The Transcriptome Landscape of Walnut Interspecies Hybrid (Juglans hindsii × Juglans regia) and Regulation of Cambial Activity in Relation to Grafting

Transcriptomic and metabolomic analyses unveil the growth advantage mechanism conferred by heterosis of Michelia 'Zhongshanhanxiao'

Michelia compressa (Maxim.) Sarg. is one of the important timber trees in Taiwan province, P. R. China. Michelia 'Zhongshanhanxiao' is a group of variants found among the progeny of M. compressa that exhibit higher growth rates compared with normal individuals, with a significantly increased stem diameter and height, as well as enlarged leaves and flowers. However, the molecular mechanisms fostering the growth advantage and morphological variations are unknown and deserve further study. Through analysing the transcriptome, metabolome and physiological processes of leaves, we identified remarkable differences in gene expression and metabolic profiles between Michelia 'Zhongshanhanxiao' and both the maternal M. compressa and its normal progeny. These differences were widely associated with a plant-pathogen interaction, phenylpropanoid biosynthesis, cyanoamino acid metabolism, carbon fixation in photosynthetic organisms and plant hormone signal transduction. Additionally, physiological measurements showed that Michelia 'Zhongshanhanxiao' possesses stronger photosynthetic capacity and higher plant hormone content. These results suggest that the heterosis of Michelia 'Zhongshanhanxiao' is regulated by candidates related to cell division, resistance to pathogens and the accumulation of organic compounds. The findings of this study provide crucial information on the molecular mechanisms underlying the growth advantages conferred by heterosis in trees.

Regulatory networks of hormone-involved transcription factors and their downstream pathways during somatic embryogenesis of Arabidopsis thaliana

Somatic embryogenesis (SE) depends on a variety of developmental pathways that are influenced by several environmental factors. Therefore, it is important to understand the relationship between environmental and genetic factors by identifying the gene networks involved in SE through gene set enrichment analysis (GSEA). For determination of SE effective transcription factors, upstream sequences of core-enriched genes were analyzed. The results indicated that response to hormones is one of the biological pathways activated by the enriched TFs at all stages of somatic embryogenesis and about half of the hormonal pathways were enriched. On the fifth day after 2,4-Dichlorophenoxyacetic acid (2,4-D) treatment, the activity of hormone-affecting genes reached its maximum. At this time, more transcription factors regulated the enriched genes compared to the other stages of somatic embryogenesis. MYBs, AT-HOOKs, and HSFs are the main families of transcription factors which affect core-enriched genes during SE. CCA1, PRR7, and TOC1 and their related genes at the center of protein-protein interaction of SE-key transcription factors, involved in the regulation of the circadian clock. Gene expression analysis of CCA1, PRR7, and TOC1 revealed that the genes involved in circadian clock reached their maximum activity when embryonic cells formed. Also, auxin response elements were identified at the upstream of SE-circadian clock transcription factors, indicating that they might mediate between auxin signaling and SE-related hormonal pathways as well as SE marker genes such as AGL15, BBM, and LECs. Based on these results, it is possible that the cellular circadian rhythm activates various developmental pathways under the influence of auxin signal transduction and their interactions determine the induction of somatic embryogenesis. According to the results of this study, modifying pathways affected by SE-related transcription factors such as circadian rhythm may result in cell reprogramming and increase somatic embryogenesis efficiency.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03546-7.

Metabolomics and Transcriptomics Provide Insights into Lipid Biosynthesis in the Embryos of Walnut (Juglans regia L.)

