Diurnal down-regulation of ethylene biosynthesis mediates biomass heterosis

Transgenerational epigenetic inheritance during plant evolution and breeding

Plants can program and reprogram their genomes to create genetic variation and epigenetic modifications, leading to phenotypic plasticity. Although consequences of genetic changes are comprehensible, the basis for transgenerational inheritance of epigenetic variation is elusive. This review addresses contributions of external (environmental) and internal (genomic) factors to the establishment and maintenance of epigenetic memory during plant evolution, crop domestication, and modern breeding. Dynamic and pervasive changes in DNA methylation and chromatin modifications provide a diverse repertoire of epigenetic variation potentially for transgenerational inheritance. Elucidating and harnessing epigenetic inheritance will help us develop innovative breeding strategies and biotechnological tools to improve crop yield and resilience in the face of environmental challenges. Beyond plants, epigenetic principles are shared across sexually reproducing organisms including humans with relevance to medicine and public health.

Circadian and photoperiodic regulation of the vegetative to reproductive transition in plants

As sessile organisms, plants must respond constantly to ever-changing environments to complete their life cycle; this includes the transition from vegetative growth to reproductive development. This process is mediated by photoperiodic response to sensing the length of night or day through circadian regulation of light-signaling molecules, such as phytochromes, to measure the length of night to initiate flowering. Flowering time is the most important trait to optimize crop performance in adaptive regions. In this review, we focus on interplays between circadian and light signaling pathways that allow plants to optimize timing for flowering and seed production in Arabidopsis, rice, soybean, and cotton. Many crops are polyploids and domesticated under natural selection and breeding. In response to adaptation and polyploidization, circadian and flowering pathway genes are epigenetically reprogrammed. Understanding the genetic and epigenetic bases for photoperiodic flowering will help improve crop yield and resilience in response to climate change.

Striking a growth-defense balance: Stress regulators that function in maize development

Maize (Zea mays) cultivation is strongly affected by both abiotic and biotic stress, leading to reduced growth and productivity. It has recently become clear that regulators of plant stress responses, including the phytohormones abscisic acid (ABA), ethylene (ET), and jasmonic acid (JA), together with reactive oxygen species (ROS), shape plant growth and development. Beyond their well established functions in stress responses, these molecules play crucial roles in balancing growth and defense, which must be finely tuned to achieve high yields in crops while maintaining some level of defense. In this review, we provide an in-depth analysis of recent research on the developmental functions of stress regulators, focusing specifically on maize. By unraveling the contributions of these regulators to maize development, we present new avenues for enhancing maize cultivation and growth while highlighting the potential risks associated with manipulating stress regulators to enhance grain yields in the face of environmental challenges.

Integrated Metabolome and Transcriptome Analysis of Gibberellins Mediated the Circadian Rhythm of Leaf Elongation by Regulating Lignin Synthesis in Maize

Plant growth exhibits rhythmic characteristics, and gibberellins (GAs) are involved in regulating cell growth, but it is still unclear how GAs crosstalk with circadian rhythm to regulate cell elongation. The study analyzed growth characteristics of wild-type (WT), zmga3ox and zmga3ox with GA3 seedlings. We integrated metabolomes and transcriptomes to study the interaction between GAs and circadian rhythm in mediating leaf elongation. The rates of leaf growth were higher in WT than zmga3ox, and zmga3ox cell length was shorter when proliferated in darkness than light, and GA3 restored zmga3ox leaf growth. The differentially expressed genes (DEGs) between WT and zmga3ox were mainly enriched in hormone signaling and cell wall synthesis, while DEGs in zmga3ox were restored to WT by GA3. Moreover, the number of circadian DEGs that reached the peak expression in darkness was more than light, and the upregulated circadian DEGs were mainly enriched in cell wall synthesis. The differentially accumulated metabolites (DAMs) were mainly attributed to flavonoids and phenolic acid. Twenty-two DAMs showed rhythmic accumulation, especially enriched in lignin synthesis. The circadian DEGs ZmMYBr41/87 and ZmHB34/70 were identified as regulators of ZmHCT8 and ZmBM1, which were enzymes in lignin synthesis. Furthermore, GAs regulated ZmMYBr41/87 and ZmHB34/70 to modulate lignin biosynthesis for mediating leaf rhythmic growth.

A truncated ETHYLENE INSENSITIVE3-like protein, GhLYI, regulates senescence in cotton

Numerous endogenous and environmental signals regulate the intricate and highly orchestrated process of plant senescence. Ethylene (ET), which accumulates as senescence progresses, is a major promoter of leaf senescence. The master transcription activator ETHYLENE INSENSITIVE3 (EIN3) activates the expression of a wide range of downstream genes during leaf senescence. Here, we found that a unique EIN3-LIKE 1 (EIL1) gene, cotton LINT YIELD INCREASING (GhLYI), encodes a truncated EIN3 protein in upland cotton (Gossypium hirsutum L.) that functions as an ET signal response factor and a positive regulator of senescence. Ectopic expression or overexpression of GhLYI accelerated leaf senescence in both Arabidopsis (Arabidopsis thaliana) and cotton. Cleavage under targets and tagmentation (CUT&Tag) analyses revealed that SENESCENCE-ASSOCIATED GENE 20 (SAG20) was a target of GhLYI. Electrophoretic mobility shift assay (EMSA), yeast 1-hybrid (Y1H), and dual-luciferase transient expression assay confirmed that GhLYI directly bound the promoter of SAG20 to activate its expression. Transcriptome analysis revealed that transcript levels of a series of senescence-related genes, SAG12, NAC-LIKE, ACTIVATED by APETALA 3/PISTILLATA (NAP/ANAC029), and WRKY53, are substantially induced in GhLYI overexpression plants compared with wild-type (WT) plants. Virus-induced gene silencing (VIGS) preliminarily confirmed that knockdown of GhSAG20 delayed leaf senescence. Collectively, our findings provide a regulatory module involving GhLYI-GhSAG20 in controlling senescence in cotton.

