De novo transcriptome assembly and comparative analysis between male and benzyladenine-induced female inflorescence buds of Plukenetia volubilis

The transcription factor MYB1 activates DGAT2 transcription to promote triacylglycerol accumulation in sacha inchi (Plukenetia volubilis L.) leaves under heat stress

Triacylglycerol (TAG) accumulation is frequently triggered in vegetative tissues experiencing heat stress, which may increases plant basal plant thermo-tolerance by sequestering the toxic lipid intermediates that contribute to membrane damage or cell death under stress conditions. However, stress-responsive TAG biosynthesis and the underlying regulatory mechanisms are not fully understood. Here, we investigated the lipidomic and transcriptomic landscape under heat stress in the leaves of sacha inchi (Plukenetia volubilis L.), an important oilseed crop in tropical regions. Under heat stress (45 °C), the content of polyunsaturated TAGs (e.g., TAG18:2 and TAG18:3) and total TAGs were significantly higher, while those of unsaturated sterol esters, including ZyE 28:4, SiE 18:2 and SiE 18:3, were dramatically lower. Transcriptome analysis showed that the expression of PvDGAT2-2, encoding a type II diacylglycerol acyltransferase (DGAT) that is critical for TAG biosynthesis, was substantially induced under heat stress. We confirmed the function of PvDGAT2-2 in TAG production by complementing a yeast mutant defective in TAG biosynthesis. Importantly, we also identified the heat-induced transcription factor PvMYB1 as an upstream activator of PvDGAT2-2 transcription. Our findings on the molecular mechanism leading to TAG biosynthesis in leaves exposed to heat stress have implications for improving the biotechnological production of TAGs in vegetative tissues, offering an alternative to seeds.

Comparative transcriptome analysis reveals candidate genes related to the sex differentiation of Schisandra chinensis

Schisandra chinensis is a monoecious plant with unisex flowers. The fruit of S. chinensis is of high medical with economic value. The yield of S. chinensis fruit is related to the ratio of its female and male flowers. However, there is little research on its floral development and sex differentiation. To elucidate the possible mechanism for the sex differentiation of S. chinensis, we collected 18 samples of female and male flowers from three developmental stages and performed a comparative RNA-seq analysis aimed at identifying differentially expressed genes (DEGs) that may be related to sex differentiation. The results showed 936, 7179, and 6890 differentially expressed genes between female and male flowers at three developmental stages, respectively, and 466 candidate genes may play roles in sex differentiation. KEGG analysis showed genes involved in the flavonoid biosynthesis pathway and DNA replication pathway were essential for the development of female flowers. 51 MADS-box genes and 10 YABBY genes were identified in S. chinensis. The DEGs analysis indicated that MADS-box and YABBY genes were strongly related to the sex determination of S. chinensis. RT-qPCR confirmed the RNA-seq results of 20 differentially expressed genes, including three male-biased genes and 17 female-biased genes. A possible regulatory model of sex differentiation in S. chinensis was proposed according to our results. This study helps reveal the sex-differentiation mechanism of S. chinensis and lays the foundation for regulating the male-female ratio of S. chinensis in the future.

Flowering induction in cassava using photoperiod extension premature pruning and plant growth regulators

Cassava (Manihot esculenta Crantz) is a vital crop for food and economic security in many regions of the world. Despite the economic and social importance of cassava, challenges persist in developing superior varieties that meet the needs of farmers in terms of agronomic performance, nutritional quality, and resistance to pests and diseases. One of the main obstacles for genetic improvement is the lack of synchronization in flowering and the abortion of young flowers, making planned crosses and progeny production difficult. Therefore, the aim of this study was to evaluate the effect of photoperiod, premature pruning, and growth regulators on cassava flowering under low-altitude conditions in Brazil. Eight cassava clones with contrasting flowering capacity were assessed in Cruz das Almas, Bahia, using two photoperiods (ambient condition and extended photoperiod with red light for 12 hours), premature pruning at the first and second branching levels (with and without pruning), and the application of growth regulators: 0.5 mM 6-benzyladenine (BA) and 4.0 mM silver thiosulfate (STS) (with and without). Plots were assessed weekly for the number of female (NFF) and male (NMF) flowers, height of the first branching (H1B, in cm), number of days to the first branching (ND1B), and the number of branching events up to 240 days after planting (NOB). The extended photoperiod did not promote an increase in the number of flowers but allowed for precocity in cassava flowering, reducing the onset of flowering by up to 35 days, and significantly increasing the number of branches, which is closely related to flowering. The use of pruning and plant growth regulators (PGR) resulted in an increase in NFF from 2.2 (control) to 4.6 and NMF from 8.1 to 21.1 flowers. Therefore, under hot and humid tropical conditions at low altitudes in the Recôncavo of Bahia, manipulating the photoperiod and using premature pruning and plant growth regulators can accelerate cassava flowering, benefiting genetic improvement programs.

