Important photosynthetic contribution of silique wall to seed yield-related traits in Arabidopsis thaliana

PSEUDO-ETIOLATION IN LIGHT proteins reduce greening by binding GLK transcription factors

Knocking out genes encoding proteins that downregulate the accumulation of pigments may lead to increases in crop quality and yield. PSEUDO-ETIOLATION IN LIGHT 1 (PEL1) downregulates the accumulation of carotenoids in carrot and chlorophyll in Arabidopsis and rice and may inhibit GOLDEN 2-LIKE (GLK) transcription factors. PEL1 belongs to a previously unstudied gene family found only in plants. We used CRISPR/Cas9 technology to knock out each member of the 4-member PEL gene family and both GLK genes in Arabidopsis. In pel mutants, chlorophyll levels were elevated in seedlings; after flowering, chloroplasts increased in size, and anthocyanin levels increased. Although the chlorophyll-deficient phenotype of glk1 glk2 was epistatic to pel1 pel2 pel3 pel4 in most of our experiments, glk1 glk2 was not epistatic to pel1 pel2 pel3 pel4 for the accumulation of anthocyanins in most of our experiments. The pel alleles attenuated growth, altered the accumulation of nutrients in seeds, disrupted an abscisic acid-inducible inhibition of seedling growth response that promotes drought tolerance, and affected the expression of genes associated with diverse biological functions, such as stress responses, cell wall metabolism hormone responses, signaling, growth, and the accumulation of phenylpropanoids and pigments. We found that PEL proteins specifically bind 6 transcription factors that influence the accumulation of anthocyanins, GLK2, and the carboxy termini of GLK1 and Arabidopsis thaliana myeloblastosis oncogene homolog 4 (AtMYB4). Our data indicate that the PEL proteins influence the accumulation of chlorophyll and many other processes, possibly by inhibiting GLK transcription factors and via other mechanisms, and that multiple mechanisms downregulate chlorophyll content.

AGAMOUS mediates timing of guard cell formation during gynoecium development

In Arabidopsis thaliana, stomata are composed of two guard cells that control the aperture of a central pore to facilitate gas exchange between the plant and its environment, which is particularly important during photosynthesis. Although leaves are the primary photosynthetic organs of flowering plants, floral organs are also photosynthetically active. In the Brassicaceae, evidence suggests that silique photosynthesis is important for optimal seed oil content. A group of transcription factors containing MADS DNA binding domains is necessary and sufficient to confer floral organ identity. Elegant models, such as the ABCE model of flower development and the floral quartet model, have been instrumental in describing the molecular mechanisms by which these floral organ identity proteins govern flower development. However, we lack a complete understanding of how the floral organ identity genes interact with the underlying leaf development program. Here, we show that the MADS domain transcription factor AGAMOUS (AG) represses stomatal development on the gynoecial valves, so that maturation of stomatal complexes coincides with fertilization. We present evidence that this regulation by AG is mediated by direct transcriptional repression of a master regulator of the stomatal lineage, MUTE, and show data that suggests this interaction is conserved among several members of the Brassicaceae. This work extends our understanding of the mechanisms underlying floral organ formation and provides a framework to decipher the mechanisms that control floral organ photosynthesis.

Analysis of Altered Flowering Related Genes in a Multi-Silique Rapeseed (Brassica napus L.) Line zws-ms Based on Combination of Genome, Transcriptome and Proteome Data

Based on previous researches, we further investigated the multi-silique trait in rapeseed (Brassica napus L.) line zws-ms. In this study, we used a relatively comprehensive list of flowering related genes in rapeseed and compared them between zws-ms and its near-isogenic line (NIL) zws-217. Genes were studied on genome, transcriptome and proteome levels and then we focused on genes with non-synonymous single nucleotide polymorphism (SNP) or frame-shift insertion-deletion (InDel), finding some genes on the list which changes their sequences. Then, combined with their annotation and the information of their orthologs, certain genes such as BnaA09g05900D, ortholog of AGAMOUS-LIKE 42 (AGL42), which encodes an MADS-box protein, were assumed as probably responsible for the multi-silique trait. Also, we analyzed the Differentially Accumulated Proteins (DAPs) between zws-ms and zws-217, revealing some genes involved in homologous recombination and mismatch repair pathways. Since the development of flowers/siliques is crucial to crops and it influences the yield of rapeseed, this study paved a way to deeply understand the mechanism of the multi-pistil flower formation, which may facilitate researches on rapeseed production in future.

