Plant gene silencing signals move from the phloem to influence gene expression in shoot apical meristems.

BACKGROUND: Small RNAs (sRNA) are potent regulators of gene expression that can diffuse short distances between cells and move long distances through plant vasculature. However, the degree to which sRNA silencing signals can move from the phloem to the shoot apical meristem (SAM) remains unclear. RESULTS: Two independent transgenic approaches were used to examine whether phloem sRNA silencing can reach different domains of the SAM and silence SAM-expressed genes. First, the phloem companion-cell specific SUCROSE-PROTON SYMPORTER2 (SUC2) promoter was used to drive expression of an inverted repeat to target the FD gene, an exclusively SAM-localized floral regulator. Second, the SUC2 promoter was used to express an artificial microRNA (aMiR) designed to target a synthetic CLAVATA3 (CLV3) transgene in SAM stem cells. Both phloem silencing signals phenocopied the loss of function of their targets and altered target gene expression suggesting that a phloem-to-SAM silencing communication axis exists, connecting distal regions of the plant to SAM stem cells. CONCLUSIONS: Demonstration of phloem-to-SAM silencing reveals a regulatory link between somatic sRNA expressed in distal regions of the plant and the growing shoot. Since the SAM stem cells ultimately produce the gametes, we discuss the intriguing possibility that phloem-to-SAM sRNA trafficking could allow transient somatic sRNA expression to manifest stable, transgenerational epigenetic changes.

Expression of coffee florigen CaFT1 reveals a sustained floral induction window associated with asynchronous flowering in tropical perennials.

The behavior of florigen(s) and environment-influenced regulatory pathways that control floral initiation in tropical perennials species with complex phenological cycles is poorly understood. Understanding the mechanisms underlying this process is important for food production in the face of climate change, thus, we used Coffea sp. L. (Rubiaceae) as a model to explore this issue. Homologs of FLOWERING LOCUS T (CaFT1) and environment-related regulators CONSTANS (CaCO), PHYTOCHROME INTERACTING FACTOR 4 (CaPIF4) and FLOWERING LOCUS C (CaFLC) were retrieved from coffee genomes and identified through phylogenetic analysis. Overexpression of CaFT1 in Arabidopsis caused early-flowering phenotype and yeast two hybrid studies indicated CaFT1 binding to bZIP floral regulator FD, which suggests that CaFT1 is a coffee florigen. Expression of CaFT1 and other floral regulators, together with carbohydrate analysis, were evaluated over one year using three contrasting genotypes, two C. arabica cultivars and C. canephora. All genotypes showed active and variable CaFT1 transcription from February until October, indicating the potential window for floral induction that reached a maximum in the cold period of June. CaCO expression, as expected, varied over a 24-hour day period and monthly with day length, whereas expression of temperature-responsive homologs, CaFLC and CaPIF4, did not correlate with temperature changes nor CaFT1 expression, suggesting alternative FT regulatory pathways in coffee. Based on our results, we suggest a continuum of floral induction that allows different starting points for floral activation, which explains developmental asynchronicity and prolonged anthesis events in tropical perennial species.

Oncolytic Orf virus licenses NK cells via cDC1 to activate innate and adaptive antitumor mechanisms and extends survival in a murine model of late-stage ovarian cancer.