Protein nonadditive expression and solubility contribute to heterosis in Arabidopsis hybrids and allotetraploids

Hybrid vigor or heterosis has been widely applied in agriculture and extensively studied using genetic and gene expression approaches. However, the biochemical mechanism underlying heterosis remains elusive. One theory suggests that a decrease in protein aggregation may occur in hybrids due to the presence of protein variants between parental alleles, but it has not been experimentally tested. Here, we report comparative analysis of soluble and insoluble proteomes in Arabidopsis intraspecific and interspecific hybrids or allotetraploids formed between A. thaliana and A. arenosa. Both allotetraploids and intraspecific hybrids displayed nonadditive expression (unequal to the sum of the two parents) of the proteins, most of which were involved in biotic and abiotic stress responses. In the allotetraploids, homoeolog-expression bias was not observed among all proteins examined but accounted for 17-20% of the nonadditively expressed proteins, consistent with the transcriptome results. Among expression-biased homoeologs, there were more A. thaliana-biased than A. arenosa-biased homoeologs. Analysis of the insoluble and soluble proteomes revealed more soluble proteins in the hybrids than their parents but not in the allotetraploids. Most proteins in ribosomal biosynthesis and in the thylakoid lumen, membrane, and stroma were in the soluble fractions, indicating a role of protein stability in photosynthetic activities for promoting growth. Thus, nonadditive expression of stress-responsive proteins and increased solubility of photosynthetic proteins may contribute to heterosis in Arabidopsis hybrids and allotetraploids and possibly hybrid crops.

MORF2-mediated plastidial retrograde signaling is involved in stress response and skotomorphogenesis beyond RNA editing

Retrograde signaling modulates the expression of nuclear genome-encoded organelle proteins to adjust organelle function in response to environmental cues. MULTIPLE ORGANELLAR RNA EDITING FACTOR 2 (MORF2) was initially recognized as a plastidial RNA-editing factor but recently shown to interact with GUN1. Given the central role of GUN1 in chloroplast retrograde signaling and the unviable phenotype of morf2 mutants that is inconsistent with many viable mutants involved in RNA editing, we hypothesized that MORF2 has functions either dosage dependent or beyond RNA editing. Using an inducible Clustered Interspaced Short Palindromic Repeat interference (iCRISPRi) approach, we were able to reduce the MORF2 transcripts in a controlled manner. In addition to MORF2-dosage dependent RNA-editing errors, we discovered that reducing MORF2 by iCRISPRi stimulated the expression of stress responsive genes, triggered plastidial retrograde signaling, repressed ethylene signaling and skotomorphogenesis, and increased accumulation of hydrogen peroxide. These findings along with previous discoveries suggest that MORF2 is an effective regulator involved in plastidial metabolic pathways whose reduction can readily activate multiple retrograde signaling molecules possibly involving reactive oxygen species to adjust plant growth. In addition, our newly developed iCRISPRi approach provided a novel genetic tool for quantitative reverse genetics studies on hub genes in plants.

Transcriptome Analysis Reveals a Major Gene Expression Pattern and Important Metabolic Pathways in the Control of Heterosis in Chinese Cabbage

Although heterosis is commonly used in Chinese cabbage, its molecular basis is poorly understood. In this study, 16Chinese cabbage hybrids were utilized as test subjects to explore the potential molecular mechanism of heterosis. RNA sequencing revealed 5815-10,252 differentially expressed genes (DEGs) (female parent vs. male parent), 1796-5990 DEGs (female parent-vs-hybrid), and 2244-7063 DEGs (male parent vs. hybrid) in 16 cross combinations at the middle stage of heading. Among of them, 72.83-84.20% DEGs conformed to the dominant expression pattern, which is the predominant expression pattern in hybrids. There were 13 pathways in which DEGs were significantly enriched in most cross combinations. Among them, the plant-pathogen interaction (ko04626) and circadian rhythm-plant (ko04712)were significantly enriched by DEGs in strong heterosis hybrids. WGCNA also proved that the two pathways were significantly related to heterosis in Chinese cabbage.

Phytochrome B mediates dim-light-reduced insect resistance by promoting the ethylene pathway in rice

Increasing planting density is one of the most effective ways to improve crop yield. However, one major factor that limits crop planting density is the weakened immunity of plants to pathogens and insects caused by dim light (DL) under shade conditions. The molecular mechanism underlying how DL compromises plant immunity remains unclear. Here, we report that DL reduces rice (Oryza sativa) resistance against brown planthopper (BPH; Nilaparvata lugens) by elevating ethylene (ET) biosynthesis and signaling in a Phytochrome B (OsPHYB)-dependent manner. The DL-reduced BPH resistance is relieved in osphyB mutants, but aggravated in OsPHYB overexpressing plants. Further, we found that DL reduces the nuclear accumulation of OsphyB, thus alleviating Phytochrome Interacting Factor Like14 (OsPIL14) degradation, consequently leading to the up-regulation of 1-Aminocyclopropane-1-Carboxylate Oxidase1 (OsACO1) and an increase in ET levels. In addition, we found that nuclear OsphyB stabilizes Ethylene Insensitive Like2 (OsEIL2) by competitively interacting with EIN3 Binding F-Box Protein (OsEBF1) to enhance ET signaling in rice, which contrasts with previous findings that phyB blocks ET signaling by facilitating Ethylene Insensitive3 (EIN3) degradation in other plant species. Thus, enhanced ET biosynthesis and signaling reduces BPH resistance under DL conditions. Our findings provide insights into the molecular mechanism of the light-regulated ET pathway and host-insect interactions and potential strategies for sustainable insect management.