Contribution of the leaf and silique photosynthesis to the seeds yield and quality of oilseed rape (Brassica napus L.) in reproductive stage

Influences of photosynthesis of leaf and silique on seeds yield and quality of oilseed rape (Brassica napus L.) were explored in this study. A field comparing experiment with several rapeseed varieties was conducted and the results showed, that the leaf area index (LAI), silique surface area index (SAI), siliques number per plant, and biological yield were statistically classified as the first principal factors which greatly influenced seeds yield, the leaf net photosynthetic rate (P_n) and silique P_n were the second principal factors; the stomatal conductance (G_s) and chlorophyll a (Chl a) content were the first principal factors which influenced leaf P_n and silique P_n. A shading experiment was conducted and the results showed that, under treatments of the ZH1, ZH2, and ZH3 (shading rapeseed plants during flowering stage, during time from initial flowering until seeds ripening, and during time from flowering ending until seeds ripening, respectively), respectively the seeds yield per plant was reduced by 34.6%, 84.3%, and 86.1%, the seed protein content was significantly increased. The treatment ZH1 Not, but the ZH2 and ZH3 caused significant decrease in both seed oil content and oleic acid (C18:1) content in seed oil, and the contents of linoleic acid (C18:2), linolenic acid (C18:3) in oil were significantly increased, gene expression of the ACCase (Acetyl-CoA carboxylase), FAD2 (fatty acid desaturase), and FAD3 (ω-3 fatty acid dehydrogenase) in green seeds was restrained/changed. Thus the LAI, SAI, siliques number per plant, biological yield per plant, leaf P_n, silique P_n, and the G_s, Chl a content of leaf and silique formed an indexes system to be used in screening rapeseed variety with higher light efficiency and seeds yield; the silique photosynthesis inhibition and the photosynthates deficiency in rapeseed plant after flowering stage predominately influenced seeds yield and quality.

Transcriptomic comparison of seeds and silique walls from two rapeseed genotypes with contrasting seed oil content

Silique walls play pivotal roles in contributing photoassimilates and nutrients to fuel seed growth. However, the interaction between seeds and silique walls impacting oil biosynthesis is not clear during silique development. Changes in sugar, fatty acid and gene expression during Brassica napus silique development of L192 with high oil content and A260 with low oil content were investigated to identify key factors affecting difference of their seed oil content. During the silique development, silique walls contained more hexose and less sucrose than seeds, and glucose and fructose contents in seeds and silique walls of L192 were higher than that of A260 at 15 DAF, and sucrose content in the silique walls of L192 were lower than that of A260 at three time points. Genes related to fatty acid biosynthesis were activated over time, and differences on fatty acid content between the two genotypes occurred after 25 DAF. Genes related to photosynthesis expressed more highly in silique walls than in contemporaneous seeds, and were inhibited over time. Gene set enrichment analysis suggested photosynthesis were activated in L192 at 25 and 35 DAF in silique walls and at both 15 and 35 DAF in the seed. Expressions of sugar transporter genes in L192 was higher than that in A260, especially at 35 DAF. Expressions of genes related to fatty acid biosynthesis, such as BCCP2s, bZIP67 and LEC1s were higher in L192 than in A260, especially at 35 DAF. Meanwhile, genes related to oil body proteins were expressed at much lower levels in L192 than in A260. According to the WGCNA results, hub modules, such as ME.turquoise relative to photosynthesis, ME.green relative to embryo development and ME.yellow relative to lipid biosynthesis, were identified and synergistically regulated seed development and oil accumulation. Our results are helpful for understanding the mechanism of oil accumulation of seeds in oilseed rape for seed oil content improvement.

