Ghd8 controls rice photoperiod sensitivity by forming a complex that interacts with Ghd7

Dominance complementation of parental heading date alleles of Hd1, Ghd7, DTH8, and PRR37 confers transgressive late maturation in hybrid rice

Rice (Oryza sativa L.) is a short-day plant whose heading date is largely determined by photoperiod sensitivity (PS). Many parental lines used in hybrid rice breeding have weak PS, but their F1 progenies have strong PS and exhibit an undesirable transgressive late-maturing phenotype. However, the genetic basis for this phenomenon is unclear. Therefore, effective methods are needed for selecting parents to create F1 hybrid varieties with the desired PS. In this study, we used bulked segregant analysis with F1 Ningyou 1179 (strong PS) and its F2 population, and through analyzing both parental haplotypes and PS data for 918 hybrid rice varieties, to identify the genetic basis of transgressive late maturation which is dependent on dominance complementation effects of Hd1, Ghd7, DTH8, and PRR37 from both parents rather than from a single parental genotype. We designed a molecular marker-assisted selection system to identify the genotypes of Hd1, Ghd7, DTH8, and PRR37 in parental lines to predict PS in F1 plants prior to crossing. Furthermore, we used CRISPR/Cas9 technique to knock out Hd1 in Ning A (sterile line) and Ning B (maintainer line) and obtained an hd1-NY material with weak PS while retaining the elite agronomic traits of NY. Our findings clarified the genetic basis of transgressive late maturation in hybrid rice and developed effective methods for parental selection and gene editing to facilitate the breeding of hybrid varieties with the desired PS for improving their adaptability.

Genetic analysis of phenotypic plasticity identifies BBX6 as the candidate gene for maize adaptation to temperate regions

Introduction

Climate changes pose a significant threat to crop adaptation and production. Dissecting the genetic basis of phenotypic plasticity and uncovering the responsiveness of regulatory genes to environmental factors can significantly contribute to the improvement of climate- resilience in crops.

Methods

We established a BC1F3:4 population using the elite inbred lines Zheng58 and PH4CV and evaluated plant height (PH) across four environments characterized by substantial variations in environmental factors. Then, we quantified the correlation between the environmental mean of PH (the mean performance in each environment) and the environmental parameters within a specific growth window. Furthermore, we performed GWAS analysis of phenotypic plasticity, and identified QTLs and candidate gene that respond to key environment index. After that, we constructed the coexpression network involving the candidate gene, and performed selective sweep analysis of the candidate gene.

Results

We found that the environmental parameters demonstrated substantial variation across the environments, and genotype by environment interaction contributed to the variations of PH. Then, we identified PTT(35-48) (PTT is the abbreviation for photothermal units), the mean PTT from 35 to 48 days after planting, as the pivotal environmental index that closely correlated with environmental mean of PH. Leveraging the slopes of the response of PH to both the environmental mean and PTT(35-48), we successfully pinpointed QTLs for phenotypic plasticity on chromosomes 1 and 2. Notably, the PH4CV genotypes at these two QTLs exhibited positive contributions to phenotypic plasticity. Furthermore, our analysis demonstrated a direct correlation between the additive effects of each QTL and PTT(35-48). By analyzing transcriptome data of the parental lines in two environments, we found that the 1009 genes responding to PTT(35-48) were enriched in the biological processes related to environmental sensitivity. BBX6 was the prime candidate gene among the 13 genes in the two QTL regions. The coexpression network of BBX6 contained other genes related to flowering time and photoperiod sensitivity. Our investigation, including selective sweep analysis and genetic differentiation analysis, suggested that BBX6 underwent selection during maize domestication.

Discussion

Th is research substantially advances our understanding of critical environmental factors influencing maize adaptation while simultaneously provides an invaluable gene resource for the development of climate-resilient maize hybrid varieties.

