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Juglans sigillata Dode is rich in flavonoids, but the low ratio of female to male flower buds limits the development of the J. sigillata industry. While the abundance of flavonoids in J. sigillata is known, whether flavonoids influence female flower bud differentiation has not been reported. In this study, we explored the regulatory mechanisms of gene expression and metabolite accumulation during female flower bud differentiation through integrated transcriptomic and metabolomic analyses. Our findings revealed that flavonoid biosynthesis is a key pathway influencing female flower bud differentiation, with metabolites primarily shifting towards the isoflavonoid, flavone, and flavonol branches. Structural genes such as chalcone synthase , dihydroflavonol 4-reductase , flavonol synthase , and flavonoid 3',5'-hydroxylase were identified as playing crucial regulatory roles. The expression of these genes promoted the accumulation of flavonoids, which in turn influenced female flower bud differentiation by modulating key regulatory genes including Suppressor of Overexpression of Constans1 , Constans , Flowering Locus T , and APETALA1 . Furthermore, transcription factors (TFs) highly expressed during the physiological differentiation of female flower buds, particularly M-type MADS, WRKY, and MYB, were positively correlated with flavonoid biosynthesis genes, indicating their significant role in the regulation of flavonoid production. These results offer valuable insights into the mechanisms of female flower bud differentiation in J. sigillata and highlight the regulatory role of flavonoids in plant bud differentiation.

Background Auxin response factor (ARF), a transcription factors that controls the expression of genes responsive to auxin, plays a key role in the regulation of plant growth and development. Analyses aimed at identifying ARF family genes and characterizing their functions in Juglans sigillata Dode are lacking. Methods and results We used bioinformatic approaches to identify members of the J. sigillata ARF gene family and analyze their evolutionary relationships, collinearity, cis -acting elements, and tissue-specific expression patterns. The expression patterns of ARF gene family members under natural drought conditions were also analyzed. The J. sigillata ARF gene family contained 31 members, which were unevenly distributed across 16 chromosomes. We constructed a phylogenetic tree of JsARF genes and other plant ARF genes. Cis -acting elements in the promoters of JsARF were predicted. JsARF28 showed higher expressions in both the roots and leaves. A heat map of the transcriptome data of the cluster analysis under drought stress indicated that JsARF3/9/11/17/20/26 are responsive to drought. The expression of the 11 ARF genes varied under PEG treatment and JsARF18 and JsARF20 were significantly up-regulated. Conclusions The interactions between abiotic stresses and plant hormones are supported by our cumulative data, which also offers a theoretical groundwork for comprehending the ARF mechanism and drought resistance in J. sigillata .

An efficient genetic transformation system is of great significance for verifying gene function and improving plant breeding efficiency by gene engineering. In this study, a stable Agrobacterium mediated genetic transformation system of Juglans sigillata Dode ‘Qianhe-7’ was investigated using callus and negative pressure-assisted and ultrasonic-assisted transformation selection. The results showed that the axillary shoot leaves were suitable to induce callus and the callus proliferation rate could reach 516.27% when induction calli were cultured on DKW medium containing 0.5 mg L−1 indole-3-butyric acid, 1.2 mg L−1 2,4-dichlorophenoxyacetic acid and 0.5 mg L−1 kinetin for 18 d. In addition, negative pressure infection was the optimal infection method with the lowest browning rate (0.00%), high GFP conversion rate (16.67%), and better growth status. To further prove the feasibility of this genetic transformation system, the flavonol synthetase (JsFLS5) gene was successfully transformed into the into leaf-derived callus of ‘Qianhe-7’. JsFLS5 expression and the content of total flavonoids in transformed callus were improved significantly compared with the untransformed callus, which proved that we had an efficient and reliable genetic transformation system using leaf-derived callus of Juglans sigillata.

Background Observations of precocious (early bearing) genotypes of walnut ( Juglans regia L.) under natural conditions encouraged us to study the origin and genetic control of these fascinating traits. Results In this study, the self-fertility, progeny performance, and simple sequence repeat (SSR) locus variation of iron walnut ( Juglans sigillata Dode), an ecotype of J. regia , were investigated. The average self-pollinated fruit set rate of J. sigillata cv. ‘Dapao’ (DP) was 7.0% annually from 1979 to 1982. The average germination rate of self-pollinated seeds was 45.2% during the 4-year period. Most progeny had inbreeding depression. Nine representative self-pollinated progeny (SP 1 –SP 9 ), with special or typical traits of DP, were selected. SP 1 –SP 4 were precocious because they initiated flowers as early as 2 years after germination, compared to the 7–10-yr period that is typical of DP. SP 9 had not flowered since 1980. Twelve SSR markers were used to analyze the SP and DP. The genome of SP had a tendency toward high levels of homozygosity. The high levels of homozygosity reported in 18 additional precocious walnut genotypes complemented the results of this study. Conclusions These results provide evidence of precocious phenotypes and genomes with high levels of homozygosity that might be generated from self-pollinating walnut. This suggests that self-pollination might facilitate the generation of unique homozygous parents for subsequent use in walnut-breeding programs. The results also indicate that more attention should be focused on adequate management of precocious walnut to avoid early depression in the production of nuts.