Walnut (Juglans regia L.) is an important woody oilseed tree species due to its commercial value. However, the regulation mechanism of walnut oil accumulation is still poorly understood, which restricted the breeding and genetic improvement of high-quality oil-bearing walnuts. In order to explore the metabolic mechanism that regulates the synthesis of walnut oil, we used transcriptome sequencing technology and metabolome technology to comprehensively analyze the key genes and metabolites involved in oil synthesis of the walnut embryo at 60, 90, and 120 days after pollination (DAP). The results showed that the oil and protein contents increased gradually during fruit development, comprising 69.61% and 18.32% of the fruit, respectively, during ripening. Conversely, the contents of soluble sugar and starch decreased gradually during fruit development, comprising 2.14% and 0.84%, respectively, during ripening. Transcriptome sequencing generated 40,631 unigenes across 9 cDNA libraries. We identified 51 and 25 candidate unigenes related to the biosynthesis of fatty acid and the biosynthesis of triacylglycerol (TAG), respectively. The expression levels of the genes encoding Acetyl-CoA carboxylase (ACCase), long-chain acyl-CoA synthetases (LACS), 3-oxoacyl-ACP synthase II (KASII), and glycerol-3-phosphate acyl transfer (GPAT) were upregulated at 60 DAP relative to the levels at 90 and 120 DAP, while the stearoyl-ACP-desaturase (SAD) and fatty acid desaturase 2 (FAD2) genes were highly abundantly expressed during all walnut developmental periods. We found that ABSCISIC ACID INSENSEITIVE3 (ABI3), WRINKLEDl (WRI1), LEAFY COTYLEDON1 (LEC1), and FUSCA3 (FUS3) may be key transcription factors involved in lipid synthesis. Additionally, the metabolomics analysis detected 706 metabolites derived from 18 samples, among which, 4 are implicated in the TAG synthesis, 2 in the glycolysis pathway, and 5 in the tricarboxylic acid cycle (TCA cycle) pathway. The combined analysis of the related genes and metabolites in TAG synthesis showed that phospholipid:diacylglycerol acyltransferase (PDAT) genes were highly abundantly expressed across walnut fruit developmental periods, and their downstream metabolite TAG gradually accumulated with the progression of fruit development. The FAD2 gene showed consistently higher expression during fruit development, and its downstream metabolites 18:2-PC and 18:3-PC gradually accumulated. The ACCase, LACS, SAD, FAD2, and PDAT genes may be crucial genes required for walnut oil synthesis. Our data will enrich public databases and provide new insights into functional genes related to lipid metabolism in walnut.

Metabolome and Transcriptome Profiling Unveil the Mechanisms of Polyphenol Synthesis in the Developing Endopleura of Walnut (Juglans regia L.)

Walnut (Juglans regia L.) is an important woody nut tree species, and its endopleura (the inner coating of a seed) is rich in many polyphenols. Thus far, the pathways and essential genes involved in polyphenol biosynthesis in developing walnut endopleura remain largely unclear. We compared metabolite differences between endopleura and embryo in mature walnuts, and analyzed the changes of metabolites in endopleura at 35, 63, 91, 119, and 147 days after pollination (DAP). A total of 760 metabolites were detected in the metabolome, and the polyphenol contents in endopleura were higher than those in embryos. A total of 15 types of procyanidins, 10 types of kaempferol glycosides, and 21 types of quercetin glycosides that accumulated during endopleura development were identified. The analysis of the phenylpropane metabolic pathway showed that phenylalanine was gradually transformed into proanthocyanidins and other secondary metabolites with the development of endopleura. A total of 49 unigenes related to polyphenol synthesis were identified by transcriptome analysis of endopleura. The expression patterns of PAL, C4H, 4CL, CHS, CHI, F3H, LDOX, and ANR were similar, and their expression levels were highest in endopleura at maturity. Transcriptome and metabolome analysis showed that endopleura rapidly synthesized and accumulated polyphenols during maturation. Moreover, the transcription factor MYB111 played an important role in synthesizing polyphenols in endopleura, and its expression pattern was positively correlated with the accumulation pattern of quercetin, kaempferol, and proanthocyanidins. MYB111 was co-expressed with NAP, NAC, ATR1, and other genes related to cell senescence and abiotic stress response. Our study analyzed the composition and molecular synthesis mechanism of polyphenols in walnut endopleura, and provided new perspectives and insights regarding the nutritional research of walnut nuts.

