A rice homeobox gene, OSH1, is expressed before organ differentiation in a specific region during early embryogenesis

Genome-wide association analysis reveals novel candidate loci and a gene regulating tiller number in orchardgrass

Tillers are specialized lateral shoots arising from axillary buds at basal nodes, and are also an important agronomic trait that determines the aboveground biomass and grain yield of various gramineous crops. So far, few genes have been reported to control tiller formation and most have been in the annual crop rice (Oryza sativa). Orchardgrass (Dactylis glomerata) is an important perennial forage crop with great economic and ecological value, but its genes regulating tillering have remained largely unknown. In the present study, we used a natural population of 264 global orchardgrass germplasms to determine genes associated with quantitative variation in tiller number through genome-wide association study analysis. A total of 19 putative loci and 55 genes associated with tiller number were thus identified. Additionally, 26 putative differentially expressed genes with tiller number, including DgCYC-C1, were identified by RNA-seq and genome-wide association study analysis. DgCYC-C1 which is involved in cell division, was overexpressed, revealing that DgCYC-C1 positively regulates tiller number. These results provide some new candidate genes or loci for the improvement of tiller number in crops, which might advance new sustainable strategies to meet global crop production challenges.

FINE CULM1 Encoding a TEOSINTE BRANCHED1-like TCP Transcription Factor Negatively Regulates Axillary Meristem Formation in Rice

Shoot branching is a critical determinant of plant architecture and a key factor affecting crop yield. The shoot branching involves two main processes: axillary meristem formation and subsequent bud outgrowth. While considerable progress has been made in elucidating the genetic mechanisms underlying the latter process, our understanding of the former process remains limited. Rice FINE CULM1 (FC1), which is an ortholog of teosinte branched1 in maize (Zea mays) and BRANCHED1/2 in Arabidopsis (Arabidopsis thaliana), is known to act in the latter process by repressing bud outgrowth. In this study, we found that FC1 also plays a role in the former process, i.e. axillary meristem formation, in rice. This study was triggered by our unexpected observation that fc1 mutation suppresses the loss of axillary meristems in the loss-of-function mutant of the rice WUSCHEL gene TILLERS ABSENT1 (TAB1). In tab1 fc1, unlike in tab1, both stem cells and undifferentiated cells were maintained during axillary meristem formation, similar to the wild type. Morphological analysis showed that axillary meristem formation was accelerated in fc1, compared to the wild type. Consistent with this, cell proliferation was more active in the region containing stem cells and undifferentiated cells during axillary meristem formation in fc1 than in the wild type. Taken altogether, these findings suggest that FC1 negatively regulates axillary meristem formation by mildly repressing cell proliferation during this process.

Upstream regulator of genomic imprinting in rice endosperm is a small RNA-associated chromatin remodeler

Genomic imprinting is observed in endosperm, a placenta-like seed tissue, where transposable elements (TEs) and repeat-derived small RNAs (sRNAs) mediate epigenetic changes in plants. In imprinting, uniparental gene expression arises due to parent-specific epigenetic marks on one allele but not on the other. The importance of sRNAs and their regulation in endosperm development or in imprinting is poorly understood in crops. Here we show that a previously uncharacterized CLASSY (CLSY)-family chromatin remodeler named OsCLSY3 is essential for rice endosperm development and imprinting, acting as an upstream player in the sRNA pathway. Comparative transcriptome and genetic analysis indicated its endosperm-preferred expression and its likely paternal imprinted nature. These important features are modulated by RNA-directed DNA methylation (RdDM) of tandemly arranged TEs in its promoter. Upon perturbation of OsCLSY3 in transgenic lines, we observe defects in endosperm development and a loss of around 70% of all sRNAs. Interestingly, well-conserved endosperm-specific sRNAs (siren) that are vital for reproductive fitness in angiosperms are also dependent on OsCLSY3. We observed that many imprinted genes and seed development-associated genes are under the control of OsCLSY3. These results support an essential role of OsCLSY3 in rice endosperm development and imprinting, and propose similar regulatory strategies involving CLSY3 homologs among other cereals.

DWARF TILLER1 regulates apical-basal pattern formation and proper orientation of rice embryos

Body axis establishment is one of the earliest patterning events in plant embryogenesis. Asymmetric zygote division is critical for apical-basal axis formation in Arabidopsis (Arabidopsis thaliana). However, how the orientation of the cell division plane is regulated and its relation to apical-basal axis establishment and proper position of embryos in grasses remain poorly understood. By characterizing mutants of 3 rice (Oryza sativa) WUSCHEL HOMEOBOX9 (WOX9) genes, whose paralogs in Arabidopsis play essential roles in zygotic asymmetric cell division and cell fate determination, we found 2 kinds of independent embryonic defects: topsy-turvy embryos, in which apical-basal axis twists from being parallel to the longitudinal axis of the seed to being perpendicular; and organ-less embryos. In contrast to their Arabidopsis orthologs, OsWOX9s displayed dynamic distribution during embryo development. Both DWT1/OsWOX9A and DWL2/WOX9C play major roles in the apical-basal axis formation and initiation of stem cells. In addition, DWT1 has a distinct function in regulating the first few embryonic cell divisions to ensure the correct orientation of the embryo in the ovary. In summary, DWT1 acts in 2 steps during rice embryo pattern formation: the initial zygotic division, and with DWL2 to establish the main body axes and stem cell fate 2 to 3 d after pollination.