Genome-Wide Identification and Expression Analysis of the RADIALIS-like Gene Family in Camellia sinensis

The RADIALIS-like (RL) proteins are v-myb avian myeloblastosis viral oncogene homolog (MYB)-related transcription factors (TFs), and are involved in many biological processes, including metabolism, development, and response to biotic and abiotic stresses. However, the studies on the RL genes of Camellia sinensis are not comprehensive enough. Therefore, we undertook this study and identified eight CsaRLs based on the typical conserved domain SANT Associated domain (SANT) of RL. These genes have low molecular weights and theoretical pI values ranging from 5.67 to 9.76. Gene structure analysis revealed that six CsaRL genes comprise two exons and one intron, while the other two contain a single exon encompassing motifs 1 and 2, and part of motif 3. The phylogenetic analysis divided one hundred and fifty-eight RL proteins into five primary classes, in which CsaRLs clustered in Group V and were homologous with CssRLs of the Shuchazao variety. In addition, we selected different tissue parts to analyze the expression profile of CsaRLs, and the results show that almost all genes displayed variable expression levels across tissues, with CsaRL1a relatively abundant in all tissues. qRT-PCR (real-time fluorescence quantitative PCR) was used to detect the relative expression levels of the CsaRL genes under various abiotic stimuli, and it was found that CsaRL1a expression levels were substantially higher than other genes, with abscisic acid (ABA) causing the highest expression. The self-activation assay with yeast two-hybrid system showed that CsaRL1a has no transcriptional activity. According to protein functional interaction networks, CsaRL1a was well connected with WIN1-like, lysine histidine transporter-1-like, β-amylase 3 chloroplastic-like, carbonic anhydrase-2-like (CA2), and carbonic anhydrase dnaJC76 (DJC76). This study adds to our understanding of the RL family and lays the groundwork for further research into the function and regulatory mechanisms of the CsaRLs gene family in Camellia sinensis.

Identifying Genes Associated with Female Flower Development of Phellodendron amurense Rupr. Using a Transcriptomics Approach

Phellodendron amurense Rupr., a species of Rutaceae, is a nationally protected and valuable medicinal plant. It is generally considered to be dioecious. With the discovery of monoecious P. amurense, the phenomenon that its sex development is regulated by epigenetics has been revealed, but the way epigenetics affects the sex differentiation of P. amurense is still unclear. In this study, we investigated the effect of DNA methylation on the sexual development of P. amurense. The young inflorescences of male plants were treated with the demethylation agent 5-azaC, and the induced female flowers were obtained. The induced female flowers’ morphological functions and transcriptome levels were close to those of normally developed plants. Genes associated with the development of female flowers were studied by comparing the differences in transcriptome levels between the male and female flowers. Referring to sex-related genes reported in other plants, 188 candidate genes related to the development of female flowers were obtained, including sex-regulating genes, genes related to the formation and development of sexual organs, genes related to biochemical pathways, and hormone-related genes. RPP0W, PAL3, MCM2, MCM6, SUP, PIN1, AINTEGUMENTA, AINTEGUMENTA-LIKE6, AGL11, SEUSS, SHI-RELATED SEQUENCE 5, and ESR2 were preliminarily considered the key genes for female flower development. This study has demonstrated that epigenetics was involved in the sex regulation of P. amurense, with DNA methylation as one of its regulatory modes. Moreover, some candidate genes related to the sexual differentiation of P. amurense were obtained with analysis. These results are of great significance for further exploring the mechanism of sex differentiation of P. amurense and studying of sex differentiation of plants.