The WD40 domain-containing protein Ehd5 positively regulates flowering in rice (Oryza sativa)

Heading date (flowering time), which greatly influences regional and seasonal adaptability in rice (Oryza sativa), is regulated by many genes in different photoperiod pathways. Here, we characterized a heading date gene, Early heading date 5 (Ehd5), using a modified bulked segregant analysis method. The ehd5 mutant showed late flowering under both short-day and long-day conditions, as well as reduced yield, compared to the wild type. Ehd5, which encodes a WD40 domain-containing protein, is induced by light and follows a circadian rhythm expression pattern. Transcriptome analysis revealed that Ehd5 acts upstream of the flowering genes Early heading date 1 (Ehd1), RICE FLOWERING LOCUS T 1 (RFT1), and Heading date 3a (Hd3a). Functional analysis showed that Ehd5 directly interacts with Rice outermost cell-specific gene 4 (Roc4) and Grain number, plant height, and heading date 8 (Ghd8), which might affect the formation of Ghd7-Ghd8 complexes, resulting in increased expression of Ehd1, Hd3a, and RFT1. In a nutshell, these results demonstrate that Ehd5 functions as a positive regulator of rice flowering and provide insight into the molecular mechanisms underlying heading date.

GWAS analysis to elucidate genetic composition underlying a photoperiod-insensitive rice population, North Korea

Heading date (Hd) is one of the main factors determining rice production and regional adaptation. To identify the genetic factors involved in the wide regional adaptability of rice, we conducted a genome-wide association study (GWAS) with 190 North Korean rice accessions selected for non-precocious flowering in the Philippines, a low-latitude region. Using both linear mixed models (LMM) and fixed and random model circulating probability unification (FarmCPU), we identified five significant loci for Hd in trials in 2018 and 2019. Among the five lead single nucleotide polymorphisms (SNPs), three were located adjacent to the known Hd genes, Heading date 3a (Hd3a), Heading date 5 (Hd5), and GF14-c. In contrast, three SNPs were located in novel loci with minor effects on heading. Further GWAS analysis for photoperiod insensitivity (PS) revealed no significant genes associated with PS, supporting that this North Korean (NK) population is largely photoperiod-insensitive. Haplotyping analysis showed that more than 80% of the NK varieties harbored nonfunctional alleles of major Hd genes investigated, of which a nonfunctional allele of Heading date 1 (Hd1) was observed in 66% of the varieties. Geographical distribution analysis of Hd allele combination types showed that nonfunctional alleles of floral repressor Hd genes enabled rice cultivation in high-latitude regions. In contrast, Hd1 alleles largely contributed to the wide regional adaptation of rice varieties. In conclusion, an allelic combination of Hd genes is critical for rice cultivation across wide areas.

Genetic basis of the early heading of high-latitude weedy rice

Japonica rice (Oryza sativa L.) is an important staple food in high-latitude regions and is widely distributed in northern China, Japan, Korea, and Europe. However, the genetic diversity of japonica rice is relatively narrow and poorly adapted. Weedy rice (Oryza sativa f. spontanea) is a semi-domesticated rice. Its headings are earlier than the accompanied japonica rice, making it a potential new genetic resource, which can make up for the defects of wild rice that are difficult to be directly applied to japonica rice improvement caused by reproductive isolation. In this study, we applied a natural population consisting of weedy rice, japonica landrace, and japonica cultivar to conduct a genome-wide association study (GWAS) of the heading date and found four loci that could explain the natural variation of the heading date in this population. At the same time, we developed recombinant inbred lines (RILs) crossed by the early-heading weedy rice WR04-6 and its accompanied japonica cultivar ShenNong 265 (SN265) to carry out a QTL mapping analysis of the heading date and mapped four quantitative trait locus (QTLs) and three epistatic effect gene pairs. The major locus on chromosome 6 overlapped with the GWAS result. Further analysis found that two genes, Hd1 and OsCCT22, on chromosome 6 (Locus 2 and Locus 3) may be the key points of the early-heading character of weedy rice. As minor effect genes, Dth7 and Hd16 also have genetic contributions to the early heading of weedy rice. In the process of developing the RIL population, we introduced fragments of Locus 2 and Locus 3 from the weedy rice into super-high-yielding japonica rice, which successfully promoted its heading date by at least 10 days and expanded the rice suitable cultivation area northward by about 400 km. This study successfully revealed the genetic basis of the early heading of weedy rice and provided a new idea for the genetic improvement of cultivated rice by weedy rice.

qHD5 encodes an AP2 factor that suppresses rice heading by down-regulating Ehd2 expression