Endosperm and Maternal-specific expression of EIN2 in the endosperm affects endosperm cellularization and seed size in Arabidopsis

Seed size is related to plant evolution and crop yield and is affected by genetic mutations, imprinting, and genome dosage. Imprinting is a widespread epigenetic phenomenon in mammals and flowering plants. ETHYLENE INSENSITIVE2 (EIN2) encodes a membrane protein that links the ethylene perception to transcriptional regulation. Interestingly, during seed development EIN2 is maternally expressed in Arabidopsis and maize, but the role of EIN2 in seed development is unknown. Here, we show that EIN2 is expressed specifically in the endosperm, and the maternal-specific EIN2 expression affects temporal regulation of endosperm cellularization. As a result, seed size increases in the genetic cross using the ein2 mutant as the maternal parent or in the ein2 mutant. The maternal-specific expression of EIN2 in the endosperm is controlled by DNA methylation but not by H3K27me3 or by ethylene and several ethylene pathway genes tested. RNA-seq analysis in the endosperm isolated by laser-capture microdissection show upregulation of many endosperm-expressed genes such as AGAMOUS-LIKEs (AGLs) in the ein2 mutant or when the maternal EIN2 allele is not expressed. EIN2 does not interact with DNA and may act through ETHYLENE INSENSITIVE3 (EIN3), a DNA-binding protein present in sporophytic tissues, to activate target genes like AGLs, which in turn mediate temporal regulation of endosperm cellularization and seed size. These results provide mechanistic insights into endosperm and maternal-specific expression of EIN2 on endosperm cellularization and seed development, which could help improve seed production in plants and crops.

HEAT-RESPONSIVE PROTEIN regulates heat stress via fine-tuning ethylene/auxin signaling pathways in cotton

Plants sense and respond to fluctuating temperature and light conditions during the circadian cycle; however, the molecular mechanism underlying plant adaptability during daytime warm conditions remains poorly understood. In this study, we reveal that the ectopic regulation of a HEAT RESPONSIVE PROTEIN (GhHRP) controls the adaptation and survival of cotton (Gossypium hirsutum) plants in response to warm conditions via modulating phytohormone signaling. Increased ambient temperature promptly enhanced the binding of the phytochrome interacting factor 4 (GhPIF4)/ethylene-insensitive 3 (GhEIN3) complex to the GhHRP promoter to increase its mRNA level. The ectopic expression of GhHRP promoted the temperature-dependent accumulation of GhPIF4 transcripts and hypocotyl elongation by triggering thermoresponsive growth-related genes. Notably, the upregulation of the GhHRP/GhPIF4 complex improved plant growth via modulating the abundance of Arabidopsis thaliana auxin biosynthetic gene YUCCA8 (AtYUC8)/1-aminocyclopropane-1-carboxylate synthase 8 (AtACS8) for fine-tuning the auxin/ethylene interplay, ultimately resulting in decreased ethylene biosynthesis. GhHRP thus protects chloroplasts from photo-oxidative bursts via repressing AtACS8 and AtACS7 and upregulating AtYUC8 and the heat shock transcription factors (HSFA2), heat shock proteins (HSP70 and HSP20). Strikingly, the Δhrp disruption mutant exhibited compromised production of HSP/YUC8 that resulted in an opposite phenotype with the loss of the ability to respond to warm conditions. Our results show that GhHRP is a heat-responsive signaling component that assists plants in confronting the dark phase and modulates auxin signaling to rescue growth under temperature fluctuations.

Dominant complementation of biological pathways in maize hybrid lines is associated with heterosis

Allele-specific expressed genes (ASEGs) are widespread in maize hybrid lines and play important roles of complementation of biological pathways in heterosis. Heterosis (hybrid vigor) is an important phenomenon with both theoretical and practical value. However, our understanding of the genetic and molecular mechanisms behind heterosis is still limited. Here, we analyzed a comprehensive dataset of maize (Zea mays L.), including RNA-seq data from three hybrid-parent triplets (HPTs) and acetylated protein data from one HPT. The gene expression patterns exhibited extensive variation between the hybrids and their parents, and a substantial number of allele-specific expressed genes (ASEGs) were identified in the hybrids. Notably, ASEGs from different HPTs were significantly enriched in various conserved pathways. The parental alleles of ASEGs with fewer deleterious single-nucleotide polymorphisms were more likely to be expressed in hybrid lines than other parental alleles. ASEGs were mainly enriched in the functional gene ontology terms protein biosynthesis, photosynthesis, and metabolism. In addition, the ASEGs across the three HPTs were involved in key photosynthetic pathways and might enhance the photosynthetic efficiency of the hybrids. These findings suggest that ASEGs involved in complementary biological pathways in maize hybrids contribute to heterosis, shedding new light on the molecular mechanism of heterosis.

ARF2-PIF5 interaction controls transcriptional reprogramming in the ABS3-mediated plant senescence pathway

One of the hallmarks of plant senescence is the global transcriptional reprogramming coordinated by a plethora of transcription factors (TFs). However, mechanisms underlying the interactions between different TFs in modulating senescence remain obscure. Previously, we discovered that plant ABS3 subfamily MATE transporter genes regulate senescence and senescence-associated transcriptional changes. In a genetic screen for mutants suppressing the accelerated senescence phenotype of the gain-of-function mutant abs3-1D, AUXIN RESPONSE FACTOR 2 (ARF2) and PHYTOCHROME-INTERACTING FACTOR 5 (PIF5) were identified as key TFs responsible for transcriptional regulation in the ABS3-mediated senescence pathway. ARF2 and PIF5 (as well as PIF4) interact directly and function interdependently to promote senescence, and they share common target genes such as key senescence promoting genes ORESARA 1 (ORE1) and STAY-GREEN 1 (SGR1) in the ABS3-mediated senescence pathway. In addition, we discovered reciprocal regulation between ABS3-subfamily MATEs and the ARF2 and PIF5/4 TFs. Taken together, our findings reveal a regulatory paradigm in which the ARF2-PIF5/4 functional module facilitates the transcriptional reprogramming in the ABS3-mediated senescence pathway.

