Is the lodicule a petal: molecular evidence?

Controlling diurnal flower-opening time by manipulating the jasmonate pathway accelerates development of indica-japonica hybrid rice breeding

Inter-subspecific indica-japonica hybrid rice (Oryza sativa) has the potential for increased yields over traditional indica intra-subspecies hybrid rice, but limited yield of F1 hybrid seed production (FHSP) hinders the development of indica-japonica hybrid rice breeding. Diurnal flower-opening time (DFOT) divergence between indica and japonica rice has been a major contributing factor to this issue, but few DFOT genes have been cloned. Here, we found that manipulating the expression of jasmonate (JA) pathway genes can effectively modulate DFOT to improve the yield of FHSP in rice. Treating japonica cultivar Zhonghua 11 (ZH11) with methyl jasmonate (MeJA) substantially advanced DFOT. Furthermore, overexpressing the JA biosynthesis gene OPDA REDUCTASE 7 (OsOPR7) and knocking out the JA inactivation gene CHILLING TOLERANCE 1 (OsHAN1) in ZH11 advanced DFOT by 1- and 2-h respectively; and knockout of the JA signal suppressor genes JASMONATE ZIM-DOMAIN PROTEIN 7 (OsJAZ7) and OsJAZ9 resulted in 50-min and 1.5-h earlier DFOT respectively. The yields of FHSP using japonica male-sterile lines GAZS with manipulated JA pathway genes were significantly higher than that of GAZS wildtype. Transcriptome analysis, cytological observations, measurements of elastic modulus and determination of cell wall components indicated that the JA pathway could affect the loosening of the lodicule cell walls by regulating their composition through controlling sugar metabolism, which in turn influences DFOT. This research has vital implications for breeding japonica rice cultivars with early DFOT to facilitate indica-japonica hybrid rice breeding.

LEAFY and WAPO1 jointly regulate spikelet number per spike and floret development in wheat

In wheat, the transition of the inflorescence meristem to a terminal spikelet (IM→TS) determines the spikelet number per spike (SNS), an important yield component. In this study, we demonstrate that the plant-specific transcription factor LEAFY (LFY) physically and genetically interacts with WHEAT ORTHOLOG OF APO1 (WAPO1) to regulate SNS and floret development. Loss-of-function mutations in either or both genes result in significant and similar reductions in SNS, as a result of a reduction in the rate of spikelet meristem formation per day. SNS is also modulated by significant genetic interactions between LFY and the SQUAMOSA MADS-box genes VRN1 and FUL2, which promote the IM→TS transition. Single-molecule fluorescence in situ hybridization revealed a downregulation of LFY and upregulation of the SQUAMOSA MADS-box genes in the distal part of the developing spike during the IM→TS transition, supporting their opposite roles in the regulation of SNS in wheat. Concurrently, the overlap of LFY and WAPO1 transcription domains in the developing spikelets contributes to normal floret development. Understanding the genetic network regulating SNS is a necessary first step to engineer this important agronomic trait.

A novel type of malformed floral organs mutant in barley was conferred by loss-of-function mutations of the MADS-box gene HvAGL6

The advanced model of floral morphogenesis is based largely on data from Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), but this process is less well understood in the Triticeae. Here, we investigated a sterile barley (Hordeum vulgare) mutant with malformed floral organs (designated mfo1), of which the paleae, lodicules, and stamens in each floret were all converted into lemma-like organs, and the ovary was abnormally shaped. Combining bulked-segregant analysis, whole-genome resequencing, and TILLING approaches, the mfo1 mutant was attributed to loss-of-function mutations in the MADS-box transcription factor gene HvAGL6, a key regulator in the ABCDE floral morphogenesis model. Through transcriptomic analysis between young inflorescences of wild-type and mfo1 plants, 380 genes were identified as differentially expressed, most of which function in DNA binding, protein dimerization, cell differentiation, or meristem determinacy. Regulatory pathway enrichment showed HvAGL6 associates with transcriptional abundance of many MADS-box genes, including the B-class gene HvMADS4. Mutants with deficiency in HvMADS4 exhibited the conversion of stamens into supernumerary pistils, producing multiple ovaries resembling the completely sterile multiple ovaries 3.h (mov3.h) mutant. These findings demonstrate that the regulatory model of floral morphogenesis is conserved across plant species and provides insights into the interactions between HvAGL6 and other MADS-box regulators.