Heading date is crucial for rice reproduction and the geographical expansion of cultivation. We fine-mapped qHD5 and identified LOC_Os05g03040, a gene that encodes an AP2 transcription factor, as the candidate gene of qHD5 in our previous study. In this article, using two near-isogenic lines NIL(BG1) and NIL(XLJ), which were derived from the progeny of the cross between BigGrain1 (BG1) and Xiaolijing (XLJ), we verified that LOC_Os05g03040 represses heading date in rice through genetic complementation and CRISPR/Cas9 gene-editing experiments. Complementary results showed that qHD5 is a semi-dominant gene and that the qHD5^XLJ and qHD5^BG1 alleles are both functional. The homozygous mutant line generated from knocking out qHD5^XLJ in NIL(XLJ) headed earlier than NIL(XLJ) under both short-day and long-day conditions. In addition, the homozygous mutant line of qHD5^BG1 in NIL(BG1) also headed slightly earlier than NIL(BG1). All of these results show that qHD5 represses the heading date in rice. Transient expression showed that the qHD5 protein localizes to the nucleus. Transactivation activity assays showed that the C-terminus is the critical site that affects self-activation in qHD5^XLJ. qRT-PCR analysis revealed that qHD5 represses flowering by down-regulating Ehd2. qHD5 may have been selected during indica rice domestication.

Bifunctional regulators of photoperiodic flowering in short day plant rice

Photoperiod is acknowledged as a crucial environmental factor for plant flowering. According to different responses to photoperiod, plants were divided into short-day plants (SDPs), long-day plants (LDPs), and day-neutral plants (DNPs). The day length measurement system of SDPs is different from LDPs. Many SDPs, such as rice, have a critical threshold for day length (CDL) and can even detect changes of 15 minutes for flowering decisions. Over the last 20 years, molecular mechanisms of flowering time in SDP rice and LDP Arabidopsis have gradually clarified, which offers a chance to elucidate the differences in day length measurement between the two types of plants. In Arabidopsis, CO is a pivotal hub in integrating numerous internal and external signals for inducing photoperiodic flowering. By contrast, Hd1 in rice, the homolog of CO, promotes and prevents flowering under SD and LD, respectively. Subsequently, numerous dual function regulators, such as phytochromes, Ghd7, DHT8, OsPRR37, OsGI, OsLHY, and OsELF3, were gradually identified. This review assesses the relationship among these regulators and a proposed regulatory framework for the reversible mechanism, which will deepen our understanding of the CDL regulation mechanism and the negative response to photoperiod between SDPs and LDPs.

Genetic dissection of grain traits and their corresponding heterosis in an elite hybrid

Rice productivity has considerably improved due to the effective employment of heterosis, but the genetic basis of heterosis for grain shape and weight remains uncertain. For studying the genetic dissection of heterosis for grain shape/weight and their relationship with grain yield in rice, quantitative trait locus (QTL) mapping was performed on 1,061 recombinant inbred lines (RILs), which was developed by crossing xian/indica rice Quan9311B (Q9311B) and Wu-shan-si-miao (WSSM). Whereas, BC₁F₁ (a backcross F₁) was developed by crossing RILs with Quan9311A (Q9311A) combined with phenotyping in Hefei (HF) and Nanning (NN) environments. Overall, 114 (main-effect, mQTL) and 359 (epistatic QTL, eQTL) were identified in all populations (RIL, BC₁F₁, and mid-parent heterosis, H_MPs) for 1000-grain weight (TGW), grain yield per plant (GYP) and grain shape traits including grain length (GL), grain width (GW), and grain length to width ratio (GLWR). Differential QTL detection revealed that all additive loci in RILs population do not show heterotic effects, and few of them affect the performance of BC₁F₁. However, 25 mQTL not only contributed to BC₁F₁'s performance but also contributed to heterosis. A total of seven QTL regions was identified, which simultaneously affected multiple grain traits (grain yield, weight, shape) in the same environment, including five regions with opposite directions and two regions with same directions of favorable allele effects, indicating that partial genetic overlaps are existed between different grain traits. This study suggested different approaches for obtaining good grain quality with high yield by pyramiding or introgressing favorable alleles (FA) with the same direction of gene effect at the QTL regions affecting grain shape/weight and grain yield distributing on different chromosomes, or introgressing or pyramiding FA in the parents instead of fixing additive effects in hybrid as well as pyramiding the polymorphic overdominant/dominant loci between the parents and eliminating underdominant loci from the parents. These outcomes offer valuable information and strategy to develop hybrid rice with suitable grain type and weight.