Heterosis and Differential DNA Methylation in Soybean Hybrids and Their Parental Lines

Heterosis is an important biological phenomenon and is widely applied to increase agricultural productivity. However, the underlying molecular mechanisms of heterosis are still unclear. Here we constructed three combinations of reciprocal hybrids of soybean, and subsequently used MethylRAD-seq to detect CCGG and CCWGG (W = A or T) methylation in the whole genome of these hybrids and their parents at the middle development period of contemporary seed. We were able to prove that changes in DNA methylation patterns occurred in immature hybrid seeds and the parental variation was to some degree responsible for differential expression between the reciprocal hybrids. Non-additive differential methylation sites (DMSs) were also identified in large numbers in hybrids. Interestingly, most of these DMSs were hyper-methylated and were more concentrated in gene regions than the natural distribution of the methylated sites. Further analysis of the non-additive DMSs located in gene regions exhibited their participation in various biological processes, especially those related to transcriptional regulation and hormonal function. These results revealed DNA methylation reprogramming pattern in the hybrid soybean, which is associated with phenotypic variation and heterosis initiation.

Circadian clock in plants: Linking timing to fitness

Endogenous circadian clock integrates cyclic signals of environment and daily and seasonal behaviors of organisms to achieve spatiotemporal synchronization, which greatly improves genetic diversity and fitness of species. This review addresses recent studies on the plant circadian system in the field of chronobiology, covering topics on molecular mechanisms, internal and external Zeitgebers, and hierarchical regulation of physiological outputs. The architecture of the circadian clock involves the autoregulatory transcriptional feedback loops, post-translational modifications of core oscillators, and epigenetic modifications of DNA and histones. Here, light, temperature, humidity, and internal elemental nutrients are summarized to illustrate the sensitivity of the circadian clock to timing cues. In addition, the circadian clock runs cell-autonomously, driving independent circadian rhythms in various tissues. The core oscillators responds to each other with biochemical factors including calcium ions, mineral nutrients, photosynthetic products, and hormones. We describe clock components sequentially expressed during a 24-h day that regulate rhythmic growth, aging, immune response, and resistance to biotic and abiotic stresses. Notably, more data have suggested the circadian clock links chrono-culture to key agronomic traits in crops.

From hybrid genomes to heterotic trait output: Challenges and opportunities

Heterosis (or hybrid vigor) has been widely used in crop seed breeding to improve many key economic traits. Nevertheless, the genetic and molecular basis of this important phenomenon has long remained elusive, constraining its flexible and effective exploitation. Advanced genomic approaches are efficient in characterizing the mechanism of heterosis. Here, we review how the omics approaches, including genomic, transcriptomic, and population genetics methods such as genome-wide association studies, can reveal how hybrid genomes outperform parental genomes in plants. This information opens up opportunities for genomic exploration and manipulation of heterosis in crop breeding.

Plant height heterosis is quantitatively associated with expression levels of plastid ribosomal proteins

The use of hybrids is widespread in agriculture, yet the molecular basis for hybrid vigor (heterosis) remains obscure. To identify molecular components that may contribute to trait heterosis, we analyzed paired proteomic and transcriptomic data from seedling leaf and mature leaf blade tissues of maize hybrids and their inbred parents. Nuclear- and plastid-encoded subunits of complexes required for protein synthesis in the chloroplast and for the light reactions of photosynthesis were expressed above midparent and high-parent levels, respectively. Consistent with previous reports in Arabidopsis, ethylene biosynthetic enzymes were expressed below midparent levels in the hybrids, suggesting a conserved mechanism for heterosis between monocots and dicots. The ethylene biosynthesis mutant, acs2/acs6, largely phenocopied the hybrid proteome, indicating that a reduction in ethylene biosynthesis may mediate the differences between inbreds and their hybrids. To rank the relevance of expression differences to trait heterosis, we compared seedling leaf protein levels to the adult plant height of 15 hybrids. Hybrid/midparent expression ratios were most positively correlated with hybrid/midparent plant height ratios for the chloroplast ribosomal proteins. Our results show that increased expression of chloroplast ribosomal proteins in hybrid seedling leaves is mediated by reduced expression of ethylene biosynthetic enzymes and that the degree of their overexpression in seedlings can quantitatively predict adult trait heterosis.

Verticillium dahliae secretory effector PevD1 induces leaf senescence by promoting ORE1-mediated ethylene biosynthesis

Leaf senescence, the final stage of leaf development, is influenced by numerous internal and environmental signals. However, how biotic stresses such as pathogen infection regulate leaf senescence remains largely unclear. In this study, we found that the premature leaf senescence in Arabidopsis caused by the soil-borne vascular fungus Verticillium dahliae was impaired by disruption of a protein elicitor from V. dahliae 1 named PevD1. Constitutive or inducible overexpression of PevD1 accelerated Arabidopsis leaf senescence. Interestingly, a senescence-associated NAC transcription factor, ORE1, was targeted by PevD1. PevD1 could interact with and stabilize ORE1 protein by disrupting its interaction with the RING-type ubiquitin E3 ligase NLA. Mutation of ORE1 suppressed the premature senescence caused by overexpressing PevD1, whereas overexpression of ORE1 or PevD1 led to enhanced ethylene production and thereby leaf senescence. We showed that ORE1 directly binds the promoter of ACS6 and promotes its expression for mediating PevD1-induced ethylene biosynthesis. Loss-of-function of ACSs could suppress V. dahliae-induced leaf senescence in ORE1-overexpressing plants. Furthermore, we found thatPevD1 also interacts with Gossypium hirsutum ORE1 (GhORE1) and that virus-induced gene silencing of GhORE1 delays V. dahliae-triggered leaf senescence in cotton, indicating a possibly conserved mechanism in plants. Taken together, these results suggest that V. dahliae induces leaf senescence by secreting the effector PevD1 to manipulate the ORE1-ACS6 cascade, providing new insights into biotic stress-induced senescence in plants.