From the floret to the canopy: High temperature tolerance during flowering

Heat waves induced by climate warming have become common in food-producing regions worldwide, frequently coinciding with high temperature (HT)-sensitive stages of many crops and thus threatening global food security. Understanding the HT sensitivity of reproductive organs is currently of great interest for increasing seed set. The responses of seed set to HT involve multiple processes in both male and female reproductive organs, but we currently lack an integrated and systematic summary of these responses for the world's three leading food crops (rice, wheat, and maize). In the present work, we define the critical high temperature thresholds for seed set in rice (37.2°C ± 0.2°C), wheat (27.3°C ± 0.5°C), and maize (37.9°C ± 0.4°C) during flowering. We assess the HT sensitivity of these three cereals from the microspore stage to the lag period, including effects of HT on flowering dynamics, floret growth and development, pollination, and fertilization. Our review synthesizes existing knowledge about the effects of HT stress on spikelet opening, anther dehiscence, pollen shedding number, pollen viability, pistil and stigma function, pollen germination on the stigma, and pollen tube elongation. HT-induced spikelet closure and arrest of pollen tube elongation have a catastrophic effect on pollination and fertilization in maize. Rice benefits from pollination under HT stress owing to bottom anther dehiscence and cleistogamy. Cleistogamy and secondary spikelet opening increase the probability of pollination success in wheat under HT stress. However, cereal crops themselves also have protective measures under HT stress. Lower canopy/tissue temperatures compared with air temperatures indicate that cereal crops, especially rice, can partly protect themselves from heat damage. In maize, husk leaves reduce inner ear temperature by about 5°C compared with outer ear temperature, thereby protecting the later phases of pollen tube growth and fertilization processes. These findings have important implications for accurate modeling, optimized crop management, and breeding of new varieties to cope with HT stress in the most important staple crops.

Organ-enriched gene expression during floral morphogenesis in wild barley

Floral morphology varies considerably between dicots and monocots. The ABCDE model explaining how floral organ development is controlled was formulated using core eudicots and applied to grass crops. Barley (Hordeum. vulgare) has unique floral morphogenesis. Wild barley (H. vulgare ssp. spontaneum), which is the immediate ancestor of cultivated barley (H. vulgare ssp. vulgare), contains a rich reservoir of genetic diversity. However, the wild barley genes involved in floral organ development are still relatively uncharacterized. In this study, we generated an organ-specific transcriptome atlas for wild barley floral organs. Genome-wide transcription profiles indicated that 22 838 protein-coding genes were expressed in at least one organ. These genes were grouped into seven clusters according to the similarities in their expression patterns. Moreover, 5619 genes exhibited organ-enriched expression, 677 of which were members of 47 transcription factor families. Gene ontology analyses suggested that the functions of the genes with organ-enriched expression influence the biological processes in floral organs. The co-expression regulatory network showed that the expression of 690 genes targeted by MADS-box proteins was highly positively correlated with the expression of ABCDE model genes during floral morphogenesis. Furthermore, the expression of 138 genes was specific to the wild barley OUH602 genome and not the Morex genome; most of these genes were highly expressed in the glume, awn, lemma, and palea. This study revealed the global gene expression patterns underlying floral morphogenesis in wild barley. On the basis of the study findings, a molecular mechanism controlling floral morphology in barley was proposed.

Identification of candidate genes responsible for chasmogamy in wheat

BACKGROUND

The flowering biology of wheat plants favours self-pollination which causes obstacles in wheat hybrid breeding. Wheat flowers can be divided into two groups, the first one is characterized by flowering and pollination within closed flowers (cleistogamy), while the second one possesses the ability to open flowers during processes mentioned above (chasmogamy). The swelling of lodicules is involved in the flowering of cereals and among others their morphology, calcium and potassium content differentiate between cleistogamic and non-cleistogamous flowers. A better understanding of the chasmogamy mechanism can lead to the development of tools for selection of plants with the desired outcrossing rate. To learn more, the sequencing of transcriptomes (RNA-Seq) and Representational Difference Analysis products (RDA-Seq) were performed to investigate the global transcriptomes of wheat lodicules in two highly chasmogamous (HCH, Piko and Poezja) and two low chasmogamous (LCH, Euforia and KWS Dacanto) varieties at two developmental stages-pre-flowering and early flowering.

