grafting onto wild plants of crop species are largely undetermined. Iron walnut (Juglans sigillata Dode) is a deciduous nut tree crop endemic to southwestern China with a long history of cultivation. Due to the rapid expansion of the walnut industry, many natural populations are now being replaced by cultivars grafted onto wild rootstocks. However, little is known about the potential genetic consequences of such action on natural populations.

RESULTS

We sampled the scion and the rootstock from each of 149 grafted individuals within nine wild populations of J. sigillata from Yunnan Province which is the center of walnut diversity and cultivation in China, and examined their genetic diversity and population structure using 31 microsatellite loci. Scions had lower genetic diversity than rootstocks, and this pattern was repeated in seven of the nine examined populations. Among those seven populations, AMOVA and clustering analyses showed a clear genetic separation between all rootstocks and all scions. However, the two remaining populations, both from northern Yunnan, showed genetic similarity between scions and rootstocks, possibly indicating that wild populations here are derived from feralized local cultivars. Moreover, our data indicated probable crop-to-wild gene flow between scions and rootstocks, across all populations.

CONCLUSIONS

Our results indicate that large-scale grafting has been causing genetic diversity erosion and genetic structure breakdown in the wild material of J. sigillata within Yunnan. To mitigate these effects, we caution against the overuse of grafting in wild populations of iron walnut and other crop species and recommend the preservation of natural genotypes through in situ  and ex situ conservation.