Enhancing the light reactions of photosynthesis: Strategies, controversies, and perspectives

Photosynthesis is central to life on Earth, employing sunlight, water, and carbon dioxide to produce chemical energy and oxygen. It is generally accepted that boosting its efficiency offers one promising way to increase crop yields under agronomically realistic conditions. Since the components, structure, and regulatory mechanisms of the light reactions of photosynthesis are well understood, concepts for enhancing the process have been suggested and partially tested. These approaches vary in complexity, from targeting single components to comprehensive redesign of the whole process on the scales from single cells or tissues to whole canopies. Attempts to enhance light utilization per leaf, by decreasing pigmentation, increasing levels of photosynthetic proteins, prolonging the lifespan of the photosynthetic machinery, or massive reconfiguration of the photosynthetic machinery and the incorporation of nanomaterials, are discussed in this review first. Secondly, strategies intended to optimize the acclimation of photosynthesis to changes in the environment are presented, including redesigning mechanisms to dissipate excess excitation energy (e.g., non-photochemical quenching) or reduction power (e.g., flavodiiron proteins). Moreover, schemes for improving acclimation, inspired by natural or laboratory-induced adaptation, are introduced. However, all these endeavors are still in an early exploratory phase and/or have not resulted in the desired outcome, largely because photosynthesis is embedded within large networks of closely interacting cellular and metabolic processes, which can vary among species and even cultivars. This explains why integrated, systems-wide approaches are required to achieve the breakthroughs required for effectively increasing crop yields.

Identification of GOLDEN2-like transcription factor genes in soybeans and their role in regulating plant development and metal ion stresses

The Golden 2-Like (G2-like or GLK) transcription factors are essential for plant growth, development, and many stress responses as well as heavy metal stress. However, G2-like regulatory genes have not been studied in soybean. This study identified the genes for 130 G2-Like candidates' in the genome of Glycine max (soybean). These GLK genes were located on all 20 chromosomes, and several of them were segmentally duplicated. Most GLK family proteins are highly conserved in Arabidopsis and soybean and were classified into five major groups based on phylogenetic analysis. These GmGLK gene promoters share cis-acting elements involved in plant responses to abscisic acid, methyl jasmonate, auxin signaling, low temperature, and biotic and abiotic stresses. RNA-seq expression data revealed that the GLK genes were classified into 12 major groups and differentially expressed in different tissues or organs. The co-expression network complex revealed that the GmGLK genes encode proteins involved in the interaction of genes related to chlorophyll biosynthesis, circadian rhythms, and flowering regulation. Real-time quantitative PCR analysis confirmed the expression profiles of eight GLK genes in response to cadmium (Cd) and copper (Cu) stress, with some GLK genes significantly induced by both Cd and Cu stress treatments, implying a functional role in defense responsiveness. Thus, we present a comprehensive perspective of the GLK genes in soybean and emphasize their important role in crop development and metal ion stresses.

Starch accumulation in bean fruit pericarp is mediated by the differentiation of chloroplasts into amyloplasts

The sucrose supply to bean fruits remains almost constant during seed development, and the early stages of this process are characterized by a significant amount of starch and soluble sugars (glucose, fructose and sucrose) accumulated in the pericarp. Bean fruits are photosynthetically active; however, our results indicated that starch synthesis in the pericarp was largely dependent on the photosynthetic activity of the leaves. The photosynthetic activity and the amount of the Rubisco large subunit were gradually reduced in the fruit pericarp, and a large increase in the amount of the ADP-glucose pyrophosphorylase small subunit (AGPase SS) was observed. These changes suggested differentiation of chloroplasts into amyloplasts. Pericarp chloroplasts imported glucose 1-P to support starch synthesis, and their differentiation into amyloplasts allowed the surplus sucrose to be used in the synthesis of starch, which was later degraded to meet the needs of fast-growing seeds. Starch stored in the bean fruit pericarp was not degraded in response to drought stress, but it was rapidly used under severe nutrient restriction. Together, this work indicated that starch accumulation in the pericarp of bean fruits is important to adjust the needs of developing seeds to the amount of sucrose that is provided to fruits.