RESULTS

The differentially expressed genes were enriched in five, main pathways: "metabolism", "organismal systems", "genetic information processing", "cellular processes" and "environmental information processing", respectively. Important genes with opposite patterns of regulation between the HCH and LCH lines have been associated with the lodicule development i.e. expression levels of MADS16 and MADS58 genes may be responsible for quantitative differences in chasmogamy level in wheat.

CONCLUSIONS

We conclude that the results provide a new insight into lodicules involvement in the wheat flowering process. This study generated important genomic information to support the exploitation of the chasmogamy in wheat hybrid breeding programs.

Methylesterification of cell-wall pectin controls the diurnal flower-opening times in rice

Flowers are the core reproductive organ of plants, and flowering is essential for cross-pollination. Diurnal flower-opening time is thus a key trait influencing reproductive isolation, hybrid breeding, and thermostability in plants. However, the molecular mechanisms controlling this trait remain unknown. Here, we report that rice Diurnal Flower Opening Time 1 (DFOT1) modulates pectin methylesterase (PME) activity to regulate pectin methylesterification levels of the lodicule cell walls, which affect lodicule swelling to control diurnal flower-opening time. DFOT1 is specifically expressed in the lodicules, and its expression gradually increases with the approach to flowering but decreases with flowering. Importantly, a knockout of DFOT1 showed earlier diurnal flower opening. We demonstrate that DFOT1 interacts directly with multiple PMEs to promote their activity. Knockout of PME40 also resulted in early diurnal flower opening, whereas overexpression of PME42 delayed diurnal flower opening. Lower PME activity was observed to be associated with higher levels of pectin methylesterification and the softening of cell walls in lodicules, which contribute to the absorption of water by lodicules and cause them to swell, thus promoting early diurnal flower opening. Higher PME activity had the opposite effect. Collectively, our work uncovers a molecular mechanism underlying the regulation of diurnal flower-opening time in rice, which would help reduce the costs of hybrid breeding and improve the heat tolerance of flowering plants by avoiding higher temperatures at anthesis.

Rice transcription factor MADS32 regulates floral patterning through interactions with multiple floral homeotic genes

Floral patterning is regulated by intricate networks of floral identity genes. The peculiar MADS32 subfamily genes, absent in eudicots but prevalent in monocots, control floral organ identity. However, how the MADS32 family genes interact with other floral homeotic genes during flower development is mostly unknown. We show here that the rice homeotic transcription factor OsMADS32 regulates floral patterning by interacting synergistically with E class protein OsMADS6 in a dosage-dependent manner. Furthermore, our results indicate important roles for OsMADS32 in defining stamen, pistil, and ovule development through physical and genetic interactions with OsMADS1, OsMADS58, and OsMADS13, and in specifying floral meristem identity with OsMADS6, OsMADS3, and OsMADS58, respectively. Our findings suggest that OsMADS32 is an important factor for floral meristem identity maintenance and that it integrates the action of other MADS-box homeotic proteins to sustain floral organ specification and development in rice. Given that OsMADS32 is an orphan gene and absent in eudicots, our data substantially expand our understanding of flower development in plants.

Spikelet movements, anther extrusion and pollen production in wheat cultivars with contrasting tendencies to cleistogamy

BACKGROUND

Cleistogamic flowers are a main barrier in pollen dispersal for cross-pollination necessary in wheat hybrid breeding. The aim of our study was to gain new knowledge on the biology of wheat flowering, in particular on the differences between the cleisto- and chasmogamic forms which has certainly cognitive significance, but it can also be used in practice when seeking a female and male ideotypes for cross hybridization.