BACKGROUND: Novel therapies are needed to improve outcomes for women diagnosed with ovarian cancer. Oncolytic viruses are multifunctional immunotherapeutic biologics that preferentially infect cancer cells and stimulate inflammation with the potential to generate antitumor immunity. Herein we describe Parapoxvirus ovis (Orf virus (OrfV)), an oncolytic poxvirus, as a viral immunotherapy for ovarian cancer. METHODS: The immunotherapeutic potential of OrfV was tested in the ID8 orthotopic mouse model of end-stage epithelial ovarian carcinoma. Immune cell profiling, impact on secondary lesion development and survival were evaluated in OrfV-treated mice as well as in Batf3 knockout, mice depleted of specific immune cell subsets and in mice where the primary tumor was removed. Finally, we interrogated gene expression datasets from primary human ovarian tumors from the International Cancer Genome Consortium database to determine whether the interplay we observed between natural killer (NK) cells, classical type 1 dendritic cells (cDC1s) and T cells exists and influences outcomes in human ovarian cancer. RESULTS: OrfV was an effective monotherapy in a murine model of advanced-stage epithelial ovarian cancer. OrfV intervention relied on NK cells, which when depleted abrogated antitumor CD8+ T-cell responses. OrfV therapy was shown to require cDC1s in experiments with BATF3 knockout mice, which do not have mature cDC1s. Furthermore, cDC1s governed antitumor NK and T-cell responses to mediate antitumor efficacy following OrfV. Primary tumor removal, a common treatment option in human patients, was effectively combined with OrfV for optimal therapeutic outcome. Analysis of human RNA sequencing datasets revealed that cDC1s correlate with NK cells in human ovarian cancer and that intratumoral NK cells correlate positively with survival. CONCLUSIONS: The data herein support the translational potential of OrfV as an NK stimulating immunotherapeutic for the treatment of advanced-stage ovarian cancer.

Patterns of stability and change in the maize genome: a case study of small RNA transcriptomes in two recombinant inbred lines and their progenitors.

Small RNAs (sRNAs) are epigenetic regulators of eukaryotic genes and transposable elements (TEs). Diverse sRNA expression patterns exist within a species, but how this diversity arises is not well understood. To provide a window into the dynamics of maize sRNA patterning, sRNA and mRNA transcriptomes were examined in two related Zea mays recombinant inbred lines (RILs) and their inbred parents. Analysis of these RILs revealed that most clusters of sRNA expression retained the parental sRNA expression level. However, expression states that differ from the parental allele were also observed, predominantly reflecting decreases in sRNA expression. When RIL sRNA expression differed from the parental allele, the new state was frequently similar between the two RILs, and similar to the expression state found at the allele in the other parent. Novel sRNA expression patterns, distinct from those of either parent, were rare. Additionally, examination of sRNA expression over TEs revealed one TE family, Gyma, which showed consistent enrichment for RIL sRNA expression differences compared to those found in parental alleles. These findings provide insights into how sRNA silencing might evolve over generations and suggest that further investigation into the molecular nature of sRNA trans regulators is warranted.

Introgressed DNA within a Zea mays near-isogenic line displays lower levels of 24nt sRNA expression than the homologous region from the recurrent parent.

Near-isogenic lines (NILs) are classical genetic tools used to dissect the actions of an allele when placed in a uniform genetic background. Although the goal of NIL creation is to examine the effects of a single allele in isolation, DNA linked to the allele is invariably retained and can confound any allele-specific effects. In addition to genetic variation, highly polymorphic species such as Zea mays will contain introgressed polymorphisms encompassing transposable elements (TEs) and the cis-acting small RNA (sRNA) that represses them. Through transcriptomics, we described the differences in sRNA and TE transcriptional expression between a W22-derived introgression and its homologous B73 region. As anticipated, many differences in sRNA expression were observed. Unexpectedly, however, 24nt sRNA expression over the introgressed region was low overall compared to both the homologous B73 region and the rest of the genome. Across the introgression, low sRNA expression was accompanied by increased TE transcription. Possible explanations for the observed trends in sRNA and TE expression across the introgression region are discussed. These findings support the notion that any introgressed allele is in an epigenetic environment distinct from that found at the allele from the recurrent parent. Additionally, these results suggest that further study of sRNA expression levels during the introgression process is warranted.

Does variable epigenetic inheritance fuel plant evolution?