Natural alleles of CIRCADIAN CLOCK ASSOCIATED1 contribute to rice cultivation by fine-tuning flowering time

The timing of flowering is a crucial factor for successful grain production at a wide range of latitudes. Domestication of rice (Oryza sativa) included selection for natural alleles of flowering-time genes that allow rice plants to adapt to broad geographic areas. Here, we describe the role of natural alleles of CIRCADIAN CLOCK ASSOCIATED1 (OsCCA1) in cultivated rice based on analysis of single-nucleotide polymorphisms deposited in the International Rice Genebank Collection Information System database. Rice varieties harboring japonica-type OsCCA1 alleles (OsCCA1a haplotype) flowered earlier than those harboring indica-type OsCCA1 alleles (OsCCA1d haplotype). In the japonica cultivar "Dongjin", a T-DNA insertion in OsCCA1a resulted in late flowering under long-day and short-day conditions, indicating that OsCCA1 is a floral inducer. Reverse transcription quantitative PCR analysis showed that the loss of OsCCA1a function induces the expression of the floral repressors PSEUDO-RESPONSE REGULATOR 37 (OsPRR37) and Days to Heading 8 (DTH8), followed by repression of the Early heading date 1 (Ehd1)-Heading date 3a (Hd3a)-RICE FLOWERING LOCUS T 1 (RFT1) pathway. Binding affinity assays indicated that OsCCA1 binds to the promoter regions of OsPRR37 and DTH8. Naturally occurring OsCCA1 alleles are evolutionarily conserved in cultivated rice (O. sativa). Oryza rufipogon-I (Or-I) and Or-III type accessions, representing the ancestors of O. sativa indica and japonica, harbored indica- and japonica-type OsCCA1 alleles, respectively. Taken together, our results demonstrate that OsCCA1 is a likely domestication locus that has contributed to the geographic adaptation and expansion of cultivated rice.

Nuclear factor OsNF-YB4 promotes flowering by negatively regulating the floral repressor gene Ghd7 in rice

Flowering time or heading date is a critical agronomic trait of rice and is regulated by numerous genes, including several genes encoding nuclear factor YB (NF-YB) in rice, NF-YB11 is one of the genes well known to be involved in the process, delaying flowering under long-day (LD) conditions but promoting flowering under short-day (SD) conditions. In this study, we identified another NF-YB gene, OsNF-YB4. Overexpression of OsNF-YB4 promoted rice flowering under both natural long-day (NLD) and natural short-day (NSD) conditions, whereas suppression or loss-of-function of this gene delayed flowering. The transcription of OsNF-YB4 exhibited an obvious circadian pattern and was induced by light under both LD and SD conditions. Expression analyses of flowering regulators in the photoperiodic flowering pathway demonstrated that up-regulation of OsNF-YB4 resulted in down-regulation of floral repressor Grain number, plant height and heading date 7 (Ghd7), and thus activating the Early heading date 1 (Ehd1)-mediated flowering pathway. Besides, OsNF-YB4 was observed to bind to the specific CCAAT-box regions in the Ghd7 promoter in vitro and interact with GHD7 in yeast. All these evidences support that OsNF-YB4 functions as a flowering promoter by negatively regulating the expression of floral repressor Ghd7 in rice photoperiodic flowering-time regulatory network.

Transcriptional and post-transcriptional regulation of heading date in rice

Rice is a facultative short day (SD) plant. In addition to serving as a model plant for molecular genetic studies of monocots, rice is a staple crop for about half of the world's population. Heading date is a critical agronomic trait, and many genes controlling heading date have been cloned over the last 2 decades. The mechanism of flowering in rice from recognition of day length by leaves to floral activation in the shoot apical meristem has been extensively studied. In this review, we summarise current progress on transcriptional and post-transcriptional regulation of heading date in rice, with emphasis on post-translational modifications of key regulators, including Heading date 1 (Hd1), Early heading date 1 (Ehd1), Grain number, plant height, and heading date7 (Ghd7). The contribution of heading date genes to heterosis and the expansion of rice cultivation areas from low-latitude to high-latitude regions are also discussed. To overcome the limitations of diverse genetic backgrounds used in heading date studies and to gain a clearer understanding of flowering in rice, we propose a systematic collection of genetic resources in a common genetic background. Strategies in breeding adapted cultivars by rational design are also discussed.