RESULTS

We characterized the most significant features defining the flowering specificity in two wheat cultivars with contrasting tendency to cleistogamy: Piko (chasmogamous) and Dacanto (cleistogamous). In the field observations we assessed diurnal pattern of anther extrusion and anther extrusion capacity. For the first time we adapted the time lapse method for measuring kinetics of the spikelet movement and 3-D image correlation technique for the non-invasive measurements of potential deformations of the spikelet lemmas. We found that the two cultivars differ in the potential of pollen dispersion for-cross-pollination and in the spikelet kinetics. We also described some anatomical traits that can have potential functional role in floret opening. None of the cultivars showed any symptoms of lemma surface deformation.

CONCLUSIONS

The cleistogamic and chasmogamic wheat cultivars differ significantly in the potential for pollen dispersion for cross-pollination, which is mainly related to anther extrusion capacity. Although none of these features differentiated the cultivars clearly, we assume, based on spikelet kinetics and the lack of lemmas surface deformation, that the water transport and turgor of cells is essential for the floret opening and anther extrusion in wheat. The search for parental ideotype should be supported by marker assisted selection, e.g. based of polymorphisms in genes related to aquaporin biosynthesis.

APETALA2 functions as a temporal factor together with BLADE-ON-PETIOLE2 and MADS29 to control flower and grain development in barley

Cereal grain develops from fertilised florets. Alterations in floret and grain development greatly influence grain yield and quality. Despite this, little is known about the underlying genetic control of these processes, especially in key temperate cereals such as barley and wheat. Using a combination of near-isogenic mutant comparisons, gene editing and genetic analyses, we reveal that HvAPETALA2 (HvAP2) controls floret organ identity, floret boundaries, and maternal tissue differentiation and elimination during grain development. These new roles of HvAP2 correlate with changes in grain size and HvAP2-dependent expression of specific HvMADS-box genes, including the B-sister gene, HvMADS29. Consistent with this, gene editing demonstrates that HvMADS29 shares roles with HvAP2 in maternal tissue differentiation. We also discovered that a gain-of-function HvAP2 allele masks changes in floret organ identity and grain size due to loss of barley LAXATUM.A/BLADE-ON-PETIOLE2 (HvBOP2) gene function. Taken together, we reveal novel pleiotropic roles and regulatory interactions for an AP2-like gene controlling floret and grain development in a temperate cereal.

Highly Conserved Evolution of Aquaporin PIPs and TIPs Confers Their Crucial Contribution to Flowering Process in Plants

Flowering is the key process for the sexual reproduction in seed plants. In gramineous crops, the process of flowering, which includes the actions of both glume opening and glume closing, is directly driven by the swelling and withering of lodicules due to the water flow into and out of lodicule cells. All these processes are considered to be controlled by aquaporins, which are the essential transmembrane proteins that facilitate the transport of water and other small molecules across the biological membranes. In the present study, the evolution of aquaporins and their contribution to flowering process in plants were investigated via an integration of genome-wide analysis and gene expression profiling. Across the barley genome, we found that HvTIP1;1, HvTIP1;2, HvTIP2;3, and HvPIP2;1 were the predominant aquaporin genes in lodicules and significantly upregulated in responding to glume opening and closing, suggesting the importance of them in the flowering process of barley. Likewise, the putative homologs of the above four aquaporin genes were also abundantly expressed in lodicules of the other monocots like rice and maize and in petals of eudicots like cotton, tobacco, and tomato. Furthermore, all of them were mostly upregulated in responding to the process of floret opening, indicating a conserved function of these aquaporin proteins in plant flowering. The phylogenetic analysis based on the OneKP database revealed that the homologs of TIP1;1, TIP1;2, TIP2;3, and PIP2;1 were highly conserved during the evolution, especially in the angiosperm species, in line with their conserved function in controlling the flowering process. Taken together, it could be concluded that the highly evolutionary conservation of TIP1;1, TIP1;2, TIP2;3 and PIP2;1 plays important roles in the flowering process for both monocots and eudicots.