Adaxial-abaxial bipolar leaf genes encode a putative cytokinin receptor and HD-Zip III, and control the formation of ectopic shoot meristems in rice

Shoot apical meristems (SAMs) continuously initiate organ formation and maintain pluripotency through dynamic genetic regulations and cell-to-cell communications. The activity of meristems directly affects the plant's structure by determining the number and arrangement of organs and tissues. We have taken a forward genetic approach to dissect the genetic pathway that controls cell differentiation around the SAM. The rice mutants, adaxial-abaxial bipolar leaf 1 and 2 (abl1 and abl2), produce an ectopic leaf that is fused back-to-back with the fourth leaf, the first leaf produced after embryogenesis. The abaxial-abaxial fusion is associated with the formation of an ectopic shoot meristem at the adaxial base of the fourth leaf primordium. We cloned the ABL1 and ABL2 genes of rice by mapping their chromosomal positions. ABL1 encodes OsHK6, a histidine kinase, and ABL2 encodes a transcription factor, OSHB3 (Class III homeodomain leucine zipper). Expression analyses of these mutant genes as well as OSH1, a rice ortholog of the Arabidopsis STM gene, unveiled a regulatory circuit that controls the formation of an ectopic meristem near the SAM at germination.

STAMENLESS1 activates SUPERWOMAN 1 and FLORAL ORGAN NUMBER 1 to control floral organ identities and meristem fate in rice

Floral patterns are unique to rice and contribute significantly to its reproductive success. SL1 encodes a C2H2 transcription factor that plays a critical role in flower development in rice, but the molecular mechanism regulated by it remains poorly understood. Here, we describe interactions of the SL1 with floral homeotic genes, SPW1, and DL in specifying floral organ identities and floral meristem fate. First, the sl1 spw1 double mutant exhibited a stamen-to-pistil transition similar to that of sl1, spw1, suggesting that SL1 and SPW1 may located in the same pathway regulating stamen development. Expression analysis revealed that SL1 is located upstream of SPW1 to maintain its high level of expression and that SPW1, in turn, activates the B-class genes OsMADS2 and OsMADS4 to suppress DL expression indirectly. Secondly, sl1 dl displayed a severe loss of floral meristem determinacy and produced amorphous tissues in the third/fourth whorl. Expression analysis revealed that the meristem identity gene OSH1 was ectopically expressed in sl1 dl in the fourth whorl, suggesting that SL1 and DL synergistically terminate the floral meristem fate. Another meristem identity gene, FON1, was significantly decreased in expression in sl1 background mutants, suggesting that SL1 may directly activate its expression to regulate floral meristem fate. Finally, molecular evidence supported the direct genomic binding of SL1 to SPW1 and FON1 and the subsequent activation of their expression. In conclusion, we present a model to illustrate the roles of SL1, SPW1, and DL in floral organ specification and regulation of floral meristem fate in rice.

The catalytic subunit of type 2A protein phosphatase negatively regulates conidiation and melanin biosynthesis in Setosphaeria turcica

Northern corn leaf blight caused by Setosphaeria turcica is a major fungal disease responsible for significant reductions in maize yield worldwide. Eukaryotic type 2A protein phosphatase (PP2A) influences growth and virulence in a number of pathogenic fungi, but little is known about its roles in S. turcica. Here, we functionally characterized S. turcica StPP2A-C, which encodes the catalytic C subunit of StPP2A. StPP2A-C deletion slowed colony growth, conidial germination, and appressorium formation but increased conidiation, melanin biosynthesis, glycerol content, and disease lesion size on maize. These effects were associated with expression changes in genes related to calcium signaling, conidiation, laccase activity, and melanin and glycerol biosynthesis, as well as changes in intra- and extracellular laccase activity. A pull-down screen for candidate StPP2A-c interactors revealed an interaction between StPP2A-c and StLac1. Theoretical modeling and yeast two-hybrid experiments confirmed that StPP2A-c interacted specifically with the copper ion binding domain of StLac1 and that Cys267 of StPP2A-c was required for this interaction. StPP2A-C expression thus appears to promote hyphal growth and reduce pathogenicity in S. turcica, at least in part by altering melanin synthesis and laccase activity; these insights may ultimately support the development of novel strategies for biological management of S. turcica.

The ldp1 Mutation Affects the Expression of Auxin-Related Genes and Enhances SAM Size in Rice

Panicle type is one of the important factors affecting rice (Oryza sativa L.) yield, and the identification of regulatory genes in panicle development can provide significant insights into the molecular network involved. This study identified a large and dense panicle 1 (ldp1) mutant produced from the Wuyunjing 7 (WYJ7) genotype, which displayed significant relative increases in panicle length, number of primary and secondary branches, number of grains per panicle, grain width, and grain yield per plant. Scanning electron microscopy results showed that the shoot apical meristem (SAM) of ldp1 was relatively larger at the bract stage (BM), with a significantly increased number of primary (PBM) and secondary branch (SBM) meristematic centers, indicating that the ldp1 mutation affects early stages in SAM development Comparative RNA-Seq analysis of meristem tissues from WYJ7 and ldp1 at the BM, PBM, and SBM developmental stages indicated that the number of differentially expressed genes (DEGs) were highest (1407) during the BM stage. Weighted gene coexpression network analysis (WGCNA) revealed that genes in one module (turquoise) are associated with the ldp1 phenotype and highly expressed during the BM stage, suggesting their roles in the identity transition and branch differentiation stages of rice inflorescences. Hub genes involved in auxin synthesis and transport pathways, such as OsAUX1, OsAUX4, and OsSAUR25, were identified. Moreover, GO and KEGG analysis of the DEGs in the turquoise module and the 1407 DEGs in the BM stage revealed that a majority of genes involved in tryptophan metabolism and auxin signaling pathway were differentially expressed between WYJ and ldp1. The genetic analysis indicated that the ldp1 phenotype is controlled by a recessive monogene (LDP1), which was mapped to a region between 16.9 and 18.1 Mb on chromosome seven. This study suggests that the ldp1 mutation may affect the expression of key genes in auxin synthesis and signal transduction, enhance the size of SAM, and thus affect panicle development. This study provides insights into the molecular regulatory network underlying rice panicle morphogenesis and lays an important foundation for further understanding the function and molecular mechanism of LDP1 during panicle development.