Plants Saline Environment in Perception with Rhizosphere Bacteria Containing 1-Aminocyclopropane-1-Carboxylate Deaminase

Soil salinity stress has become a serious roadblock for food production worldwide since it is one of the key factors affecting agricultural productivity. Salinity and drought are predicted to cause considerable loss of crops. To deal with this difficult situation, a variety of strategies have been developed, including plant breeding, plant genetic engineering, and a wide range of agricultural practices, including the use of plant growth-promoting rhizobacteria (PGPR) and seed biopriming techniques, to improve the plants' defenses against salinity stress, resulting in higher crop yields to meet future human food demand. In the present review, we updated and discussed the negative effects of salinity stress on plant morphological parameters and physio-biochemical attributes via various mechanisms and the beneficial roles of PGPR with 1-Aminocyclopropane-1-Carboxylate(ACC) deaminase activity as green bio-inoculants in reducing the impact of saline conditions. Furthermore, the applications of ACC deaminase-producing PGPR as a beneficial tool in seed biopriming techniques are updated and explored. This strategy shows promise in boosting quick seed germination, seedling vigor and plant growth uniformity. In addition, the contentious findings of the variation of antioxidants and osmolytes in ACC deaminase-producing PGPR treated plants are examined.

Advances in Research on the Mechanism of Heterosis in Plants

Heterosis is a common biological phenomenon in nature. It substantially contributes to the biomass yield and grain yield of plants. Moreover, this phenomenon results in high economic returns in agricultural production. However, the utilization of heterosis far exceeds the level of theoretical research on this phenomenon. In this review, the recent progress in research on heterosis in plants was reviewed from the aspects of classical genetics, parental genetic distance, quantitative trait loci, transcriptomes, proteomes, epigenetics (DNA methylation, histone modification, and small RNA), and hormone regulation. A regulatory network of various heterosis-related genes under the action of different regulatory factors was summarized. This review lays a foundation for the in-depth study of the molecular and physiological aspects of this phenomenon to promote its effects on increasing the yield of agricultural production.

LCM and RNA-seq analyses revealed roles of cell cycle and translational regulation and homoeolog expression bias in cotton fiber cell initiation

Background

Cotton fibers provide a powerful model for studying cell differentiation and elongation. Each cotton fiber is a singular and elongated cell derived from epidermal-layer cells of a cotton seed. Efforts to understand this dramatic developmental shift have been impeded by the difficulty of separation between fiber and epidermal cells.

Results

Here we employed laser-capture microdissection (LCM) to separate these cell types. RNA-seq analysis revealed transitional differences between fiber and epidermal-layer cells at 0 or 2 days post anthesis. Specifically, down-regulation of putative cell cycle genes was coupled with upregulation of ribosome biosynthesis and translation-related genes, which may suggest their respective roles in fiber cell initiation. Indeed, the amount of fibers in cultured ovules was increased by cell cycle progression inhibitor, Roscovitine, and decreased by ribosome biosynthesis inhibitor, Rbin-1. Moreover, subfunctionalization of homoeologs was pervasive in fiber and epidermal cells, with expression bias towards 10% more D than A homoeologs of cell cycle related genes and 40–50% more D than A homoeologs of ribosomal protein subunit genes. Key cell cycle regulators were predicted to be epialleles in allotetraploid cotton. MYB-transcription factor genes displayed expression divergence between fibers and ovules. Notably, many phytohormone-related genes were upregulated in ovules and down-regulated in fibers, suggesting spatial-temporal effects on fiber cell development.

Conclusions

Fiber cell initiation is accompanied by cell cycle arrest coupled with active ribosome biosynthesis, spatial-temporal regulation of phytohormones and MYB transcription factors, and homoeolog expression bias of cell cycle and ribosome biosynthesis genes. These valuable genomic resources and molecular insights will help develop breeding and biotechnological tools to improve cotton fiber production.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07579-1.

Temporal Regulation of the Metabolome and Proteome in Photosynthetic and Photorespiratory Pathways Contributes to Maize Heterosis

Heterosis or hybrid vigor is widespread in plants and animals. Although the molecular basis for heterosis has been extensively studied, metabolic and proteomic contributions to heterosis remain elusive. Here we report an integrative analysis of time-series metabolome and proteome data in maize (Zea mays) hybrids and their inbred parents. Many maize metabolites and proteins are diurnally regulated, and many of these show nonadditive abundance in the hybrids, including key enzymes and metabolites involved in carbon assimilation. Compared with robust trait heterosis, metabolic heterosis is relatively mild. Interestingly, most amino acids display negative mid-parent heterosis (MPH), i.e., having lower values than the average of the parents, while sugars, alcohols, and nucleoside metabolites show positive MPH. From the network perspective, metabolites in the photosynthetic pathway show positive MPH, whereas metabolites in the photorespiratory pathway show negative MPH, which corresponds to nonadditive protein abundance and enzyme activities of key enzymes in the respective pathways in the hybrids. Moreover, diurnally expressed proteins that are upregulated in the hybrids are enriched in photosynthesis-related gene-ontology terms. Hybrids may more effectively remove toxic metabolites generated during photorespiration, and thus maintain higher photosynthetic efficiency. These metabolic and proteomic resources provide unique insight into heterosis and its utilization for high yielding maize and other crop plants.