Epigenetic changes influence gene expression and contribute to the modulation of biological processes in response to the environment. Transgenerational epigenetic changes in gene expression have been described in many eukaryotes. However, plants appear to have a stronger propensity for inheriting novel epialleles. This mini-review discusses how plant traits, such as meristematic growth, totipotency, and incomplete epigenetic erasure in gametes promote epiallele inheritance. Additionally, we highlight how plant biology may be inherently tailored to reap the benefits of epigenetic metastability. Importantly, environmentally triggered small RNA expression and subsequent epigenetic changes may allow immobile plants to adapt themselves, and possibly their progeny, to thrive in local environments. The change of epigenetic states through the passage of generations has ramifications for evolution in the natural and agricultural world. In populations containing little genetic diversity, such as elite crop germplasm or habitually self-reproducing species, epigenetics may provide an important source of heritable phenotypic variation. Basic understanding of the processes that direct epigenetic shifts in the genome may allow for breeding or bioengineering for improved plant traits that do not require changes to DNA sequence.

Distinct gene networks modulate floral induction of autonomous maize and photoperiod-dependent teosinte.

Temperate maize was domesticated from its tropical ancestor, teosinte. Whereas temperate maize is an autonomous day-neutral plant, teosinte is an obligate short-day plant that requires uninterrupted long nights to induce flowering. Leaf-derived florigenic signals trigger reproductive growth in both teosinte and temperate maize. To study the genetic mechanisms underlying floral inductive pathways in maize and teosinte, mRNA and small RNA genome-wide expression analyses were conducted on leaf tissue from plants that were induced or not induced to flower. Transcriptome profiles reveal common differentially expressed genes during floral induction, but a comparison of candidate flowering time genes indicates that photoperiod and autonomous pathways act independently. Expression differences in teosinte are consistent with the current paradigm for photoperiod-induced flowering, where changes in circadian clock output trigger florigen production. Conversely, differentially expressed genes in temperate maize link carbon partitioning and flowering, but also show altered expression of circadian clock genes that are distinct from those altered upon photoperiodic induction in teosinte. Altered miRNA399 levels in both teosinte and maize suggest a novel common connection between flowering and phosphorus perception. These findings provide insights into the molecular mechanisms underlying a strengthened autonomous pathway that enabled maize growth throughout temperate regions.

Florigen-Encoding Genes of Day-Neutral and Photoperiod-Sensitive Maize Are Regulated by Different Chromatin Modifications at the Floral Transition.

The activity of the maize (Zea mays) florigen gene ZEA CENTRORADIALIS8 (ZCN8) is associated with the floral transition in both day-neutral temperate maize and short-day (SD)-requiring tropical maize. We analyzed transcription and chromatin modifications at the ZCN8 locus and its nearly identical paralog ZCN7 during the floral transition. This analysis was performed with day-neutral maize (Zea mays ssp. mays), where flowering is promoted almost exclusively via the autonomous pathway through the activity of the regulatory gene indeterminate1 (id1), and tropical teosinte (Zea mays ssp. parviglumis) under floral inductive and noninductive photoperiods. Comparison of ZCN7/ZCN8 histone modification profiles in immature leaves of nonflowering id1 mutants and teosinte grown under floral inhibitory photoperiods reveals that both id1 floral inductive activity and SD-mediated induction result in histone modification patterns that are compatible with the formation of transcriptionally competent chromatin environments. Specific histone modifications are maintained during leaf development and may represent a chromatin signature that favors the production of processed ZCN7/ZCN8 messenger RNA in florigen-producing mature leaf. However, whereas id1 function promotes histone H3 hyperacetylation, SD induction is associated with increased histone H3 dimethylation and trimethylation at lysine-4. In addition, id1 and SD differently affect the production of ZCN7/ZCN8 antisense transcript. These observations suggest that distinct mechanisms distinguish florigen regulation in response to autonomous and photoperiod pathways. Finally, the identical expression and histone modification profiles of ZCN7 and ZCN8 in response to floral induction suggest that ZCN7 may represent a second maize florigen.

Nitrogen limitation and high density responses in rice suggest a role for ethylene under high density stress.