Transcriptome Profiling Provides Insights into the Early Development of Tiller Buds in High- and Low-Tillering Orchardgrass Genotypes

Orchardgrass (Dactylis glomerata L.) is among the most economically important perennial cool-season grasses, and is considered an excellent hay, pasture, and silage crop in temperate regions worldwide. Tillering is a vital feature that dominates orchardgrass regeneration and biomass yield. However, transcriptional dynamics underlying early-stage bud development in high- and low-tillering orchardgrass genotypes are unclear. Thus, this study assessed the photosynthetic parameters, the partially essential intermediate biomolecular substances, and the transcriptome to elaborate the early-stage profiles of tiller development. Photosynthetic efficiency and morphological development significantly differed between high- (AKZ-NRGR667) and low-tillering genotypes (D20170203) at the early stage after tiller formation. The 206.41 Gb of high-quality reads revealed stage-specific differentially expressed genes (DEGs), demonstrating that signal transduction and energy-related metabolism pathways, especially photosynthetic-related processes, influence tiller induction and development. Moreover, weighted correlation network analysis (WGCNA) and functional enrichment identified distinctively co-expressed gene clusters and four main regulatory pathways, including chlorophyll, lutein, nitrogen, and gibberellic acid (GA) metabolism pathways. Therefore, photosynthesis, carbohydrate synthesis, nitrogen efficient utilization, and phytohormone signaling pathways are closely and intrinsically linked at the transcriptional level. These findings enhance our understanding of tillering in orchardgrass and perennial grasses, providing a new breeding strategy for improving forage biomass yield.

Mutation in Polycomb repressive complex 2 gene OsFIE2 promotes asexual embryo formation in rice

Prevention of autonomous division of the egg apparatus and central cell in a female gametophyte before fertilization ensures successful reproduction in flowering plants. Here we show that rice ovules of Polycomb repressive complex 2 (PRC2) Osfie1 and Osfie2 double mutants exhibit asexual embryo and autonomous endosperm formation at a high frequency, while ovules of single Osfie2 mutants display asexual pre-embryo-like structures at a lower frequency without fertilization. Earlier onset, higher penetrance and better development of asexual embryos in the double mutants compared with those in Osfie2 suggest that the autonomous endosperm facilitated asexual embryo development. Transcriptomic analysis showed that male genome-expressed OsBBM1 and OsWOX8/9 were activated in the asexual embryos. Similarly, the maternal alleles of the paternally expressed imprinted genes were activated in the autonomous endosperm, suggesting that the egg apparatus and central cell convergently adopt PRC2 to maintain the non-dividing state before fertilization, possibly through silencing of the maternal alleles of male genome-expressed genes.

Biocuration of a Transcription Factors Network Involved in Submergence Tolerance during Seed Germination and Coleoptile Elongation in Rice (Oryza sativa)

Modeling biological processes and genetic-regulatory networks using in silico approaches provides a valuable framework for understanding how genes and associated allelic and genotypic differences result in specific traits. Submergence tolerance is a significant agronomic trait in rice; however, the gene-gene interactions linked with this polygenic trait remain largely unknown. In this study, we constructed a network of 57 transcription factors involved in seed germination and coleoptile elongation under submergence. The gene-gene interactions were based on the co-expression profiles of genes and the presence of transcription factor binding sites in the promoter region of target genes. We also incorporated published experimental evidence, wherever available, to support gene-gene, gene-protein, and protein-protein interactions. The co-expression data were obtained by re-analyzing publicly available transcriptome data from rice. Notably, this network includes OSH1, OSH15, OSH71, Sub1B, ERFs, WRKYs, NACs, ZFP36, TCPs, etc., which play key regulatory roles in seed germination, coleoptile elongation and submergence response, and mediate gravitropic signaling by regulating OsLAZY1 and/or IL2. The network of transcription factors was manually biocurated and submitted to the Plant Reactome Knowledgebase to make it publicly accessible. We expect this work will facilitate the re-analysis/re-use of OMICs data and aid genomics research to accelerate crop improvement.

Genetic basis controlling rice plant architecture and its modification for breeding

The shoot and root system architectures are fundamental for crop productivity. During the history of artificial selection of domestication and post-domestication breeding, the architecture of rice has significantly changed from its wild ancestor to fulfil requirements in agriculture. We review the recent studies on developmental biology in rice by focusing on components determining rice plant architecture; shoot meristems, leaves, tillers, stems, inflorescences and roots. We also highlight natural variations that affected these structures and were utilized in cultivars. Importantly, many core regulators identified from developmental mutants have been utilized in breeding as weak alleles moderately affecting these architectures. Given a surge of functional genomics and genome editing, the genetic mechanisms underlying the rice plant architecture discussed here will provide a theoretical basis to push breeding further forward not only in rice but also in other crops and their wild relatives.

Genetic and molecular pathways controlling rice inflorescence architecture

Rice inflorescence is one of the major organs in determining grain yield. The genetic and molecular regulation on rice inflorescence architecture has been well investigated over the past years. In the present review, we described genes regulating rice inflorescence architecture based on their roles in meristem activity maintenance, meristem identity conversion and branch elongation. We also introduced the emerging regulatory pathways of phytohormones involved in rice inflorescence development. These studies show the intricacies and challenges of manipulating inflorescence architecture for rice yield improvement.