Molecular Mechanisms of Pollination Biology

Pollination is the transfer of pollen grains from the stamens to the stigma, an essential requirement of sexual reproduction in flowering plants. Cross-pollination increases genetic diversity and is favored by selection in the majority of situations. Flowering plants have evolved a wide variety of traits that influence pollination success, including those involved in optimization of self-pollination, attraction of animal pollinators, and the effective use of wind pollination. In this review we discuss our current understanding of the molecular basis of the development and production of these various traits. We conclude that recent integration of molecular developmental studies with population genetic approaches is improving our understanding of how selection acts on key floral traits in taxonomically diverse species, and that further work in nonmodel systems promises to provide exciting insights in the years to come.

Gene duplication led to divergence of expression patterns, protein-protein interaction patterns and floral development functions of AGL6-like genes in the basal angiosperm Magnolia wufengensis (Magnoliaceae)

The MADS-box family genes play critical roles in the regulation of growth and development of flowering plants. AGAMOUS-LIKE 6 (AGL6)-like genes are one of the most enigmatic subfamilies of the MADS-box family because of highly variable expression patterns and ambiguous functions, which have long puzzled researchers. A lot of AGL6 homologs have been identified from gymnosperms and angiosperms. However, only a few have been characterized, especially for basal angiosperm taxa. Magnolia wufengensis is a woody basal angiosperm from the family Magnoliaceae. In the current study, the phylogenesis, expression and protein-protein interaction (PPI) patterns, and functions of two AGL6 homologs from M. wufengensis, MawuAGL6-1 and MawuAGL6-2, were analyzed. Phylogenetic analysis indicated that the two AGL6 duplicates may have arisen by gene duplication before the divergence of Magnoliaceae and Lauraceae, with the diversification of their expression and PPI patterns after gene duplication. Functional analysis revealed that, in addition to common functions in accelerating flowering, MawuAGL6-1 might be responsible for flower meristem determinacy, while MawuAGL6-2 is preferentially recruited to regulate tepal morphogenesis. These findings further advance our understanding of the evolution of phylogenesis, expression, interaction and functions of AGL6 lineage genes from basal angiosperms, as well as the entire AGL6 lineage genes, and the significance of AGL6 lineage genes in the evolution and biological diversity.

Photosynthetic activity of reproductive organs

During seed development, carbon is reallocated from maternal tissues to support germination and subsequent growth. As this pool of resources is depleted post-germination, the plant begins autotrophic growth through leaf photosynthesis. Photoassimilates derived from the leaf are used to sustain the plant and form new organs, including other vegetative leaves, stems, bracts, flowers, fruits, and seeds. In contrast to the view that reproductive tissues act only as resource sinks, many studies demonstrate that flowers, fruits, and seeds are photosynthetically active. The photosynthetic contribution to development is variable between these reproductive organs and between species. In addition, our understanding of the developmental control of photosynthetic activity in reproductive organs is vastly incomplete. A further complication is that reproductive organ photosynthesis (ROP) appears to be particularly important under suboptimal growth conditions. Therefore, the topic of ROP presents the community with a challenge to integrate the fields of photosynthesis, development, and stress responses. Here, we attempt to summarize our understanding of the contribution of ROP to development and the molecular mechanisms underlying its control.

Understanding the role of floral development in the evolution of angiosperm flowers: clarifications from a historical and physico-dynamic perspective

Flower morphology results from the interaction of an established genetic program, the influence of external forces induced by pollination systems, and physical forces acting before, during and after initiation. Floral ontogeny, as the process of development from a meristem to a fully developed flower, can be approached either from a historical perspective, as a "recapitulation of the phylogeny" mainly explained as a process of genetic mutations through time, or from a physico-dynamic perspective, where time, spatial pressures, and growth processes are determining factors in creating the floral morphospace. The first (historical) perspective clarifies how flower morphology is the result of development over time, where evolutionary changes are only possible using building blocks that are available at a certain stage in the developmental history. Flowers are regulated by genetically determined constraints and development clarifies specific transitions between different floral morphs. These constraints are the result of inherent mutations or are induced by the interaction of flowers with pollinators. The second (physico-dynamic) perspective explains how changes in the physical environment of apical meristems create shifts in ontogeny and this is reflected in the morphospace of flowers. Changes in morphology are mainly induced by shifts in space, caused by the time of initiation (heterochrony), pressure of organs, and alterations of the size of the floral meristem, and these operate independently or in parallel with genetic factors. A number of examples demonstrate this interaction and its importance in the establishment of different floral forms. Both perspectives are complementary and should be considered in the understanding of factors regulating floral development. It is suggested that floral evolution is the result of alternating bursts of physical constraints and genetic stabilization processes following each other in succession. Future research needs to combine these different perspectives in understanding the evolution of floral systems and their diversification.