BACKGROUND: High density stress, also known as intraspecies competition, causes significant yield losses in a wide variety of crop plants. At the same time, increases in density tolerance through selective breeding and the concomitant ability to plant crops at a higher population density has been one of the most important factors in the development of high yielding modern cultivars. RESULTS: Physiological changes underlying high density stress were examined in Oryza sativa plants over the course of a life cycle by assessing differences in gene expression and metabolism. Moreover, the nitrogen limitation was examined in parallel with high density stress to gain a better understanding of physiological responses specific to high density stress. While both nitrogen limitation and high density resulted in decreased shoot fresh weight, tiller number, plant height and chlorophyll content, high density stress alone had a greater impact on physiological factors. Decreases in aspartate and glutamate concentration were found in plants grown under both stress conditions; however, high density stress had a more significant effect on the concentration of these amino acids. Global transcriptome analysis revealed a large proportion of genes with altered expression in response to both stresses. The presence of ethylene-associated genes in a majority of density responsive genes was investigated further. Expression of ethylene biosynthesis genes ACC synthase 1, ACC synthase 2 and ACC oxidase 7 were found to be upregulated in plants under high density stress. Plants at high density were also found to up regulate ethylene-associated genes and senescence genes, while cytokinin response and biosynthesis genes were down regulated, consistent with higher ethylene production. CONCLUSIONS: High density stress has similar but greater impact on plant growth and development compared to nitrogen limitation. Global transcriptome changes implicate ethylene as a volatile signal used to communicate proximity in under dense population growth condition and suggest a role for phytohormones in high density stress response in rice plants.

Putative sugarcane FT/TFL1 genes delay flowering time and alter reproductive architecture in Arabidopsis.

Agriculturally important grasses such as rice, maize, and sugarcane are evolutionarily distant from Arabidopsis, yet some components of the floral induction process are highly conserved. Flowering in sugarcane is an important factor that negatively affects cane yield and reduces sugar/ethanol production from this important perennial bioenergy crop. Comparative studies have facilitated the identification and characterization of putative orthologs of key flowering time genes in sugarcane, a complex polyploid plant whose genome has yet to be sequenced completely. Using this approach we identified phosphatidylethanolamine-binding protein (PEBP) gene family members in sugarcane that are similar to the archetypical FT and TFL1 genes of Arabidopsis that play an essential role in controlling the transition from vegetative to reproductive growth. Expression analysis of ScTFL1, which falls into the TFL1-clade of floral repressors, showed transcripts in developing leaves surrounding the shoot apex but not at the apex itself. ScFT1 was detected in immature leaves and apical regions of vegetatively growing plants and, after the floral transition, expression also occurred in mature leaves. Ectopic over-expression of ScTFL1 in Arabidopsis caused delayed flowering in Arabidopsis, as might be expected for a gene related to TFL1. In addition, lines with the latest flowering phenotype exhibited aerial rosette formation. Unexpectedly, over-expression of ScFT1, which has greatest similarity to the florigen-encoding FT, also caused a delay in flowering. This preliminary analysis of divergent sugarcane FT and TFL1 gene family members from Saccharum spp. suggests that their expression patterns and roles in the floral transition has diverged from the predicted role of similar PEBP family members.

The SEPALLATA MADS-box protein SLMBP21 forms protein complexes with JOINTLESS and MACROCALYX as a transcription activator for development of the tomato flower abscission zone.