Co-option of the SHOOT MERISTEMLESS network regulates protocorm-like body development in Phalaenopsis aphrodite

The protocorm is a structure that is formed upon germination of an orchid seed. It lacks cotyledons and is ovoid in shape. The protocorm-like body (PLB), on the other hand, is a protocorm-like organ induced from somatic tissues. PLBs have been widely used for orchid micropropagation. Because of its unique structure and its application in the orchid industry, PLB development has drawn considerable interest from orchid and developmental biologists. Our previous genome-wide comparative transcriptome study demonstrated that protocorms and PLBs share similar molecular signatures and suggested that SHOOT MERISTEMLESS (STM)-dependent organogenesis is important for PLB development. Here, we show that overexpression of Phalaenopsis aphrodite STM (PaSTM) greatly enhances PLB regeneration from vegetative tissue-based explants of Phalaenopsis orchids, confirming its regulatory role in PLB development. Expression of PaSTM restored shoot meristem function of the Arabidopsis (Arabidopsis thaliana) stm-2 mutant. Moreover, we identified class S11 MYB transcription factors (TFs) as targets downstream of PaSTM. A cis-acting element, TTGACT, identified in the promoters of S11 MYB TFs was found to be important for PaSTM binding and activation. Overexpression of PaSTM or its downstream targets, PaMYB13, PaMYB14, and PaMYB17, enhanced de novo shoot regeneration in Arabidopsis, indicating the active role of the PaSTM-S11 PaMYB module in organogenesis. In summary, our data demonstrate that PaSTM is important for PLB development. The STM-S11 MYB regulatory module is evolutionarily conserved and may regulate shoot or shoot-related organ development in plants.

A coiled-coil protein associates Polycomb Repressive Complex 2 with KNOX/BELL transcription factors to maintain silencing of cell differentiation-promoting genes in the shoot apex

Polycomb repressive complex 2 (PRC2), which mediates the deposition of H3K27me3 histone marks, is important for developmental decisions in animals and plants. In the shoot apical meristem (SAM), Three Amino acid Loop Extension family KNOTTED-LIKE HOMEOBOX /BEL-like (KNOX/BELL) transcription factors are key regulators of meristem cell pluripotency and differentiation. Here, we identified a PRC2-associated coiled-coil protein (PACP) that interacts with KNOX/BELL transcription factors in rice (Oryza sativa) shoot apex cells. A loss-of-function mutation of PACP resulted in differential gene expression similar to that observed in PRC2 gene knockdown plants, reduced H3K27me3 levels, and reduced genome-wide binding of the PRC2 core component EMF2b. The genomic binding of PACP displayed a similar distribution pattern to EMF2b, and genomic regions with high PACP- and EMF2b-binding signals were marked by high levels of H3K27me3. We show that PACP is required for the repression of cell differentiation-promoting genes targeted by a rice KNOX1 protein in the SAM. PACP is involved in the recruitment or stabilization of PRC2 to genes targeted by KNOX/BELL transcription factors to maintain H3K27me3 and gene repression in dividing cells of the shoot apex. Our results provide insight into PRC2-mediated maintenance of H3K27me3 and the mechanism by which KNOX/BELL proteins regulate SAM development.

SWI2/SNF2 chromatin remodeling ATPases SPLAYED and BRAHMA control embryo development in rice

Chromatin remodeling ATPases OsSYD and OsBRM are involved in shoot establishment, and both affect OSH gene transcription. OsSYD protein interacts with RFL, but OsBRM does not. In plants, SPLAYED (SYD) and BRAHMA (BRM) encode chromatin remodeling ATPases that use the energy derived from ATP hydrolysis to restructure nucleosomes and render certain genomic regions available to transcription factors. However, the function of SYD and BRM on rice growth and development is unknown. Here, we constructed ossyd and osbrm mutants using CRISPR/Cas9 technology and analyzed the effects of mutations on rice embryo development. We discovered that the ossyd and osbrm mutants exhibited severe defects during embryonic development, whereas endosperm development was normal. These results indicated that the development of the embryo and endosperm is independent of each other. Consequently, the ossyd- and osbrm-null mutants did not germinate due to the abnormal embryos. Furthermore, we observed the embryos of ossyd- and osbrm-null mutants, and they indeed had distinct differentiation defects in shoot establishment, acquired during embryogenesis. To verify the function of OsSYD and OsBRM in embryogenesis, we measured the transcript levels of marker genes at different stages. Compared with wild type, the expression levels of multiple OSH genes were significantly reduced in the mutants, which was consistent with the defective shoot establishment phenotypes. The interaction between SYD and RICE FLORICAULA/LFY (RFL) was revealed using a yeast two-hybrid screening system, suggesting that the interaction between the LFY homolog and chromatin remodeling ATPases is ubiquitous in plants. Collectively, our findings provide the basis for elucidating the function of OsSYD and OsBRM during embryo development in rice.

Flower meristem maintenance by TILLERS ABSENT 1 is essential for ovule development in rice

Plant development depends on the activity of pluripotent stem cells in meristems, such as the shoot apical meristem and the flower meristem. In Arabidopsis thaliana, WUSCHEL (WUS) is essential for stem cell homeostasis in meristems and integument differentiation in ovule development. In rice (Oryza sativa), the WUS ortholog TILLERS ABSENT 1 (TAB1) promotes stem cell fate in axillary meristem development, but its function is unrelated to shoot apical meristem maintenance in vegetative development. In this study, we examined the role of TAB1 in flower development. The ovule, which originates directly from the flower meristem, failed to differentiate in tab1 mutants, suggesting that TAB1 is required for ovule formation. Expression of a stem cell marker was completely absent in the flower meristem at the ovule initiation stage, indicating that TAB1 is essential for stem cell maintenance in the 'final' flower meristem. The ovule defect in tab1 was partially rescued by floral organ number 2 mutation, which causes overproliferation of stem cells. Collectively, it is likely that TAB1 promotes ovule formation by maintaining stem cells at a later stage of flower development.

A SNP Mutation in Homeodomain-DDT (HD-DDT) Transcription Factor Results in Multiple Trichomes (mt) in Cucumber (Cucumis sativus L.)