Key message Walnut hybrid proline-rich protein gene JsPRP1 was induced by Colletotrichum gloeosporioides and signaling molecules. JsPRP1 protein inhibited pathogenic fungi. JsPRP1 overexpression in tobacco conferred strong resistance to abiotic and biotic stresses. Walnut ( Juglans sigillata Dode) is one of the important sources of nuts and timbers in the world. With the expansion of walnut cultivation area, its disease hazards have become a serious problem. In this study, a novel hybrid proline-rich cell-wall protein gene JsPRP1 was investigated and isolated from Juglans sigillata Dode induced by Colletotrichum gloeosporioides . qRT-PCR (quantitative reverse transcription-PCR) analysis showed that JsPRP1 transcriptional level was up-regulated in J. sigillata Dode following jasmonic acid, salicylic acid, ethylene, and hydrogen peroxide treatments, and C. gloeosporioides inoculation. The JsPRP1 gene was further fused with green fluorescent protein (GFP) gene in a plant expression vector and transferred into onion ( Allium cepa ) epidermal cells. Laser scanning confocal microscope confirmed that JsPRP1 was expressed in the cell wall. Moreover, the JsPRP1 protein was expressed in Escherichia coli , and the recombinant protein purified by Ni–NTA column showed in vitro antifungal activity to C. gloeosporioides, Gibberella moniliformis, Botryosphaeria dothidea , and Fusarium solani . Furthermore, the plant overexpression vector of JsPRP1 was constructed and transferred into tobacco. The T 2 transgenic tobacco showed high tolerance to drought, CdCl 2 , and C. gloeosporioides infection, meanwhile the accumulation of reactive oxygen species (ROS) in transgenic tobacco was significantly lower than wild-type plant under stress. The above results indicate that JsPRP1 is an important defense gene in J. sigillata Dode against drought, CdCl 2 stress, and C. gloeosporioides infection.

Iron walnut, Juglans sigillata Dode, restricted to southwestern China, has its centre of distribution in Yunnan Province which has a varied climate, geography and rich plant diversity. Yunnan contains abundant J. sigillata germplasm. In this study, a provincial-scale set of walnut germplasm resources (14 populations comprising 1122 individuals) was evaluated for genetic diversity based on 20 simple sequence repeat (SSR) loci. The number of SSR alleles per locus ranged from 7 to 27, with an average of 17.55. Mean allelic richness and mean private allelic richness ranged from 3.40 to 4.62 and 0.11 to 0.36, with average of 3.93 and 0.30, respectively. Expected heterozygosity ( H e ) varied from 0.26 to 0.78, with an average of 0.57. Polymorphism information content ranged from 0.22 to 0.79, with an average of 0.57. Genetic differentiation ( F ST ) was 0.05, indicating that only 5% of total genetic variability was inter-populational, a finding supported by an analysis of molecular variance and STRUCTURE analysis. Relatively high gene flow ( N m = 6.70) was observed between populations. A unweighted pair-group method with arithmetic analysis classified the 14 populations into two major groups. Mantel testing uncovered a significant correlation between geographic distance and genetic distance ( r = 0.33, P = 0.04). Overall, the research revealed a moderately high level of genetic diversity in the germplasm and low genetic differentiation among populations, which showed great potential for further development and exploitation of this resource.

A new caffeoylated galloyl glucoside, 1,3-di-O-galloyl-6-O-trans-caffeoyl-β-D-glucoside (1), was isolated and purified, together with three known galloyl glucosides, 1,2,4,6-tetra-O-galloyl-β-D-glucoside (2), 1,2,6-tri-O-galloyl-β-D-glucoside (3), and 1,2,3,4,6-penta-O-galloyl-β-D-glucoside (4), from the leaves of Juglans sigillata. Chemical structures of the compounds were mainly established on the basis of spectroscopic methods, including 1D and 2D NMR and MS analyses.