Organ abscission is a key step in a plant's life cycle and is one of the most important agronomic traits for crops. In tomato, two MADS-box genes, JOINTLESS (J) and MACROCAYLYX (MC), have been shown to be implicated in development of the flower abscission zone (AZ), but the molecular mechanisms underlying this process are not well known. We report here that the SEPALLATA (SEP) MADS-box gene SLMBP21 acts as an additional factor for development of the AZ in tomato. We show that knockdown of SLMBP21 abolishes development of the flower AZ, while overexpression of SLMBP21 produces small cells at the proximal section of the pedicel and the peduncle. Bimolecular fluorescence complementation analysis confirms that SLMBP21 interacts with J and MC, and co-immunoprecipitation assays further demonstrates that these three proteins may form higher-order protein complexes. In situ hybridization shows that SLMBP21, J, and MC transcripts accumulate in distinct regions, but overlap at the AZ vasculature. In addition, transactivation assays in yeast show that, of the three interacting proteins, only SLMBP21 can activate reporter gene transcription. RNA-seq analysis furthermore reveals that loss of function of SLMBP21, J, or MC affects a common subset of meristem activity genes including LeWUS and LATERAL SUPPRESSOR that were specifically expressed in the AZ on the tomato flower pedicel. Since SLMBP21 belongs to the FBP9/23 subclade of the SEP gene family, which is absent in Arabidopsis, the SLMBP21-J-MC complex may represent a distinct mechanism for development of the AZ in plants.

Nitrogen transporter and assimilation genes exhibit developmental stage-selective expression in maize (Zea mays L.) associated with distinct cis-acting promoter motifs.

Nitrogen is considered the most limiting nutrient for maize (Zea mays L.), but there is limited understanding of the regulation of nitrogen-related genes during maize development. An Affymetrix 82K maize array was used to analyze the expression of ≤ 46 unique nitrogen uptake and assimilation probes in 50 maize tissues from seedling emergence to 31 d after pollination. Four nitrogen-related expression clusters were identified in roots and shoots corresponding to, or overlapping, juvenile, adult, and reproductive phases of development. Quantitative real time PCR data was consistent with the existence of these distinct expression clusters. Promoters corresponding to each cluster were screened for over-represented cis-acting elements. The 8-bp distal motif of the Arabidopsis 43-bp nitrogen response element (NRE) was over-represented in nitrogen-related maize gene promoters. This conserved motif, referred to here as NRE43-d8, was previously shown to be critical for nitrate-activated transcription of nitrate reductase (NIA1) and nitrite reductase (NIR1) by the NIN-LIKE PROTEIN 6 (NLP6) in Arabidopsis. Here, NRE43-d8 was over-represented in the promoters of maize nitrate and ammonium transporter genes, specifically those that showed peak expression during early-stage vegetative development. This result predicts an expansion of the NRE-NLP6 regulon and suggests that it may have a developmental component in maize. We also report leaf expression of putative orthologs of nitrite transporters (NiTR1), a transporter not previously reported in maize. We conclude by discussing how each of the four transcriptional modules may be responsible for the different nitrogen uptake and assimilation requirements of leaves and roots at different stages of maize development.

A developmental transcriptional network for maize defines coexpression modules.

Here, we present a genome-wide overview of transcriptional circuits in the agriculturally significant crop species maize (Zea mays). We examined transcript abundance data at 50 developmental stages, from embryogenesis to senescence, for 34,876 gene models and classified genes into 24 robust coexpression modules. Modules were strongly associated with tissue types and related biological processes. Sixteen of the 24 modules (67%) have preferential transcript abundance within specific tissues. One-third of modules had an absence of gene expression in specific tissues. Genes within a number of modules also correlated with the developmental age of tissues. Coexpression of genes is likely due to transcriptional control. For a number of modules, key genes involved in transcriptional control have expression profiles that mimic the expression profiles of module genes, although the expression of transcriptional control genes is not unusually representative of module gene expression. Known regulatory motifs are enriched in several modules. Finally, of the 13 network modules with more than 200 genes, three contain genes that are notably clustered (P < 0.05) within the genome. This work, based on a carefully selected set of major tissues representing diverse stages of maize development, demonstrates the remarkable power of transcript-level coexpression networks to identify underlying biological processes and their molecular components.

Physiological and genetic analysis of Arabidopsis thaliana anthocyanin biosynthesis mutants under chronic adverse environmental conditions.