Maize Golden2-like transcription factors boost rice chloroplast development, photosynthesis, and grain yield

Chloroplasts are the sites for photosynthesis, and two Golden2-like factors act as transcriptional activators of chloroplast development in rice (Oryza sativa L.) and maize (Zea mays L.). Rice OsGLK1 and OsGLK2 are orthologous to maize ZmGLK1 (ZmG1) and ZmGLK2 (ZmG2), respectively. However, while rice OsGLK1 and OsGLK2 act redundantly to regulate chloroplast development in mesophyll cells, maize ZmG1 and ZmG2 are functionally specialized and expressed in different cell-specific manners. To boost rice chloroplast development and photosynthesis, we generated transgenic rice plants overexpressing ZmG1 and ZmG2, individually or simultaneously, with constitutive promoters (pZmUbi::ZmG1 and p35S::ZmG2) or maize promoters (pZmG1::ZmG1, pZmG2::ZmG2, and pZmG1::ZmG1/pZmG2::ZmG2). Both ZmG1 and ZmG2 genes were highly expressed in transgenic rice leaves. Moreover, ZmG1 and ZmG2 showed coordinated expression in pZmG1::ZmG1/pZmG2::ZmG2 plants. All Golden2-like (GLK) transgenic plants had higher chlorophyll and protein contents, Rubisco activities and photosynthetic rates per unit leaf area in flag leaves. However, the highest grain yields occurred when maize promoters were used; pZmG1::ZmG1, pZmG2::ZmG2, and pZmG1::ZmG1/pZmG2::ZmG2 transgenic plants showed increases in grain yield by 51%, 47%, and 70%, respectively. In contrast, the pZmUbi::ZmG1 plant produced smaller seeds without yield increases. Transcriptome analysis indicated that maize GLKs act as master regulators promoting the expression of both photosynthesis-related and stress-responsive regulatory genes in both rice shoot and root. Thus, by promoting these important functions under the control of their own promoters, maize GLK1 and GLK2 genes together dramatically improved rice photosynthetic performance and productivity. A similar approach can potentially improve the productivity of many other crops.

Mutations in a�Golden2-Like�Gene Cause Reduced Seed Weight in�Barley�albino lemma 1�Mutants

The albino lemma 1 (alm1) mutants of barley (Hordeum vulgare L.) exhibit obvious chlorophyll-deficient hulls. Hulls are seed-enclosing tissues on the spike, consisting of the lemma and palea. The alm1 phenotype is also expressed in the pericarp, culm nodes and basal leaf sheaths, but leaf blades and awns are normal green. A single recessive nuclear gene controls tissue-specific alm1 phenotypic expression. Positional cloning revealed that the ALM1 gene encodes a Golden 2-like (GLK) transcription factor, HvGLK2, belonging to the GARP subfamily of Myb transcription factors. This finding was validated by genetic evidence indicating that all 10 alm1 mutants studied had a lesion in functionally important regions of HvGLK2, including the three alpha-helix domains, an AREAEAA motif and the GCT box. Transmission electron microscopy revealed that, in lemmas of the alm1.g mutant, the chloroplasts lacked thylakoid membranes, instead of stacked thylakoid grana in wild-type chloroplasts. Compared with wild type, alm1.g plants showed similar levels of leaf photosynthesis but reduced spike photosynthesis by 34%. The alm1.g mutant and the alm1.a mutant showed a reduction in 100-grain weight by 15.8% and 23.1%, respectively. As in other plants, barley has HvGLK2 and a paralog, HvGLK1. In flag leaves and awns, HvGLK2 and HvGLK1 are expressed at moderate levels, but in hulls, HvGLK1 expression was barely detectable compared with HvGLK2. Barley alm1/Hvglk2 mutants exhibit more severe phenotypes than glk2 mutants of other plant species reported to date. The severe alm1 phenotypic expression in multiple tissues indicates that HvGLK2 plays some roles that are nonredundant with HvGLK1.