Dissecting the role of MADS-box genes in monocot floral development and diversity

Many monocot plants have high social and economic value. These include grasses such as rice (Oryza sativa), wheat (Triticum aestivum), and barley (Hordeum vulgare), which produce soft commodities for many food and beverage industries, and ornamental flowers such ase lily (Lilium longiflorum) and orchid (Oncidium Gower Ramsey), which represent an important component of international flower markets. There is constant pressure to improve the development and diversity of these species, with a significant emphasis on flower development, and this is particularly relevant considering the impact of changing environments on reproduction and thus yield. MADS-box proteins are a family of transcription factors that contain a conserved 60 amino acid MADS-box motif. In plants, attention has been devoted to characterization of this family due to their roles in inflorescence and flower development, which holds promise for the modification of floral architecture for plant breeding. This has been explored in diverse angiosperms, but particularly the dicot model Arabidopsis thaliana. The focus of this review is on the less well characterized roles of the MADS-box proteins in monocot flower development and how changes in MADS-box proteins throughout evolution may have contributed to creating a diverse range of flowers. Examining these changes within the monocots can identify the importance of certain genes and pinpoint those which might be useful in future crop improvement and breeding strategies.

MADS-box genes and crop domestication: the jack of all traits

MADS-box genes are key regulators of virtually every aspect of plant reproductive development. They play especially prominent roles in flowering time control, inflorescence architecture, floral organ identity determination, and seed development. The developmental and evolutionary importance of MADS-box genes is widely acknowledged. However, their role during flowering plant domestication is less well recognized. Here, we provide an overview illustrating that MADS-box genes have been important targets of selection during crop domestication and improvement. Numerous examples from a diversity of crop plants show that various developmental processes have been shaped by allelic variations in MADS-box genes. We propose that new genomic and genome editing resources provide an excellent starting point for further harnessing the potential of MADS-box genes to improve a variety of reproductive traits in crops. We also suggest that the biophysics of MADS-domain protein-protein and protein-DNA interactions, which is becoming increasingly well characterized, makes them especially suited to exploit coding sequence variations for targeted breeding approaches.

The superwoman1-cleistogamy2 mutant is a novel resource for gene containment in rice

Outcrossing between cultivated plants and their related wild species may result in the loss of favourable agricultural traits in the progeny or escape of transgenes in the environment. Outcrossing can be physically prevented by using cleistogamous (i.e. closed-flower) plants. In rice, flower opening is dependent on the mechanical action of fleshy organs called lodicules, which are generally regarded as the grass petal equivalents. Lodicule identity and development are specified by the action of protein complexes involving the SPW1 and OsMADS2 transcription factors. In the superwoman1-cleistogamy1 (spw1-cls1) mutant, SPW1 is impaired for heterodimerization with OsMADS2 and consequently spw1-cls1 shows thin, ineffective lodicules. However, low temperatures help stabilise the mutated SPW1/OsMADS2 heterodimer and lodicule development is restored when spw1-cls1 is grown in a cold environment, resulting in the loss of the cleistogamous phenotype. To identify a novel, temperature-stable cleistogamous allele of SPW1, targeted and random mutations were introduced into the SPW1 sequence and their effects over SPW1/OsMADS2 dimer formation were assessed in yeast two-hybrid experiments. In parallel, a novel cleistogamous allele of SPW1 called spw1-cls2 was isolated from a forward genetic screen. In spw1-cls2, a mutation leading to a change of an amino acid involved in DNA binding by the transcription factor was identified. Fertility of spw1-cls2 is somewhat decreased under low temperatures but unlike for spw1-cls1, the cleistogamous phenotype is maintained, making the line a safer and valuable genetic resource for gene containment.