Forecasting rice latitude adaptation through a daylength-sensing-based environment adaptation simulator

Global climate change necessitates crop varieties with good environmental adaptability. As a proxy for climate adaptation, crop breeders could select for adaptability to different latitudes, but the lengthy procedures for that slow development. Here, we combined molecular technologies with a streamlined in-house screening method to facilitate rapid selection for latitude adaptation. We established the daylength-sensing-based environment adaptation simulator (DEAS) to assess rice latitude adaptation status via the transcriptional dynamics of florigen genes at different latitudes. The DEAS predicted the florigen expression profiles in rice varieties with high accuracy. Furthermore, the DEAS showed potential for application in different crops. Incorporating the DEAS into conventional breeding programmes would help to develop cultivars for climate adaptation.

Characterization of the Ghd8 Flowering Time Gene in a Mini-Core Collection of Miscanthus sinensis

The optimal flowering time for bioenergy crop Miscanthus is essential for environmental adaptability and biomass accumulation. However, little is known about how genes controlling flowering in other grasses contribute to flowering regulation in Miscanthus. Here, we report on the sequence characterization and gene expression of Miscanthus sinensisGhd8, a transcription factor encoding a HAP3/NF-YB DNA-binding domain, which has been identified as a major quantitative trait locus in rice, with pleiotropic effects on grain yield, heading date and plant height. In M. sinensis, we identified two homoeologous loci, MsiGhd8A located on chromosome 13 and MsiGhd8B on chromosome 7, with one on each of this paleo-allotetraploid species’ subgenomes. A total of 46 alleles and 28 predicted protein sequence types were identified in 12 wild-collected accessions. Several variants of MsiGhd8 showed a geographic and latitudinal distribution. Quantitative real-time PCR revealed that MsiGhd8 expressed under both long days and short days, and MsiGhd8B showed a significantly higher expression than MsiGhd8A. The comparison between flowering time and gene expression indicated that MsiGhd8B affected flowering time in response to day length for some accessions. This study provides insight into the conserved function of Ghd8 in the Poaceae, and is an important initial step in elucidating the flowering regulatory network of Miscanthus.

Strong photoperiod sensitivity is controlled by cooperation and competition among Hd1, Ghd7 and DTH8 in rice heading

Rice (Oryza sativa) is a short-day (SD) plant originally having strong photoperiod sensitivity (PS), with SDs promoting and long days (LDs) suppressing flowering. Although the evolution of PS in rice has been extensively studied, there are few studies that combine the genetic effects and underlying mechanism of different PS gene combinations with variations in PS. We created a set of isogenic lines among the core PS-flowering genes Hd1, Ghd7 and DTH8 using CRISPR mutagenesis, to systematically dissect their genetic relationships under different day-lengths. We investigated their monogenic, digenic, and trigenic effects on target gene regulation and PS variation. We found that Hd1 and Ghd7 have the primary functions for promoting and repressing flowering, respectively, regardless of day-length. However, under LD conditions, Hd1 promotes Ghd7 expression and is recruited by Ghd7 and/or DTH8 to form repressive complexes that collaboratively suppress the Ehd1-Hd3a/RFT1 pathway to block heading, but under SD conditions Hd1 competes with the complexes to promote Hd3a/RFT1 expression, playing a tradeoff relationship with PS flowering. Natural allelic variations of Hd1, Ghd7 and DTH8 in rice populations have resulted in various PS performances. Our findings reveal that rice PS flowering is controlled by crosstalk of two modules - Hd1-Hd3a/RFT1 in SD conditions and (Hd1/Ghd7/DTH8)-Ehd1-Hd3a/RFT1 in LD conditions - and the divergences of these genes provide the basis for rice adaptation to broad regions.

What is going on with the hormonal control of flowering in plants?

Molecular genetic studies using Arabidopsis thaliana as a model system have overwhelmingly revealed many important molecular mechanisms underlying the control of various biological events, including floral induction in plants. The major genetic pathways of flowering have been characterized in-depth, and include the photoperiod, vernalization, autonomous and gibberellin pathways. In recent years, novel flowering pathways are increasingly being identified. These include age, thermosensory, sugar, stress and hormonal signals to control floral transition. Among them, hormonal control of flowering except the gibberellin pathway is not formally considered a major flowering pathway per se, due to relatively weak and often pleiotropic genetic effects, complex phenotypic variations, including some controversial ones. However, a number of recent studies have suggested that various stress signals may be mediated by hormonal regulation of flowering. In view of molecular diversity in plant kingdoms, this review begins with an assessment of photoperiodic flowering, not in A. thaliana, but in rice (Oryza sativa); rice is a staple crop for human consumption worldwide, and is a model system of short-day plants, cereals and breeding crops. The rice flowering pathway is then compared with that of A. thaliana. This review then aims to update our knowledge on hormonal control of flowering, and integrate it into the entire flowering gene network.