TOC1 in Nicotiana attenuata regulates efficient allocation of nitrogen to defense metabolites under herbivory stress

The circadian clock contextualizes plant responses to environmental signals. Plants use temporal information to respond to herbivory, but many of the functional roles of circadian clock components in these responses, and their contribution to fitness, remain unknown. We investigate the role of the central clock regulator TIMING OF CAB EXPRESSION 1 (TOC1) in Nicotiana attenuata's defense responses to the specialist herbivore Manduca sexta under both field and glasshouse conditions. We utilize 15 N pulse-labeling to quantify nitrogen incorporation into pools of three defense compounds: caffeoylputrescine (CP), dicaffeoyl spermidine (DCS) and nicotine. Nitrogen incorporation was decreased in CP and DCS and increased in nicotine pools in irTOC1 plants compared to empty vector (EV) under control conditions, but these differences were abolished after simulated herbivory. Differences between EV and irTOC1 plants in nicotine, but not phenolamide production, were abolished by treatment with the ethylene agonist 1-methylcyclopropene. Using micrografting, TOC1's effect on nicotine was isolated to the root and did not affect the fitness of heterografts under field conditions. These results suggest that the circadian clock contributes to plant fitness by balancing production of metabolically expensive nitrogen-rich defense compounds and mediating the allocation of resources between vegetative biomass and reproduction.

Insights into the genes involved in the ethylene biosynthesis pathway in Arabidopsis thaliana and Oryza sativa

Background

Ethylene is a gaseous plant hormone that acts as a requisite role in many aspects of the plant life cycle, and it is also a regulator of plant responses to abiotic and biotic stresses. In this study, we attempt to provide comprehensive information through analyses of existing data using bioinformatics tools to compare the identified ethylene biosynthesis genes between Arabidopsis (as dicotyledonous) and rice (as monocotyledonous).

Results

The results exposed that the Arabidopsis proteins of the ethylene biosynthesis pathway had more potential glycosylation sites than rice, and 1-aminocyclopropane-1-carboxylate oxidase proteins were less phosphorylated than 1-aminocyclopropane-1-carboxylate synthase and S-adenosylmethionine proteins. According to the gene expression patterns, S-adenosylmethionine genes were more involved in the rice-ripening stage while in Arabidopsis, ACS2, and 1-aminocyclopropane-1-carboxylate oxidase genes were contributed to seed maturity. Furthermore, the result of miRNA targeting the transcript sequences showed that ath-miR843 and osa-miR1858 play a key role to regulate the post-transcription modification of S-adenosylmethionine genes in Arabidopsis and rice, respectively. The discovered cis- motifs in the promoter site of all the ethylene biosynthesis genes of A. thaliana genes were engaged to light-induced response in the cotyledon and root genes, sulfur-responsive element, dehydration, cell cycle phase-independent activation, and salicylic acid. The ACS4 protein prediction demonstrated strong protein-protein interaction in Arabidopsis, as well as, SAM2, Os04T0578000, Os01T0192900, and Os03T0727600 predicted strong protein-protein interactions in rice.

Conclusion

In the current study, the complex between miRNAs with transcript sequences of ethylene biosynthesis genes in A. thaliana and O. sativa were identified, which could be helpful to understand the gene expression regulation after the transcription process. The binding sites of common transcription factors such as MYB, WRKY, and ABRE that control target genes in abiotic and biotic stresses were generally distributed in promoter sites of ethylene biosynthesis genes of A. thaliana. This was the first time to wide explore the ethylene biosynthesis pathway using bioinformatics tools that markedly showed the capability of the in silico study to integrate existing data and knowledge and furnish novel insights into the understanding of underlying ethylene biosynthesis pathway genes that will be helpful for more dissection.

Parental legacy and regulatory novelty in Brachypodium diurnal transcriptomes accompanying their polyploidy

Polyploidy is a widespread phenomenon in eukaryotes that can lead to phenotypic novelty and has important implications for evolution and diversification. The modification of phenotypes in polyploids relative to their diploid progenitors may be associated with altered gene expression. However, it is largely unknown how interactions between duplicated genes affect their diurnal expression in allopolyploid species. In this study, we explored parental legacy and hybrid novelty in the transcriptomes of an allopolyploid species and its diploid progenitors. We compared the diurnal transcriptomes of representative Brachypodium cytotypes, including the allotetraploid Brachypodium hybridum and its diploid progenitors Brachypodium distachyon and Brachypodium stacei. We also artificially induced an autotetraploid B. distachyon. We identified patterns of homoeolog expression bias (HEB) across Brachypodium cytotypes and time-dependent gain and loss of HEB in B. hybridum. Furthermore, we established that many genes with diurnal expression experienced HEB, while their expression patterns and peak times were correlated between homoeologs in B. hybridum relative to B. distachyon and B. stacei, suggesting diurnal synchronization of homoeolog expression in B. hybridum. Our findings provide insight into the parental legacy and hybrid novelty associated with polyploidy in Brachypodium, and highlight the evolutionary consequences of diurnal transcriptional regulation that accompanied allopolyploidy.

Single-cell RNA-seq analysis reveals ploidy-dependent and cell-specific transcriptome changes in Arabidopsis female gametophytes

BACKGROUND

Polyploidy provides new genetic material that facilitates evolutionary novelty, species adaptation, and crop domestication. Polyploidy often leads to an increase in cell or organism size, which may affect transcript abundance or transcriptome size, but the relationship between polyploidy and transcriptome changes remains poorly understood. Plant cells often undergo endoreduplication, confounding the polyploid effect.