Trichome is a natural physical barrier protecting plants against environmental stresses, natural infestations, ultraviolet rays and pathogenicity. Trichome also helps plants in maintaining appropriate water content by reducing transpiration rate. The molecular mechanism regulating unicellular trichome development in Arabidopsis has been extensively elucidated, but the molecular mechanism regulating multicellular trichome development remains unclear. In this study, we identified a multiple trichomes (mt) mutant from a cucumber EMS (Ethylmethylsulfone) mutagenesis population. Genetic analysis indicated that an incomplete dominant gene controls the mt trait. Using a combination of map-based cloning and BSA-seq (Bulked Segregant Analysis -Sequencing), we identified the candidate gene, CsaV3_6G050410, responsible for the mt mutation. Sequence alignment revealed one base substitution in gene CsaV3_6G050410, resulting in an amino acid substitution. The deduced amino acid sequence of CsaV3_6G050410 encodes a HD-DDT (homeodomain-DDT) transcriptional regulatory protein containing a conserved homeobox domain and a DDT domain. Gene expression analysis revealed that the expression level of CsaV3_6G050410 in the mt mutant was similar to that in the WT (wild type). Transcriptome analysis indicated that the mt gene may regulate the development of the epidermis by influencing plant hormone signaling pathways or participating in several transcription factor pathways. The results of this study are fundamental for a better understanding of the function of the HD-DDT transcription factor in the trichome development of cucumber.

Cadmium stress suppresses the tillering of perennial ryegrass and is associated with the transcriptional regulation of genes controlling axillary bud outgrowth

Perennial ryegrass (Lolium perenne L.), a grass species with superior tillering capacity, plays a potential role in the phytoremediation of cadmium (Cd)-contaminated soils. Tiller production is inhibited in response to serious Cd stress. However, the regulatory mechanism of Cd stress-induced inhibition of tiller development is not well documented. To address this issue, we investigated the phenotype, the expression levels of genes involved in axillary bud initiation and bud outgrowth, and endogenous hormone biosynthesis and signaling pathways in seedlings of perennial ryegrass under Cd stress. The results showed that the number of tillers and axillary buds in the Cd-treated seedlings decreased by 67% and 21%, respectively. The suppression of tiller production in the Cd-treated seedlings was more closely associated with the inhibition of axillary bud outgrowth than with bud initiation. Cd stress upregulated the expression level of genes related to axillary bud dormancy and downregulated bud activity genes. Additionally, genes involved in strigolactone biosynthesis and signaling, auxin transport and signaling, and cytokinin degradation were upregulated in Cd-treated seedlings, and cytokinin biosynthesis gene expression were decreased by Cd stress. The content of zeatin in the Cd-treated pants was significantly reduced by 69~85% compared to the control plants. The content of indole-3-acetic acid (IAA) remains constant under Cd stress. Overall, Cd stress induced axillary bud dormancy and subsequently inhibited axillary bud outgrowth. The decrease of zeatin content and upregulation of genes involved in strigolactone signaling and bud dormancy might be responsible for the inhibition of axillary bud outgrowth.

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.

Molecular and Genetic Aspects of Grain Number Determination in Rice (Oryza sativa L.)

Rice grain yield is a complex trait determined by three components: panicle number, grain number per panicle (GNPP) and grain weight. GNPP is the major contributor to grain yield and is crucial for its improvement. GNPP is determined by a series of physiological and biochemical steps, including inflorescence development, formation of rachis branches such as primary rachis branches and secondary rachis branches, and spikelet specialisation (lateral and terminal spikelets). The molecular genetic basis of GNPP determination is complex, and it is regulated by numerous interlinked genes. In this review, panicle development and the determination of GNPP is described briefly, and GNPP-related genes that influence its determination are categorised according to their regulatory mechanisms. We introduce genes related to rachis branch development and their regulation of GNPP, genes related to phase transition (from rachis branch meristem to spikelet meristem) and their regulation of GNPP, and genes related to spikelet specialisation and their regulation of GNPP. In addition, we describe other GNPP-related genes and their regulation of GNPP. Research on GNPP determination suggests that it is possible to cultivate rice varieties with higher grain yield by modifying GNPP-related genes.

The HK5 and HK6 cytokinin receptors mediate diverse developmental pathways in rice

The phytohormone cytokinin regulates diverse aspects of plant growth and development. Our understanding of the metabolism and perception of cytokinin has made great strides in recent years, mostly from studies of the model dicot Arabidopsis Here, we employed a CRISPR/Cas9-based approach to disrupt a subset of cytokinin histidine kinase (HK) receptors in rice (Oryza sativa) in order to explore the role of cytokinin in a monocot species. In hk5 and hk6 single mutants, the root growth, leaf width, inflorescence architecture and/or floral development were affected. The double hk5 hk6 mutant showed more substantial defects, including severely reduced root and shoot growth, a smaller shoot apical meristem, and an enlarged root cap. Flowering was delayed in the hk5 hk6 mutant and the panicle was significantly reduced in size and infertile due to multiple defects in floral development. The hk5 hk6 mutant also exhibited a severely reduced cytokinin response, consistent with the developmental phenotypes arising from a defect in cytokinin signaling. These results indicate that HK5 and HK6 act as cytokinin receptors, with overlapping functions to regulate diverse aspects of rice growth and development.

Identification and fine mapping of qSB.A09, a major QTL that controls shoot branching in Brassica rapa ssp. chinensis Makino

QTL mapping plus bulked segregant analysis revealed a major QTL for shoot branching in non-heading Chinese cabbage. The candidate gene was then identified using sequence alignment and expression analysis. Shoot branching is a complex quantitative trait that contributes to plant architecture and ultimately yield. Although many studies have examined branching in grain crops, the genetic control of shoot branching in vegetable crops such as Brassica rapa L. ssp. chinensis remains poorly understood. In this study, we used bulked segregant analysis (BSA) of an F₂ population to detect a major quantitative trait locus (QTL) for shoot branching, designated shoot branching 9 (qSB.A09) on the long arm of chromosome A09 in Brassica rapa L. ssp. chinensis. In addition, traditional QTL mapping of the F₂ population revealed six QTLs in different regions. Of these, the mapping region on chromosome A09 was consistent with the results of BSA-seq analysis, as well as being stable over the 2-year study period, explaining 19.37% and 22.18% of the phenotypic variation across multiple genetic backgrounds. Using extreme recombinants, qSB.A09 was further delimited to a 127-kb genomic region harboring 28 annotated genes. We subsequently identified the GRAS transcription factor gene Bra007056 as a potential candidate gene; Bra007056 is an ortholog of MONOCULM 1 (MOC1), the key gene that controls tillering in rice. Quantitative RT-PCR further revealed that expression of Bra007056 was positively correlated with the shoot branching phenotype. Furthermore, an insertion/deletion marker specific to Bra007056 co-segregated with the shoot branching trait in the F₂ populations. Overall, these results provide the basis for elucidating the molecular mechanism of shoot branching in Brassica rapa ssp. chinensis Makino.