Environmental Signal-Dependent Regulation of Flowering Time in Rice

The transition from the vegetative to the reproductive stage of growth is a critical event in the lifecycle of a plant and is required for the plant’s reproductive success. Flowering time is tightly regulated by an internal time-keeping system and external light conditions, including photoperiod, light quality, and light quantity. Other environmental factors, such as drought and temperature, also participate in the regulation of flowering time. Thus, flexibility in flowering time in response to environmental factors is required for the successful adaptation of plants to the environment. In this review, we summarize our current understanding of the molecular mechanisms by which internal and environmental signals are integrated to regulate flowering time in Arabidopsis thaliana and rice (Oryza sativa).

Epistatic interactions between PHOTOPERIOD1, CONSTANS1 and CONSTANS2 modulate the photoperiodic response in wheat

In Arabidopsis, CONSTANS (CO) integrates light and circadian clock signals to promote flowering under long days (LD). In the grasses, a duplication generated two paralogs designated as CONSTANS1 (CO1) and CONSTANS2 (CO2). Here we show that in tetraploid wheat plants grown under LD, combined loss-of-function mutations in the A and B-genome homeologs of CO1 and CO2 (co1 co2) result in a small (3 d) but significant (P<0.0001) acceleration of heading time both in PHOTOPERIOD1 (PPD1) sensitive (Ppd-A1b, functional ancestral allele) and insensitive (Ppd-A1a, functional dominant allele) backgrounds. Under short days (SD), co1 co2 mutants headed 13 d earlier than the wild type (P<0.0001) in the presence of Ppd-A1a. However, in the presence of Ppd-A1b, spikes from both genotypes failed to emerge by 180 d. These results indicate that CO1 and CO2 operate mainly as weak heading time repressors in both LD and SD. By contrast, in ppd1 mutants with loss-of-function mutations in both PPD1 homeologs, the wild type Co1 allele accelerated heading time >60 d relative to the co1 mutant allele under LD. We detected significant genetic interactions among CO1, CO2 and PPD1 genes on heading time, which were reflected in complex interactions at the transcriptional and protein levels. Loss-of-function mutations in PPD1 delayed heading more than combined co1 co2 mutations and, more importantly, PPD1 was able to perceive and respond to differences in photoperiod in the absence of functional CO1 and CO2 genes. Similarly, CO1 was able to accelerate heading time in response to LD in the absence of a functional PPD1. Taken together, these results indicate that PPD1 and CO1 are able to respond to photoperiod in the absence of each other, and that interactions between these two photoperiod pathways at the transcriptional and protein levels are important to fine-tune the flowering response in wheat.

Understanding Past, and Predicting Future, Niche Transitions based on Grass Flowering Time Variation

Since their origin in the early Cretaceous, grasses have diversified across every continent on Earth, with a handful of species (rice [Oryza sativa], maize [Zea mays], and wheat [Triticum aestivum]) providing most of the caloric intake of contemporary humans and their livestock. The ecological dominance of grasses can be attributed to a number of physiological innovations, many of which contributed to shifts from closed to open habitats that incur daily (e.g. tropical mountains) and/or seasonal extremes in temperature (e.g. temperate/continental regions) and precipitation (e.g. tropical savannas). In addition to strategies that allow them to tolerate or resist periodically stressful environments, plants can adopt escape behaviors by modifying the relative timing of distinct development phases. Flowering time is one of these behaviors that can also act as a postzygotic barrier to reproduction and allow temporal partitioning of resources to promote coexistence. In this review, we explore what is known about the phylogenetic pattern of flowering control in grasses, and how this relates to broad- and fine-scale niche transitions within the family. We then synthesize recent findings on the genetic basis of flowering time evolution as a way to begin deciphering why certain aspects of flowering are seemingly so conserved, and what the implications of this are for future adaptation under climate change.