RESULTS

To mitigate these effects, we select female gametic cells that are developmentally stable and void of endoreduplication. Using single-cell RNA sequencing (scRNA-seq) in Arabidopsis thaliana tetraploid lines and isogenic diploids, we show that transcriptome abundance doubles in the egg cell and increases approximately 1.6-fold in the central cell, consistent with cell size changes. In the central cell of tetraploid plants, DEMETER (DME) is upregulated, which can activate PRC2 family members FIS2 and MEA, and may suppress the expression of other genes. Upregulation of cell size regulators in tetraploids, including TOR and OSR2, may increase the size of reproductive cells. In diploids, the order of transcriptome abundance is central cell, synergid cell, and egg cell, consistent with their cell size variation. Remarkably, we uncover new sets of female gametophytic cell-specific transcripts with predicted biological roles; the most abundant transcripts encode families of cysteine-rich peptides, implying roles in cell-cell recognition during double fertilization.

CONCLUSIONS

Transcriptome in single cells doubles in tetraploid plants compared to diploid, while the degree of change and relationship to the cell size depends on cell types. These scRNA-seq resources are free of cross-contamination and are uniquely valuable for advancing plant hybridization, reproductive biology, and polyploid genomics.

In Arabidopsis thaliana Heterosis Level Varies among Individuals in an F1 Hybrid Population

Heterosis or hybrid vigour is a phenomenon in which hybrid progeny exhibit superior yield and biomass to parental lines and has been used to breed F₁ hybrid cultivars in many crops. A similar level of heterosis in all F₁ individuals is expected as they are genetically identical. However, we found variation in rosette size in individual F₁ plants from a cross between C24 and Columbia-0 accessions of Arabidopsis thaliana. Big-sized F₁ plants had 26.1% larger leaf area in the first and second leaves than medium-sized F₁ plants at 14 days after sowing in spite of the identical genetic background. We identified differentially expressed genes between big- and medium-sized F₁ plants by microarray; genes involved in the category of stress response were overrepresented. We made transgenic plants overexpressing 21 genes, which were differentially expressed between the two size classes, and some lines had increased plant size at 14 or 21 days after sowing but not at all time points during development. Change of expression levels in stress-responsive genes among individual F₁ plants could generate the variation in plant size of individual F₁ plants in A. thaliana.

Root ethylene mediates rhizosphere microbial community reconstruction when chemically detecting cyanide produced by neighbouring plants

BACKGROUND

Stress-induced hormones are essential for plants to modulate their microbiota and dynamically adjust to the environment. Despite the emphasis of the role of the phytohormone ethylene in the plant physiological response to heterospecific neighbour detection, less is known about how this activated signal mediates focal plant rhizosphere microbiota to enhance plant fitness. Here, using 3 years of peanut (Arachis hypogaea L.), a legume, and cyanide-containing cassava (Manihot esculenta Crantz) intercropping and peanut monocropping field, pot and hydroponic experiments in addition to exogenous ethylene application and soil incubation experiments, we found that ethylene, a cyanide-derived signal, is associated with the chemical identification of neighbouring cassava and the microbial re-assemblage in the peanut rhizosphere.

RESULTS

Ethylene production in peanut roots can be triggered by cyanide production of neighbouring cassava plants. This gaseous signal alters the microbial composition and re-assembles the microbial co-occurrence network of peanut by shifting the abundance of an actinobacterial species, Catenulispora sp., which becomes a keystone in the intercropped peanut rhizosphere. The re-assembled rhizosphere microbiota provide more available nutrients to peanut roots and support seed production.

CONCLUSIONS

Our findings suggest that root ethylene acts as a signal with a dual role. It plays a role in perceiving biochemical cues from interspecific neighbours, and also has a regulatory function in mediating the rhizosphere microbial assembly, thereby enhancing focal plant fitness by improving seed production. This discovery provides a promising direction to develop novel intercropping strategies for targeted manipulations of the rhizosphere microbiome through phytohormone signals. Video abstract.

In Arabidopsis hybrids and Hybrid Mimics, up-regulation of cell wall biogenesis is associated with the increased plant size

Hybrid breeding is of economic importance in agriculture for increasing yield, yet the basis of heterosis is not well understood. In Arabidopsis, crosses between different accessions produce hybrids with different levels of heterosis relative to parental phenotypes in biomass. In all hybrids, the advantage of the F1 hybrid in both phenotypic uniformity and yield gain is lost in the heterogeneous F2. F5/F6 Hybrid Mimics generated from a cross between C24 and Landsberg erecta (Ler) ecotypes demonstrated that the large plant phenotype of the F1 hybrids can be stabilized. Hybrid Mimic selection was applied to Wassilewskija (Ws)/Ler and Col/Ler hybrids. The two hybrids show different levels of heterosis. The Col/Ler hybrid generated F7 Hybrid Mimics with rosette diameter and fresh weight equivalent to the F1 hybrid at 30 DAS; F7 Ws/Ler Hybrid Mimics outperformed the F1 hybrid in both the rosette size and biomass. Transcriptome analysis revealed up-regulation of cell wall biosynthesis, and cell wall expansion genes could be a common pathway in increased size in the Arabidopsis hybrids and Hybrid Mimics. Intercross of two independent Hybrid Mimic lines can further increase the biomass gain. Our results encourage the use of Hybrid Mimics for breeding and for investigating the molecular basis of heterosis.