Anthocyanin production is a characteristic response of flowering plants to unfavourable environmental conditions. The potential roles of flavonoids and anthocyanins in plant growth were investigated by growing Arabidopsis thaliana anthocyanin production mutants (transparent testa) under limiting nitrogen and high light conditions. Inability to produce kaempferol or subsequent intermediate compounds by some transparent testa lines was correlated with less biomass accumulation in mature plants compared with wild-type control plants under all growth conditions tested. However, under both limiting nitrogen and high light chronic stress conditions, mutant lines defective in later steps of the anthocyanin production pathway produced the same or more biomass than wild-type plants. No difference in senescence between transparent testa and wild-type plants was found using chlorophyll catabolism and SAG12 expression measurements, and no mutants were impaired in the ability to remobilize nutrients from the vegetative to reproductive tissues. Moreover, the absence of anthocyanin and/or upstream flavonoids does not affect the ability of plants to respond to limiting nitrogen by reducing photosynthetic capacity. These results support a role for kaempferol and quercetin accumulation in normal plant growth and development. Further, the absence of anthocyanins has no effect on plant growth under the chronic stress conditions tested.

Transcript and metabolite signature of maize source leaves suggests a link between transitory starch to sucrose balance and the autonomous floral transition.

Little is known about the nature of floral inductive cues in day-neutral plants that are insensitive to photoperiod variations and, therefore, rely on endogenous signals to initiate reproductive growth. The INDETERMINATE1 (ID1) transcription factor is a key regulator of the transition to flowering in day-neutral maize. The ID1 gene is expressed exclusively in developing leaves, where it controls the production or transmission of leaf-derived florigenic signals. Florigen-producing source leaves were compared with mature leaves of late-flowering id1 plants, and metabolite and gene expression differences associated with the floral transition in maize were observed. While id1 mutants have a similar capacity for photosynthesis to wild-type siblings, id1 source leaves show quantitative differences in carbohydrate allocation prior to the floral transition stage, with a marked increase in sucrose and other soluble sugars, accompanied by a decrease in tricarboxylic acid (TCA) cycle organic acids. Importantly, source leaves of autonomous-flowering maize are typified by a higher transitory starch to sucrose ratio and a transcript profile enriched for sucrose synthesis and starch metabolism-related gene function. Finally, similar changes in transitory starch and sucrose are not observed in teosinte, the tropical progenitor of maize that requires short-day photoperiods to induce flowering. Together, these data define a transcript and metabolite signature associated with the autonomous floral transition in temperate maize leaves.

Activity of transcription factor JACKDAW is essential for SHR/SCR-dependent activation of SCARECROW and MAGPIE and is modulated by reciprocal interactions with MAGPIE, SCARECROW and SHORT ROOT.

Two GRAS family transcription factors, SHORT-ROOT (SHR) and SCARECROW (SCR), are required for ground tissue and quiescent center formation in Arabidopsis roots. The action of SHR and SCR is regulated by two INDETERMINATE DOMAIN (IDD) family proteins, JACKDAW (JKD) and MAGPIE (MGP). Although the reciprocal interaction of these transcription factors is considered to be involved in the modulation of SHR and SCR action by JKD and MGP, the underlying mechanism remains unclear. In this study, we use a transient assay with Arabidopsis culture cells to show that the physical interaction of these transcription factors modulate their transcriptional activity. Transient expression of LUC reporter genes with the proximal sequences upstream from the ATG codon of SCR and MGP in protoplasts were activated by JKD. Moreover, promoter activities were enhanced further by the addition of SHR and SCR to JKD, but not by the combination of SHR and SCR in the absence of JKD. Yeast one-hybrid analysis showed that JKD binds to the SCR and MGP promoter sequences, indicating the existence of another binding sequences of JKD different from the previously determined IDD binding sequence. Our findings suggest that JKD directly regulates SCR and MGP expression in cooperation with SHR, SCR and MGP.