Background Persian walnut, Juglans regia , occurs naturally from Greece to western China, while its closest relative, the iron walnut, Juglans sigillata , is endemic in southwest China; both species are cultivated for their nuts and wood. Here, we infer their demographic histories and the time and direction of possible hybridization and introgression between them. Results We use whole-genome resequencing data, different population-genetic approaches (PSMC and GONE), and isolation-with-migration models (IMa3) on individuals from Europe, Iran, Kazakhstan, Pakistan, and China. IMa3 analyses indicate that the two species diverged from each other by 0.85 million years ago, with unidirectional gene flow from eastern J. regia and its ancestor into J. sigillata , including the shell-thickness gene. Within J. regia , a western group, located from Europe to Iran, and an eastern group with individuals from northern China, experienced dramatically declining population sizes about 80 generations ago (roughly 2400 to 4000 years), followed by an expansion at about 40 generations, while J. sigillata had a constant population size from about 100 to 20 generations ago, followed by a rapid decline. Conclusions Both J. regia and J. sigillata appear to have suffered sudden population declines during their domestication, suggesting that the bottleneck scenario of plant domestication may well apply in at least some perennial crop species. Introgression from introduced J. regia appears to have played a role in the domestication of J. sigillata.

Aims A comprehensive understanding of the genetic variation of walnuts (Juglans regia and J. sigillata) in the Himalaya and its potential drivers are essential for the conservation and sustainable utilization of these plant genetic resources. In this study, we aimed to uncover the genetic landscape of walnuts and potential drivers in the Himalaya for better utilization, awareness, sustainable management and conservation of the extant genetic resources of walnuts in the vast Himalayan landscape. Location The Himalaya biodiversity hotspot. Methods A total of 1410 wild or feral walnut trees from 65 populations of J. regia and J. sigillata across four countries in the Himalaya were collected. The genetic diversity, population structure, hybridization and gene flow were thoroughly investigated based on the chloroplast genome and 31 nuclear microsatellite markers. The patterns and drivers of the walnut genetic landscape were further explored. Results We detected three genetic groups of J. regia (JR1, JR2, JR3), one of J. sigillata (JS) and two hybrid types (JR1 × JS and JR3 × JS) of walnut in the Himalaya, with the western Himalaya identified as the genetic diversity hotspot of J. regia. The spatial genetic pattern of the J. regia was significantly influenced by geographic and climatic factors. Human‐mediated dispersal probably promoted the hybridization and gene introgression between J. regia and J. sigillata, which reshaped the genetic landscape of walnut populations in the Himalaya. Main Conclusions The extant genetic landscape of walnuts in the Himalaya was driven by natural and anthropogenic forces. Regarding conservation, the western and eastern Himalaya are the genetic reservoir of J. regia and J. sigillata, and hence, pure individuals should be urgently protected from frequent hybridization and introgression. In addition, we propose the utilization of natural hybrid resources coupled with new breeding techniques that combine genomic and phenotypic data.

Background The foundation for ensuring walnuts of high yield and quality is excellent rootstock. Numerous studies have shown that the walnut rootstock has a major impact on the grafted varieties’ quality, yield, stress resistance, growth, and development. Elucidating the mechanism of grafting affinity in walnuts is crucial, as it paves the way for selecting premium rootstocks to achieve increased production and efficiency. In this study, a combined analysis of the transcriptome, miRNA, and metabolome was conducted to elucidate the molecular mechanisms underlying graft compatibility between different walnut rootstocks ( Juglans sigillata cv. ‘Qianhe 7’ and ‘Panzhou’). Results A total of 44, 41, 50, and 28 differential miRNAs were identified between ‘Qianhe 7’ vs ‘Panzhou’ at 1st d, 11th d, 17th d, and 30th d after grafting, respectively. These miRNAs target genes associated with plant hormone signaling pathways, particularly those related to IAA, SA, and ABA, such as ARF, SAUR, and RAS1-like genes. The most dominant metabolites in both J. sigillata rootstocks (cv. ‘Qianhe 7’ and ‘Panzhou’) at 1st d after grafting were salicylic acid (SA) and Indole-3-acetic acid (IAA). The concentration of SA increased with the extension of time in ‘Qianhe 7’, while IAA decreased significantly on the 30th d. The IAA and abscisic acid (ABA) contents of ‘Qianhe 7’ were markedly higher than those observed in ‘Panzhou’ at 11th d and 30th d post-grafting, respectively. But the concentration of jasmonic acid (JA) in the ‘Panzhou’ was significantly higher than that observed in the ‘Qianhe 7’ at 17th d post-grafting. Conclusions These results indicated that SA, ABA, and IAA may play a pivotal role in the adaptability of grafted J. sigillata (‘Qianhe 7’ and ‘Panzhou’), which provides a foundation for understanding walnut graft affinity. A deeper exploration of the synergistic regulatory network between the identified miRNAs and plant hormones is warranted to clarify the underlying mechanisms, thereby informing hormone-based breeding strategies for walnut rootstocks and facilitating the sustainable advancement of the walnut industry.