Genome-Wide Identification of Splicing Quantitative Trait Loci (sQTLs) in Diverse Ecotypes of Arabidopsis thaliana

Alternative splicing (AS) of pre-mRNAs contributes to transcriptome diversity and enables plants to generate different protein isoforms from a single gene and/or fine-tune gene expression during different development stages and environmental changes. Although AS is pervasive, the genetic basis for differential isoform usage in plants is still emerging. In this study, we performed genome-wide analysis in 666 geographically distributed diverse ecotypes of Arabidopsis thaliana to identify genomic regions [splicing quantitative trait loci (sQTLs)] that may regulate differential AS. These ecotypes belong to different microclimatic conditions and are part of the relict and non-relict populations. Although sQTLs were spread across the genome, we observed enrichment for trans-sQTL (trans-sQTLs hotspots) on chromosome one. Furthermore, we identified several sQTL (911) that co-localized with trait-linked single nucleotide polymorphisms (SNP) identified in the Arabidopsis genome-wide association studies (AraGWAS). Many sQTLs were enriched among circadian clock, flowering, and stress-responsive genes, suggesting a role for differential isoform usage in regulating these important processes in diverse ecotypes of Arabidopsis. In conclusion, the current study provides a deep insight into SNPs affecting isoform ratios/genes and facilitates a better mechanistic understanding of trait-associated SNPs in GWAS studies. To the best of our knowledge, this is the first report of sQTL analysis in a large set of Arabidopsis ecotypes and can be used as a reference to perform sQTL analysis in the Brassicaceae family. Since whole genome and transcriptome datasets are available for these diverse ecotypes, it could serve as a powerful resource for the biological interpretation of trait-associated loci, splice isoform ratios, and their phenotypic consequences to help produce more resilient and high yield crop varieties.

Senescence and Defense Pathways Contribute to Heterosis

Hybrids are used extensively in agriculture due to their superior performance in seed yield and plant growth, yet the molecular mechanisms underpinning hybrid performance are not well understood. Recent evidence has suggested that a decrease in basal defense response gene expression regulated by reduced levels of salicylic acid (SA) may be important for vigor in certain hybrid combinations. Decreasing levels of SA in the Arabidopsis (Arabidopsis thaliana) accession C24 through the introduction of the SA catabolic enzyme salicylate1 hydroxylase (NahG) increases plant size, phenocopying the large-sized C24/Landsberg erecta (Ler) F1 hybrids. C24♀ × Ler♂ F1 hybrids and C24 NahG lines shared differentially expressed genes and pathways associated with plant defense and leaf senescence including decreased expression of SA biosynthetic genes and SA response genes. The expression of TL1 BINDING TRANSCRIPTION FACTOR1, a key regulator in resource allocation between growth and defense, was decreased in both the F1 hybrid and the C24 NahG lines, which may promote growth. Both C24 NahG lines and the F1 hybrids showed decreased expression of the key senescence-associated transcription factors WRKY53, NAC-CONTAINING PROTEIN29, and ORESARA1 with a delayed onset of senescence compared to C24 plants. The delay in senescence resulted in an extension of the photosynthetic period in the leaves of F1 hybrids compared to the parental lines, potentially allowing each leaf to contribute more resources toward growth.

Interactive roles of chromatin regulation and circadian clock function in plants

Circadian rhythms in transcription ultimately result in oscillations of key biological processes. Understanding how transcriptional rhythms are generated in plants provides an opportunity for fine-tuning growth, development, and responses to the environment. Here, we present a succinct description of the plant circadian clock, briefly reviewing a number of recent studies but mostly emphasizing the components and mechanisms connecting chromatin remodeling with transcriptional regulation by the clock. The possibility that intergenomic interactions govern hybrid vigor through epigenetic changes at clock loci and the function of epialleles controlling clock output traits during crop domestication are also discussed.

Light Modulates Ethylene Synthesis, Signaling, and Downstream Transcriptional Networks to Control Plant Development

The inhibition of hypocotyl elongation by ethylene in dark-grown seedlings was the basis of elegant screens that identified ethylene-insensitive Arabidopsis mutants, which remained tall even when treated with high concentrations of ethylene. This simple approach proved invaluable for identification and molecular characterization of major players in the ethylene signaling and response pathway, including receptors and downstream signaling proteins, as well as transcription factors that mediate the extensive transcriptional remodeling observed in response to elevated ethylene. However, the dark-adapted early developmental stage used in these experiments represents only a small segment of a plant's life cycle. After a seedling's emergence from the soil, light signaling pathways elicit a switch in developmental programming and the hormonal circuitry that controls it. Accordingly, ethylene levels and responses diverge under these different environmental conditions. In this review, we compare and contrast ethylene synthesis, perception, and response in light and dark contexts, including the molecular mechanisms linking light responses to ethylene biology. One powerful method to identify similarities and differences in these important regulatory processes is through comparison of transcriptomic datasets resulting from manipulation of ethylene levels or signaling under varying light conditions. We performed a meta-analysis of multiple transcriptomic datasets to uncover transcriptional responses to ethylene that are both light-dependent and light-independent. We identified a core set of 139 transcripts with robust and consistent responses to elevated ethylene across three root-specific datasets. This "gold standard" group of ethylene-regulated transcripts includes mRNAs encoding numerous proteins that function in ethylene signaling and synthesis, but also reveals a number of previously uncharacterized gene products that may contribute to ethylene response phenotypes. Understanding these light-dependent differences in ethylene signaling and synthesis will provide greater insight into the roles of ethylene in growth and development across the entire plant life cycle.

A Functional Connection between the Circadian Clock and Hormonal Timing in Arabidopsis

The rotation of the Earth entails changes in environmental conditions that pervasively influence an organism's physiology and metabolism. An internal cellular mechanism known as the circadian clock acts as an internal timekeeper that is able to perceive the changes in environmental cues to generate 24-h rhythms in synchronization with daily and seasonal fluctuations. In plants, the circadian clock function is particularly important and regulates nearly every aspect of plant growth and development as well as proper responses to stresses. The circadian clock does not function in isolation but rather interconnects with an intricate network of different pathways, including those of phytohormones. Here, we describe the interplay of the circadian clock with a subset of hormones in Arabidopsis. The molecular components directly connecting the circadian and hormone pathways are described, highlighting the biological significance of such connections in the control of growth, development, fitness, and survival. We focus on the overlapping as well as contrasting circadian and hormonal functions that together provide a glimpse on how the Arabidopsis circadian system regulates hormone function in response to endogenous and exogenous cues. Examples of feedback regulation from hormone signaling to the clock are also discussed.