Sterility Caused by Floral Organ Degeneration and Abiotic Stresses in Arabidopsis and Cereal Grains

Natural floral organ degeneration or abortion results in unisexual or fully sterile flowers, while abiotic stresses lead to sterility after initiation of floral reproductive organs. Since normal flower development is essential for plant sexual reproduction and crop yield, it is imperative to have a better understanding of plant sterility under regular and stress conditions. Here, we review the functions of ABC genes together with their downstream genes in floral organ degeneration and the formation of unisexual flowers in Arabidopsis and several agriculturally significant cereal grains. We further explore the roles of hormones, including auxin, brassinosteroids, jasmonic acid, gibberellic acid, and ethylene, in floral organ formation and fertility. We show that alterations in genes affecting hormone biosynthesis, hormone transport and perception cause loss of stamens/carpels, abnormal floral organ development, poor pollen production, which consequently result in unisexual flowers and male/female sterility. Moreover, abiotic stresses, such as heat, cold, and drought, commonly affect floral organ development and fertility. Sterility is induced by abiotic stresses mostly in male floral organ development, particularly during meiosis, tapetum development, anthesis, dehiscence, and fertilization. A variety of genes including those involved in heat shock, hormone signaling, cold tolerance, metabolisms of starch and sucrose, meiosis, and tapetum development are essential for plants to maintain normal fertility under abiotic stress conditions. Further elucidation of cellular, biochemical, and molecular mechanisms about regulation of fertility will improve yield and quality for many agriculturally valuable crops.

OsCHX14 is Involved in the K+ Homeostasis in Rice (Oryza sativa) Flowers

Previously we showed in the osjar1 mutants that the lodicule senescence which controls the closing of rice flowers was delayed. This resulted in florets staying open longer when compared with the wild type. The gene OsJAR1 is silenced in osjar1 mutants and is a key member of the jasmonic acid (JA) signaling pathway. We found that K concentrations in lodicules and flowers of osjar1-2 were significantly elevated compared with the wild type, indicating that K⁺ homeostasis may play a role in regulating the closure of rice flowers. The cation/H⁺ exchanger (CHX) family from rice was screened for potential K⁺ transporters involved as many members of this family in Arabidopsis were exclusively or preferentially expressed in flowers. Expression profiling confirmed that among 17 CHX genes in rice, OsCHX14 was the only member that showed an expression polymorphism, not only in osjar1 mutants but also in RNAi (RNA interference) lines of OsCOI1, another key member of the JA signaling pathway. This suggests that the expression of OsCHX14 is regulated by the JA signaling pathway. Green fluorescent protein (GFP)-tagged OsCHX14 protein was preferentially localized to the endoplasmic reticulum. Promoter-β-glucuronidase (GUS) analysis of transgenic rice revealed that OsCHX14 is mainly expressed in lodicules and the region close by throughout the flowering process. Characterization in yeast and Xenopus laevis oocytes verified that OsCHX14 is able to transport K⁺, Rb⁺ and Cs⁺ in vivo. Our data suggest that OsCHX14 may play an important role in K⁺ homeostasis during flowering in rice.

A stranger in a strange land: the utility and interpretation of heterologous expression

One of the major goals of the modern study of evodevo is to understand the evolution of gene function across a range of contexts, including sub/neofunctionalization, co-option of genetic modules, and the evolution of morphological novelty. To these ends, comparative studies of gene expression can be useful for constructing hypotheses, but cannot provide direct evidence of functional evolution. Unfortunately, determining endogenous gene function in non-model species is often not an option. Faced with this dilemma, a common approach is to use heterologous expression (HE) in genetically tractable model species as a proxy for functional analyses. Such experiments have important limitations, however, and require caution in the interpretation of their results. How do we dissociate biochemical function from its original genomic context? In the end, what does HE actually tell us? Here, I argue that HE only sheds light on specific types of biochemical conservation, but can be useful when experiments are carefully interpreted.