Whole-transcriptome analysis of differentially expressed genes between ray and disc florets and identification of flowering regulatory genes in Chrysanthemum morifolium

Chrysanthemum morifolium has ornamental and economic values. However, there has been minimal research on the morphology of the chrysanthemum florets and related genes. In this study, we used the leaves as a control to screen for differentially expressed genes between ray and disc florets in chrysanthemum flowers. A total of 8,359 genes were differentially expressed between the ray and disc florets, of which 3,005 were upregulated and 5,354 were downregulated in the disc florets. Important regulatory genes that control flower development and flowering determination were identified. Among them, we identified a TM6 gene (CmTM6-mu) that belongs to the Class B floral homeotic MADS-box transcription factor family, which was specifically expressed in disc florets. We isolated this gene and found it was highly similar to other typical TM6 lineage genes, but a single-base deletion at the 3' end of the open reading frame caused a frame shift that generated a protein in which the TM6-specific paleoAP3 motif was missing at the C terminus. The CmTM6-mu gene was ectopically expressed in Arabidopsis thaliana. Petal and stamen developmental processes were unaffected in transgenic A. thaliana lines; however, the flowering time was earlier than in the wild-type control. Thus, the C-terminal of paleoAP3 appears to be necessary for the functional performance in regulating the development of petals or stamens and CmTM6-mu may be involved in the regulation of flowering time in chrysanthemum. The results of this study will be useful for future research on flowering molecular mechanisms and for the breeding of novel flower types.

Transcriptome profiling of differentially expressed genes of male and female inflorescences in spinach (Spinacia oleracea L.)

Spinach (Spinacia oleracea L.) is commonly considered a dioecious plant with heterogametic (XY) and homogametic (XX) sex chromosomes. The characteristic is also utilized for the production of spinach hybrid seeds. However, the molecular mechanisms of sex determination in spinach are still unclear because of a lack of genomic and transcriptomic information. In this study, RNA sequencing (RNA-seq) was performed in male and female inflorescences to provide insight into the molecular basis of sex determination in spinach. Comparative transcriptome analyses showed that 2278 differentially expressed genes (DEGs) were identified between male and female inflorescences. A high correlation between the RNA-Seq and qRT-PCR validation for DEGs was observed. Among these, 182 DEGs were annotated to transcription factors including the MYB family protein, bHLH family, and MADS family, suggesting these factors might play a vital role in sex determination. Moreover, 26 DEGs related to flower development, including nine ABCE class genes, were detected. Expression analyses of hormone pathways showed that brassinosteroids may be key hormones related to sex determination in spinach. Overall, this study provides a large amount of DEGs related to sexual expression and lays a foundation for unraveling the regulatory mechanism of sex determination in spinach.

Sacha inchi (Plukenetia volubilis L.)-from lost crop of the Incas to part of the solution to global challenges?

The underutilized, oleaginous crop Plukenetia volubilis L. has a remarkable lipid composition and a large potential for further domestication, alleviation of malnutrition, and integration into sustainable food production systems. Current global challenges include climate change, increasing population size, lack of food security, malnutrition, and degradation of arable lands. In this context, a reformation of our food production systems is imperative. Underutilized crops, or orphan crops, can provide valuable traits for this purpose, e.g., climate change resilience, nutritional benefits, cultivability on marginal lands, and improvement of income opportunities for smallholders. Plukenetia volubilis L. (Euphorbiaceae)-sacha inchi-is a 'lost crop' of the Incas native to the Amazon basin. Its oleaginous seeds are large, with a high content of ω-3, and -6 fatty acids (ca. 50.5, and 34.1%, of the lipid fraction, respectively), protein, and antioxidants. Culinarily, the seeds are nut-like and the crop has been associated with humans since Incan times. Research has particularly been undertaken in seed biochemistry, and to some extent in phylogeny, genetics, and cultivation ecology, but attention has been unevenly distributed, causing knowledge gaps in areas such as ethnobotany, allergenicity, and sustainable cultivation practices. Recently, seed size evolution and molecular drivers of the fatty acid synthesis and composition have been studied, however, further research into the lipid biosynthesis is desirable. Targeted breeding has not been undertaken but might be especially relevant for yield, sensory qualities, and cultivation with low environmental impact. Similarly, studies of integration into sustainable management systems are of highest importance. Here, present knowledge on P. volubilis is reviewed and a general framework for conducting research on underutilized crops with the aim of integration into sustainable food production systems is presented.