Both overlapping and independent loci underlie seed number per pod and seed weight in Brassica napus by comparative quantitative trait loci analysis

Seed number per pod (SNPP) and seed weight (SW) are two components of seed yield in rapeseed (Brassica napus). Here, a natural population of rapeseed was employed for genome-wide association analysis for SNPP and SW across multi-years. A total of 101 and 77 SNPs significantly associated with SNPP and SW with the phenotypic variances (R2) ranging from 1.35 to 29.47% and from 0.78 to 34.58%, respectively. And 43 and 33 homologs of known genes from model plants were located in the 65 and 49 haplotype blocks (HBs) for SNPP and SW, respectively. Notably, we found 5 overlapping loci and 3 sets of loci with collinearity for both SNPP and SW, of which 4 overlapping loci harbored the haplotypes with the same direction of genetic effects on SNPP and SW, indicating high possibility to simultaneously improve SNPP and SW in rapeseed. Our findings revealed both overlapping and independent loci controlling seed number per pod and seed weight in rapeseed.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-021-01232-1.

Comparative transcriptome analysis identifies genes associated with chlorophyll levels and reveals photosynthesis in green flesh of radish taproot

The flesh of the taproot of Raphanus sativus L. is rich in chlorophyll (Chl) throughout the developmental process, which is why the flesh is green. However, little is known about which genes are associated with Chl accumulation in this non-foliar, internal green tissue and whether the green flesh can perform photosynthesis. To determine these aspects, we measured the Chl content, examined Chl fluorescence, and carried out comparative transcriptome analyses of taproot flesh between green-fleshed "Cuishuai" and white-fleshed "Zhedachang" across five developmental stages. Numerous genes involved in the Chl metabolic pathway were identified. It was found that Chl accumulation in radish green flesh may be due to the low expression of Chl degradation genes and high expression of Chl biosynthesis genes, especially those associated with Part Ⅳ (from Protoporphyrin Ⅸ to Chl a). Bioinformatics analysis revealed that differentially expressed genes between "Cuishuai" and "Zhedachang" were significantly enriched in photosynthesis-related pathways, such as photosynthesis, antenna proteins, porphyrin and Chl metabolism, carbon fixation, and photorespiration. Twenty-five genes involved in the Calvin cycle were highly expressed in "Cuishuai". These findings suggested that photosynthesis occurred in the radish green flesh, which was also supported by the results of Chl fluorescence. Our study provides transcriptome data on radish taproots and provides new information on the formation and function of radish green flesh.

Combining quantitative trait locus and co-expression analysis allowed identification of new candidates for oil accumulation in rapeseed

In crops there are quantitative trait loci (QTLs) in which some of the causal quantitative trait genes (QTGs) have not been functionally characterized even in the model plant Arabidopsis. We propose an approach to delineate QTGs in rapeseed by coordinating expression of genes located within QTLs and known orthologs related to traits from Arabidopsis. Using this method in developing siliques 15 d after pollination in 71 lines of rapeseed, we established an acyl-lipid metabolism co-expression network with 21 modules composed of 270 known acyl-lipid genes and 3503 new genes. The core module harbored 76 known genes involved in fatty acid and triacylglycerol biosynthesis and 671 new genes involved in sucrose transport, carbon metabolism, amino acid metabolism, seed storage protein processes, seed maturation, and phytohormone metabolism. Moreover, the core module closely associated with the modules of photosynthesis and carbon metabolism. From the co-expression network, we selected 12 hub genes to identify their putative Arabidopsis orthologs. These putative orthologs were functionally analysed using Arabidopsis knockout and overexpression lines. Four knockout mutants exhibited lower seed oil content, while the seed oil content in 10 overexpression lines was significantly increased. Therefore, combining gene co-expression network analysis and QTL mapping, this study provides new insights into the detection of QTGs and into acyl-lipid metabolism in rapeseed.