Interactions of OsMADS1 with Floral Homeotic Genes in Rice Flower Development

During reproductive development, rice plants develop unique flower organs which determine the final grain yield. OsMADS1, one of SEPALLATA-like MADS-box genes, has been unraveled to play critical roles in rice floral organ identity specification and floral meristem determinacy. However, the molecular mechanisms underlying interactions of OsMADS1 with other floral homeotic genes in regulating flower development remains largely elusive. In this work, we studied the genetic interactions of OsMADS1 with B-, C-, and D-class genes along with physical interactions among their proteins. We show that the physical and genetic interactions between OsMADS1 and OsMADS3 are essential for floral meristem activity maintenance and organ identity specification; while OsMADS1 physically and genetically interacts with OsMADS58 in regulating floral meristem determinacy and suppressing spikelet meristem reversion. We provided important genetic evidence to support the neofunctionalization of two rice C-class genes (OsMADS3 and OsMADS58) during flower development. Gene expression profiling and quantitative RT-PCR analyses further revealed that OsMADS1 affects the expression of many genes involved in floral identity and hormone signaling, and chromatin immunoprecipitation (ChIP)-PCR assay further demonstrated that OsMADS17 is a direct target gene of OsMADS1. Taken together, these results reveal that OsMADS1 has diversified regulatory functions in specifying rice floral organ and meristem identity, probably through its genetic and physical interactions with different floral homeotic regulators.

Grass flower development

Grasses bear unique flowers lacking obvious petals and sepals in special inflorescence units, the florets and the spikelet. Despite this, grass floral organs such as stamens and lodicules (petal homologs) are specified by ABC homeotic genes encoding MADS domain transcription factors, suggesting that the ABC model of eudicot flower development is largely applicable to grass flowers. However, some modifications need to be made for the model to fit grasses well: for example, a YABBY gene plays an important role in carpel specification. In addition, a number of genes are involved in the development of the lateral organs that constitute the spikelet. In this review, we discuss recent progress in elucidating the genes required for flower and spikelet development in grasses, together with those involved in fate determination of the spikelet and flower meristems.

An alternative mechanism for cleistogamy in barley

Cleistogamy in barley is genetically determined by the presence of the recessive allele cly1, but the dominant allele at the linked locus Cly2 is epistatic over cly1. Although the molecular basis for cly1 action is well understood, that of Cly2 is not. Here we show that anther non-extrusion can occur not just when the lodicules fail to expand adequately (a trait which is fully determined by the allelic state at the cly1 locus), but by the premature timing of anthesis before the spike has emerged from the boot. The transcription of HvAP2 at cly1 is unaffected by the timing of anthesis. Where this occurs prematurely, by the time that the spike has emerged from the boot, the lodicules have already become shrunken and have lost the capacity to push the lemma and palea apart. Premature anthesis appears to be governed by a dominant gene, probably Cly2. Of the three phases of development of a non-cleistogamous barley floret (spike emergence from the boot, floret gaping induced by lodicule expansion and anther extrusion), genetic variation is available regarding at least the former two.

Homeotic Genes and the ABCDE Model for Floral Organ Formation in Wheat

Floral organ formation has been the subject of intensive study for over 20 years, particularly in the model dicot species Arabidopsis thaliana. These studies have led to the establishment of a general model for the development of floral organs in higher plants, the so-called ABCDE model, in which floral whorl-specific combinations of class A, B, C, D, or E genes specify floral organ identity. In Arabidopsis, class A, B, C, D, E genes encode MADS-box transcription factors except for the class A gene APETALA2. Mutation of these genes induces floral organ homeosis. In this review, I focus on the roles of these homeotic genes in bread wheat (Triticum aestivum), particularly with respect to the ABCDE model. Pistillody, the homeotic transformation of stamens into pistil-like structures, occurs in cytoplasmic substitution (alloplasmic) wheat lines that have the cytoplasm of the related wild species Aegilops crassa. This phenomenon is a valuable tool for analysis of the wheat ABCDE model. Using an alloplasmic line, the wheat ortholog of DROOPING LEAF (TaDL), a member of the YABBY gene family, has been shown to regulate pistil specification. Here, I describe the current understanding of the ABCDE model for floral organ formation in wheat.