Developmental basis for flower sex determination and effects of cytokinin on sex determination in Plukenetia volubilis (Euphorbiaceae)

KEY MESSAGE

Cytokinin might be an important factor to regulate floral sex at the very early stage of flower development in sacha inchi. Sacha inchi (Plukenetia volubilis, Euphorbiaceae) is characterized by having female and male flowers in a thyrse with particular differences. The mechanisms involved in the development of unisexual flowers are very poorly understood. In this study, the inflorescence and flower development of P. volubilis were investigated using light microscopy and scanning electron microscopy. We also investigated the effects of cytokinin on flower sex determination by exogenous application of 6-benzyladenine (BA) in P. volubilis. The floral development of P. volubilis was divided into eight stages, and the first morphological divergence between the male and female flowers was found to occur at stage 3. Both female and male flowers can be structurally distinguished by differences in the shape and size of the flower apex after sepal primordia initiation. There are no traces of gynoecia in male flowers or of androecia in female flowers. Exogenous application of BA effectively induced gynoecium primordia initiation and female flower development, especially at the early flower developmental stages. We propose that flower sex is determined earlier and probably occurs before flower initiation, either prior to or at inflorescence development due to the difference in the position of the female and male primordia in the inflorescence and in the time of the female and male primordia being initiated. The influence of cytokinin on female primordia during flower development in P. volubilis strongly suggests a feminization role for cytokinin in sex determination. These results indicate that cytokinin could modify the fate of the apical meristem of male flower and promote the formation of carpel primordia in P. volubilis.

Comparative transcriptome analysis reveals the regulatory networks of cytokinin in promoting the floral feminization in the oil plant Sapium sebiferum

BACKGROUND

Sapium sebiferum, whose seeds contain high level of fatty acids, has been considered as one of the most important oil plants. However, the high male to female flower ratio limited the seed yield improvement and its industrial potentials. Thus, the study of the sex determination in S. sebiferum is of significant importance in increasing the seed yield.

RESULTS

In this study, we demonstrated that in S. sebiferum, cytokinin (CK) had strong feminization effects on the floral development. Exogenous application with 6-benzylaminopurine (6-BA) or thidiazuron (TDZ) significantly induced the development of female flowers and increased the fruit number. Interestingly, the feminization effects of cytokinin were also detected on the androecious genotype of S. sebiferum which only produce male flowers. To further investigate the mechanism underlying the role of cytokinin in the flower development and sex differentiation, we performed the comparative transcriptome analysis of the floral buds of the androecious plants subjected to 6-BA. The results showed that there were separately 129, 352 and 642 genes differentially expressed at 6 h, 12 h and 24 h after 6-BA treatment. Functional analysis of the differentially expressed genes (DEGs) showed that many genes are related to the hormonal biosynthesis and signaling, nutrients translocation and cell cycle. Moreover, there were twenty one flowering-related genes identified to be differentially regulated by 6-BA treatment. Specifically, the gynoecium development-related genes SPATULA (SPT), KANADI 2 (KAN2), JAGGED (JAG) and Cytochrome P450 78A9 (CYP79A9) were significantly up-regulated, whereas the expression of PISTILLATA (PI), TATA Box Associated Factor II 59 (TAFII59) and MYB Domain Protein 108 (MYB108) that were important for male organ development was down-regulated in response to 6-BA treatment, demonstrating that cytokinin could directly target the floral organ identity genes to regulate the flower sex.

CONCLUSIONS

Our work demonstrated that cytokinin is a potential regulator in female flower development in S. sebiferum. The transcriptome analysis of the floral sex transition from androecious to monoecious in response to cytokinin treatment on the androecious S. sebiferum provided valuable information related to the mechanism of sex determination in the perennial woody plants.