Two SLENDER AND CRINKLY LEAF dioxygenases play an essential role in rice shoot development

It is clear that 2-oxoglutarate-dependent dioxygenases have critical functions in salicylic acid (SA) metabolism in plants, yet their role in SA biosynthesis is poorly understood. Here, we report that two dioxygenase-encoding genes, SLENDER AND CRINKLY LEAF1 (SLC1) and SLC2, play essential roles in shoot development and SA production in rice. Overexpression of SLC1 (SLC1-OE) or SLC2 (SLC2-OE) in rice produced infertile plants with slender and crinkly leaves. Disruption of SLC1 or SLC2 led to dwarf plants, while simultaneous down-regulation of SLC1 and SLC2 resulted in a severe defect in early leaf development. Enhanced SA levels in SLC1-OE plants and decreased SA levels in slc1 and slc2 mutants were observed. Accordingly, these lines all showed altered expression of a set of SA-related genes. We demonstrated that SLC1 interacts with homeobox1 (OSH1), and that either the knotted1-like homeobox (KNOX1) or glutamate, leucine, and lysine (ELK) domain of OSH1 is sufficient for accomplishing this interaction. Collectively, our data reveal the importance of SLC1 and SLC2 in rice shoot development.

Integrated Analysis of the Transcriptome and Metabolome Revealed the Molecular Mechanisms Underlying the Enhanced Salt Tolerance of Rice Due to the Application of Exogenous Melatonin

High salinity is one of the major abiotic stresses limiting rice production. Melatonin has been implicated in the salt tolerance of rice. However, the molecular basis of melatonin-mediated salt tolerance in rice remains unclear. In the present study, we performed an integrated transcriptome and metabolome profiling of rice seedlings treated with salt, melatonin, or salt + melatonin. The application of exogenous melatonin increased the salt tolerance of rice plants by decreasing the sodium content to maintain Na⁺/K⁺ homeostasis, alleviating membrane lipid oxidation, and enhancing chlorophyll contention. A comparative transcriptome analysis revealed that complex molecular pathways contribute to melatonin-mediated salt tolerance. More specifically, the AP2/EREBP-HB-WRKY transcriptional cascade and phytohormone (e.g., auxin and abscisic acid) signaling pathways were activated by an exogenous melatonin treatment. On the basis of metabolome profiles, 64 metabolites, such as amino acids, organic acids, nucleotides, and secondary metabolites, were identified with increased abundances only in plants treated with salt + melatonin. Several of these metabolites including endogenous melatonin and its intermediates (5-hydroxy-L-tryptophan, N ¹-acetyl-N ²-formyl-5-methoxykynuramine), gallic acid, diosmetin, and cyanidin 3-O-galactoside had antioxidant functions, suggesting melatonin activates multiple antioxidant pathways to alleviate the detrimental effects of salt stress. Combined transcriptome and metabolome analyses revealed a few gene-metabolite networks related to various pathways, including linoleic acid metabolism and amino acid metabolism that are important for melatonin-mediated salt tolerance. The data presented herein may be useful for further elucidating the multiple regulatory roles of melatonin in plant responses to abiotic stresses.

Antagonistic action of TILLERS ABSENT1 and FLORAL ORGAN NUMBER2 regulates stem cell maintenance during axillary meristem development in rice

Shoot branches are formed from the axillary meristem and their formation is a key process in plant development. Although our understanding of the mechanisms underlying stem cell maintenance in the shoot apical meristem (SAM) is progressing, our knowledge of these mechanisms during the process of axillary meristem development is insufficient. To elucidate the genetic mechanisms underlying axillary meristem development in rice (Oryza sativa), we undertook a molecular genetic analysis focusing on TILLERS ABSENT1 (TAB1) and FLORAL ORGAN NUMBER2 (FON2), respective orthologs of the WUSCHEL and CLAVATA3 genes involved in SAM maintenance in Arabidopsis (Arabidopsis thaliana). We revealed that stem cells were established at an early stage of axillary meristem development in the wild-type, but were not maintained in tab1. By contrast, the stem cell region and TAB1 expression domain were expanded in fon2, and FON2 overexpression inhibited axillary meristem formation. These results indicate that TAB1 is required to maintain stem cells during axillary meristem development, whereas FON2 negatively regulates stem cell fate by restricting TAB1 expression. Thus, the genetic pathway regulating SAM maintenance in Arabidopsis seems to have been recruited to play a specific role within a narrow developmental window - namely, axillary meristem establishment - in rice.