Transcriptome Analysis of Walnut (Juglans regia L.) Embryos Reveals Key Developmental Stages and Genes Involved in Lipid Biosynthesis and Polyunsaturated Fatty Acid Metabolism

Walnut (Juglans regia L.) is a widely cultivated woody oilseed tree species, and its embryo is rich in polyunsaturated fatty acids. Thus far, the pathways and essential genes involved in oil biosynthesis in developing walnut embryos remain largely unclear. Our analyses revealed that a mature walnut embryo accumulated 69% oil, in which 71% were polyunsaturated fatty acids with 64% linoleic acid and 7% linolenic acid. RNA sequencing generated 39 384 unigenes in 24 cDNA libraries prepared from walnut embryos collected at 49, 63, 77, 91, 105, 119, 133, and 147 days after pollination (DAP). The principal components analysis (PCA) of samples and cluster analysis of differentially expressed genes (DEGs) showed that the total samples were divided into three main groups: 49 DAP, 63-119 DAP, and 133-147 DAP. We identified 108 unigenes associated with lipid biosynthesis, including 60 unigenes for fatty acid biosynthesis, 33 for triacylglycerol biosynthesis, 7 for oil bodies, and 8 for transcription factors. The expression levels of the genes encoding WRI1, ACCase, ACP, KASII, SAD, FAD2, FAD3, and PDAT were upregulated at 63-119 DAP relative to the levels at 49 DAP. Additionally, the lipid biosynthesis in walnut embryos began to increase while oil contents increased from 15 to 69%. We identified eight SAD, three FAD2, one FAD3, one FAD5, one FAD6, and three FAD7/8 genes. In addition, SAD, FAD2, and FAD3 were highly abundantly expressed in the walnut embryo, and their FPKM values achieved were 834, 2205, and 9038, respectively. High expression levels of FAD2 and FAD3 may be the reason why walnuts are rich in polyunsaturated fatty acids. Subcellular localization confirmed that the JrFAD3 protein played a role in the endoplasmic reticulum rather than the plastid, suggesting that linolenic acid was mainly synthesized in the endoplasmic reticulum. Weighted gene coexpression network analysis (WGCNA) showed that ACP, ENO, VAMP727, and IDD14 were coexpressed with WRI1. Our study provides large-scale and comprehensive transcriptome data of walnut embryo development. These data lay the foundation for the metabolic engineering of walnuts to increase oil contents and modify fatty acid compositions.

Effects of Cutting, Pruning, and Grafting on the Expression of Age-Related Genes in Larix kaempferi

Grafting, cutting, and pruning are important horticultural techniques widely used in the establishment of clonal forestry. After the application of these techniques, some properties of the plants change, however, the underlying molecular mechanisms are still unclear. In our previous study, 27 age-related transcripts were found to be expressed differentially between the juvenile vegetative (1- and 2-year-old) and adult reproductive (25- and 50-year-old) phases of Larix kaempferi. Here, we re-analyzed the 27 age-related transcripts, cloned their full-length cDNA sequences, and measured their responses to grafting, cutting, and pruning. After sequence analysis and cloning, 20 transcription factors were obtained and annotated, most of which were associated with reproductive development, and six (LaAGL2-1, LaAGL2-2, LaAGL2-3, LaSOC1-1, LaAGL11, and LaAP2-2) showed regular expression patterns with L. kaempferi aging. Based on the expression patterns of these transcription factors in L. kaempferi trees subjected to grafting, cutting, and pruning, we concluded that (1) cutting and pruning rejuvenate the plants and change their expression, and the effects of cutting on gene expression are detectable within 14 years, although the cutting seedlings are still maturing during these years; (2) within three months after grafting, the rootstock is more sensitive to grafting than the scion and readily becomes mature with the effect of the scion, while the scion is not readily rejuvenated by the effect of the rootstock; and (3) LaAGL2-2 and LaAGL2-3 are more sensitive to grafting, while LaAP2-2 is impervious to it. These findings not only provide potential molecular markers to assess the state of plants but also aid in studies of the molecular mechanisms of rejuvenation.