Grazing Intensity Alters Leaf and Spike Photosynthesis, Transpiration, and Related Parameters of Three Grass Species on an Alpine Steppe in the Qilian Mountains

The effect of grazing on leaf photosynthesis has been extensively studied. However, the influence of grazing on photosynthesis in other green tissues, especially spike, has remained poorly understood. This study investigated the impact of different grazing intensities (light grazing (LG), medium grazing (MG), and heavy grazing (HG)) on leaf and spike photosynthesis parameters and photosynthetic pigments of three grass species (Stipa purpurea, Achnatherum inebrians, and Leymus secalinus) on an alpine steppe in the Qilian Mountains. Grazing promoted leaf photosynthesis rate in S. purpurea and L. secalinus but reduced it in A. inebrians. Conversely, spike photosynthesis rate decreased in S. purpurea and L. secalinus under intense grazing, while there was no significant difference in spike photosynthesis rate in A. inebrians. The leaf and spike net photosynthetic rate (Pn) and transpiration rate (Tr) in S. purpurea were the greatest among the three species, while their organ temperatures were the lowest. On the other hand, grazing stimulated leaf chlorophyll biosynthesis in S. purpurea and L. secalinus but accelerated leaf chlorophyll degradation in A. inebrians. Furthermore, spike chlorophyll biosynthesis was inhibited in the three species under grazing, and only L. secalinus had the ability to recover from the impairment. Grazing had a positive effect on leaf photosynthesis parameters of S. purpurea and L. secalinus but a negative effect on those of A. inebrians. However, spike photosynthesis parameters were negatively influenced by grazing. Among the three species investigated, S. purpurea displayed the greatest ability for leaf and spike photosynthesis to withstand and acclimate to grazing stress. This study suggests that moderate grazing enhanced leaf photosynthetic capacity of S. purpurea and L. secalinus but reduced it in A. inebrians. However, spike photosynthetic capacity of three grass species decreased in response to grazing intensities.

Arabidopsis Plant Natriuretic Peptide Is a Novel Interactor of Rubisco Activase

Plant natriuretic peptides (PNPs) are a group of systemically acting peptidic hormones affecting solute and solvent homeostasis and responses to biotrophic pathogens. Although an increasing body of evidence suggests PNPs modulate plant responses to biotic and abiotic stress, which could lead to their potential biotechnological application by conferring increased stress tolerance to plants, the exact mode of PNPs action is still elusive. In order to gain insight into PNP-dependent signalling, we set out to identify interactors of PNP present in the model plant Arabidopsis thaliana, termed AtPNP-A. Here, we report identification of rubisco activase (RCA), a central regulator of photosynthesis converting Rubisco catalytic sites from a closed to an open conformation, as an interactor of AtPNP-A through affinity isolation followed by mass spectrometric identification. Surface plasmon resonance (SPR) analyses reveals that the full-length recombinant AtPNP-A and the biologically active fragment of AtPNP-A bind specifically to RCA, whereas a biologically inactive scrambled peptide fails to bind. These results are considered in the light of known functions of PNPs, PNP-like proteins, and RCA in biotic and abiotic stress responses.

Identification of genomic regions associated with multi-silique trait in Brassica napus

BACKGROUND

Although rapeseed (Brassica napus L.) mutant forming multiple siliques was morphologically described and considered to increase the silique number per plant, an important agronomic trait in this crop, the molecular mechanism underlying this beneficial trait remains unclear. Here, we combined bulked-segregant analysis (BSA) and whole genome re-sequencing (WGR) to map the genomic regions responsible for the multi-silique trait using two pools of DNA from the near-isogenic lines (NILs) zws-ms (multi-silique) and zws-217 (single-silique). We used the Euclidean Distance (ED) to identify genomic regions associated with this trait based on both SNPs and InDels. We also conducted transcriptome sequencing to identify differentially expressed genes (DEGs) between zws-ms and zws-217.

RESULTS

Genetic analysis using the ED algorithm identified three SNP- and two InDel-associated regions for the multi-silique trait. Two highly overlapped parts of the SNP- and InDel-associated regions were identified as important intersecting regions, which are located on chromosomes A09 and C08, respectively, including 2044 genes in 10.20-MB length totally. Transcriptome sequencing revealed 129 DEGs between zws-ms and zws-217 in buds, including 39 DEGs located in the two abovementioned associated regions. We identified candidate genes involved in multi-silique formation in rapeseed based on the results of functional annotation.

CONCLUSIONS

This study identified the genomic regions and candidate genes related to the multi-silique trait in rapeseed.