Gibberellic acid inhibition of tillering in tall fescue involving crosstalks with cytokinins and transcriptional regulation of genes controlling axillary bud outgrowth

Tiller production in grass species is controlled by both axillary bud initiation and bud outgrowth, which may be regulated by plant hormones. However, how gibberellic acid (GA) affects tillering in perennial grass species is still unclear. This study aims to elucidate the roles and the underlying mechanisms of GA in regulating tiller development. Tall fescue seedlings were treated with different concentrations of GA3 by foliar application, dose-dependent inhibitory effects of GA on tiller production were observed. GA3 (25 μM) slowed down the transition from axillary buds to tillers by specifically inhibiting the outgrowth of axillary buds. GA-inhibition of tillering were not related to endogenous content for auxin or strigolactone, but was mainly due to the antagonistic interaction with cytokinins (CK), as shown by the decreased CK content and up-regulation expression of CK degradation genes in GA3-treated plants. Furthermore, GA could act through regulating the expression of FaTB1 specifically expressed in axillary buds to repress bud outgrowth. These results provide insights for the regulatory mechanisms of GA for tiller bud outgrowth through crosstalks with CK and signaling of FaTB1 expression.

Transcriptional Corepressor ASP1 and CLV-Like Signaling Regulate Meristem Maintenance in Rice

Stem cell homeostasis is maintained by the WUSCHEL-CLAVATA (WUS-CLV) negative feedback loop in Arabidopsis (Arabidopsis thaliana). In rice (Oryza sativa), FLORAL ORGAN NUMBER2 (FON2) functions in the negative regulation of stem cell proliferation, similar to Arabidopsis CLV3 In this study, through genetic enhancer analysis, we found that loss of function of ABERRANT SPIKELET AND PANICLE1 (ASP1), encoding an Arabidopsis TOPLESS (TPL)-like transcriptional corepressor, enhances the fon2 flower phenotype, which displayed an increase in floral organ number. In the fon2 asp1 double mutant, the inflorescence was severely affected, resulting in bifurcation of the main axis (rachis), a phenotype that has not previously been reported. The stem cells showed marked overproliferation in fon2 asp1, resulting in extreme enlargement and splitting of the inflorescence meristem. These results suggest that ASP1 and FON2 synergistically regulate stem cell maintenance in rice. Unexpectedly, genetic analysis indicated that TILLERS ABSENT1, the rice ortholog of WUS, is not involved in promoting stem cell proliferation in this meristem. Transcriptome analysis suggested that ASP1 and FON signaling negatively regulate a set of genes with similar functions, and they act on these genes in concert. Taken together, our results suggest that TPL-like corepressor activity plays a crucial role in meristem maintenance, and that stem cell proliferation is properly maintained via the cooperation of ASP1 and FON2.

Gene expression and localization of arabinogalactan proteins during the development of anther, ovule, and embryo in rice

Arabinogalactan proteins (AGPs) are hyperglycosylated members of the hydroxyproline-rich glycoprotein (HRGP) superfamily and are widely distributed throughout the plant kingdom. In Oryza sativa (rice), the gene expression and biological function of AGPs only have received minimal research attention. Here, we used qRT-PCR to detect the expression patterns of OsAGP genes in various organs, and found that six genes were preferentially expressed in panicles, three genes were specifically expressed in anthers, and one gene in the stigma. Furthermore, using four specific monoclonal antibodies (JIM8, JIM13, LM2, MAC207), we observed the distribution of AGPs in rice anthers, ovules, and embryos. In anthers, the strong fluorescence signals of AGPs were present in tapetum cells, pollen mother cells, and mature pollens, suggesting that AGPs might be related to the development of anther and pollen. In ovules, signals of AGPs were specifically distributed in the three micropylar megaspores of the tetrad, and with intense signals in the egg cell and synergid cells in the mature embryo sac. This suggests that AGPs may be involved in megaspore determination and double fertilization. In embryos, the immunological signals of AGPs appeared in peripheral and inner cells at the early stage, and in the scutellum, plumule, and radicle at the late stage, indicating that AGPs may be associated with organ differentiation and maturation of embryos. In this study, we revealed that AGPs were widely distributed in rice anthers, ovules, and embryos, which lays a foundation for the functional investigation of AGPs in various processes of sexual reproduction.

Overexpression of a CPYC-Type Glutaredoxin, OsGrxC2.2, Causes Abnormal Embryos and an Increased Grain Weight in Rice

Glutaredoxins (Grxs) are a ubiquitous group of oxidoreductase enzymes that are important in plant growth and development; however, the functions of rice Grxs have not been fully elucidated. In this paper, we showed that one of the Grxs, encoded by OsGrxC2.2, exhibited Grx activity. Furthermore, we demonstrated that OsGrxC2.2 was able to regulate embryo development during embryogenesis. Transgenic rice lines overexpressing OsGrxC2.2 unexpectedly exhibited degenerate embryos as well as embryoless seeds. Our data indicated that the embryonic abnormalities occurred at an early stage during embryogenesis. We found that the expression of several endodermal layer marker genes for embryo development, such as OSH1 (apical region marker), OsSCR (L2 ground tissue marker), and OsPNH1 (L3 vascular tissue marker), were significantly decreased in the OsGrxC2.2-overexpressed transgenic rice lines. In contrast, the transcript levels of the majority of protodermal layer markers, including HAZ1, ROC2, ROC3, and RAmy1A, and the shoot apical meristem marker HB, showed little change between the wild-type (WT) and OsGrxC2.2-overexpressing embryos. Surprisingly, the seed weight of the overexpressed transgenic rice was remarkably increased in comparison to that of the WT. These results indicate that the overexpression of OsGrxC2.2 interferes with the normal embryogenesis of rice embryos and leads to increased grain weight. To the best of our knowledge, this is the first report that OsGrxC2.2 is a rice embryo development-associated gene.

Specification of basal region identity after asymmetric zygotic division requires mitogen-activated protein kinase 6 in rice

Asymmetric cell division is a key step in cellular differentiation in multicellular organisms. In plants, asymmetric zygotic division produces the apical and basal cells. The mitogen-activated protein kinase (MPK) cascade in Arabidopsis acts in asymmetric divisions such as zygotic division and stomatal development, but whether the effect on cellular differentiation of this cascade is direct or indirect following asymmetric division is not clear. Here, we report the analysis of a rice mutant, globular embryo 4 (gle4). In two- and four-cell-stage embryos, asymmetric zygotic division and subsequent cell division patterns were indistinguishable between the wild type and gle4 mutants. However, marker gene expression and transcriptome analyses showed that specification of the basal region was compromised in gle4 We found that GLE4 encodes MPK6 and that GLE4/MPK6 is essential in cellular differentiation rather than in asymmetric zygotic division. Our findings provide a new insight into the role of MPK in plant development. We propose that the regulation of asymmetric zygotic division is separate from the regulation of cellular differentiation that leads to apical-basal polarity.