ZCN8 encodes a potential orthologue of Arabidopsis FT florigen that integrates both endogenous and photoperiod flowering signals in maize.

Higher plants use multiple perceptive measures to coordinate flowering time with environmental and endogenous cues. Physiological studies show that florigen is a mobile factor that transmits floral inductive signals from the leaf to the shoot apex. Arabidopsis FT protein is widely regarded as the archetype florigen found in diverse plant species, particularly in plants that use inductive photoperiods to flower. Recently, a large family of FT homologues in maize, the Zea CENTRORADIALIS (ZCN) genes, was described, suggesting that maize also contains FT-related proteins that act as a florigen. The product of one member of this large family, ZCN8, has several attributes that make it a good candidate as a maize florigen. Mechanisms underlying the floral transition in maize are less well understood than those of other species, partly because flowering in temperate maize is dependent largely on endogenous signals. The maize indeterminate1 (id1) gene is an important regulator of maize autonomous flowering that acts in leaves to mediate the transmission or production of florigenic signals. This study finds that id1 acts upstream of ZCN8 to control its expression, suggesting a possible new link to flowering in day-neutral maize. Moreover, in teosinte, a tropical progenitor of maize that requires short-day photoperiods to induce flowering, ZCN8 is highly up-regulated in leaves under inductive photoperiods. Finally, vascular-specific expression of ZCN8 in Arabidopsis complements the ft-1 mutation, demonstrating that leaf-specific expression of ZCN8 can induce flowering. These results suggest that ZCN8 may encode a florigen that integrates both endogenous and environmental signals in maize.

The role of epigenetic processes in controlling flowering time in plants exposed to stress.

Plants interact with their environment by modifying gene expression patterns. One mechanism for this interaction involves epigenetic modifications that affect a number of aspects of plant growth and development. Thus, the epigenome is highly dynamic in response to environmental cues and developmental changes. Flowering is controlled by a set of genes that are affected by environmental conditions through an alteration in their expression pattern. This ensures the production of flowers even when plants are growing under adverse conditions, and thereby enhances transgenerational seed production. In this review recent findings on the epigenetic changes associated with flowering in Arabidopsis thaliana grown under abiotic stress conditions such as cold, drought, and high salinity are discussed. These epigenetic modifications include DNA methylation, histone modifications, and the production of micro RNAs (miRNAs) that mediate epigenetic modifications. The roles played by the phytohormones abscisic acid (ABA) and auxin in chromatin remodelling are also discussed. It is shown that there is a crucial relationship between the epigenetic modifications associated with floral initiation and development and modifications associated with stress tolerance. This relationship is demonstrated by the common epigenetic pathways through which plants control both flowering and stress tolerance, and can be used to identify new epigenomic players.

Rice Indeterminate 1 (OsId1) is necessary for the expression of Ehd1 (Early heading date 1) regardless of photoperiod.

Indeterminate 1 (Id1), a classical flowering gene first reported in 1946, is one of the earliest genes whose expression in leaf tissues affects the floral transition in the shoot meristem. How Id1 is integrated into the flowering process is largely unknown. In this study, we examined the genetic action of the rice (Oryza sativa) ortholog OsId1. In rice, OsId1 is preferentially expressed in young leaves, but the overall expression pattern is broader than that in maize (Zea mays). OsId1 is able to activate transcription in yeast. RNAi mutants show a delay in flowering under both short-day (SD) and long-day (LD) conditions. OsId1 regulates the expression of Ehd1 (Early heading date 1) and its downstream genes, including Hd3a (a rice ortholog of FT) and RFT1 (Rice Flowering Locus T1), under both SD and LD conditions. In rice, the expression of Ehd1 is also controlled by the photoperiodic flowering genes OsGI (a rice ortholog of GI) and OsMADS51. However, the expression of OsId1 is independent of OsGI, OsMADS51, and OsMADS50 (a rice SOC1 ortholog). This study demonstrates that the activation of Ehd1 by OsId1 is required for the promotion of flowering.