OsPINOID Regulates Stigma and Ovule Initiation through Maintenance of the Floral Meristem by Auxin Signaling

Stigma and ovule initiation is essential for sexual reproduction in flowering plants. However, the mechanism underlying the initiation of stigma and ovule primordia remains elusive. We identified a stigma-less mutant of rice (Oryza sativa) and revealed that it was caused by the mutation in the PINOID (OsPID) gene. Unlike the pid mutant that shows typical pin-like inflorescences in maize (Zea mays) and Arabidopsis (Arabidopsis thaliana), the ospid mutant does not display any defects in inflorescence development and flower initiation, and fails to develop normal ovules in most spikelets. The auxin activity in the young pistil of ospid was lower than that in the wild-type pistil. Furthermore, the expression of most auxin response factor genes was down-regulated, and OsETTIN1, OsETTIN2, and OsMONOPTEROS lost their rearrangements of expression patterns during pistil and stamen primordia development in ospid Moreover, the transcription of the floral meristem marker gene, OSH1, was down-regulated and FLORAL ORGAN NUMBER4, the putative ortholog of Arabidopsis CLAVATA3, was up-regulated in the pistil primordium of ospid These results suggested that the meristem proliferation in the pistil primordium might be arrested prematurely in ospid Based on these results, we propose that the OsPID-mediated auxin signaling pathway plays a crucial role in the regulation of rice stigma and ovule initiation by maintaining the floral meristem.

Rice Flower Development Revisited: Regulation of Carpel Specification and Flower Meristem Determinacy

The ABC model in flower development represents a milestone of plant developmental studies and is essentially conserved across a wide range of angiosperm species. Despite this overall conservation, individual genes in the ABC model are not necessarily conserved and sometimes play a species-specific role, depending on the plant. We previously reported that carpels are specified by the YABBY gene DROOPING LEAF (DL) in rice (Oryza sativa), which bears flowers that are distinct from those of eudicots. In contrast, another group reported that carpels are specified by two class C genes, OsMADS3 and OsMADS58. Here, we have addressed this controversial issue by phenotypic characterization of floral homeotic gene mutants. Analysis of a complete loss-of-function mutant of OsMADS3 and OsMADS58 revealed that carpel-like organs expressing DL were formed in the absence of the two class C genes. Furthermore, no known flower organs including carpels were specified in a double mutant of DL and SUPERWOMAN1 (a class B gene), which expresses only class C genes in whorls 3 and 4. These results suggest that, in contrast to Arabidopsis, class C genes are not a key regulator for carpel specification in rice. Instead, they seem to be involved in the elaboration of carpel morphology rather than its specification. Our phenotypic analysis also revealed that, similar to its Arabidopsis ortholog CRABS CLAW, DL plays an important function in regulating flower meristem determinacy in addition to carpel specification.

BELL1-like homeobox genes regulate inflorescence architecture and meristem maintenance in rice

Inflorescence architecture is diverse in angiosperms, and is mainly determined by the arrangement of the branches and flowers, known as phyllotaxy. In rice (Oryza sativa), the main inflorescence axis, called the rachis, generates primary branches in a spiral phyllotaxy, and flowers (spikelets) are formed on these branches. Here, we have studied a classical mutant, named verticillate rachis (ri), which produces branches in a partially whorled phyllotaxy. Gene isolation revealed that RI encodes a BELL1-type homeodomain transcription factor, similar to Arabidopsis PENNYWISE/BELLRINGER/REPLUMLESS, and is expressed in the specific regions within the inflorescence and branch meristems where their descendant meristems would soon initiate. Genetic combination of an ri homozygote and a mutant allele of RI-LIKE1 (RIL1) (designated ri ril1/+ plant), a close paralog of RI, enhanced the ri inflorescence phenotype, including the abnormalities in branch phyllotaxy and rachis internode patterning. During early inflorescence development, the timing and arrangement of primary branch meristem (pBM) initiation were disturbed in both ri and ri ril1/+ plants. These findings suggest that RI and RIL1 were involved in regulating the phyllotactic pattern of the pBMs to form normal inflorescences. In addition, both RI and RIL1 seem to be involved in meristem maintenance, because the ri ril1 double-mutant failed to establish or maintain the shoot apical meristem during embryogenesis.

Genetic Regulation of Shoot Architecture

Shoot architecture is determined by the organization and activities of apical, axillary, intercalary, secondary, and inflorescence meristems and by the subsequent development of stems, leaves, shoot branches, and inflorescences. In this review, we discuss the unifying principles of hormonal and genetic control of shoot architecture including advances in our understanding of lateral branch outgrowth; control of stem elongation, thickness, and angle; and regulation of inflorescence development. We focus on recent progress made mainly in Arabidopsis thaliana, rice, pea, maize, and tomato, including the identification of new genes and mechanisms controlling shoot architecture. Key advances include elucidation of mechanisms by which strigolactones, auxins, and genes such as IDEAL PLANT ARCHITECTURE1 and TEOSINTE BRANCHED1 control shoot architecture. Knowledge now available provides a foundation for rational approaches to crop breeding and the generation of ideotypes with defined architectural features to improve performance and productivity.

OsGCD1 is essential for rice fertility and required for embryo dorsal-ventral pattern formation and endosperm development

Rice fertility is critical for rice reproduction and is thus a focus of interest. Most studies have addressed male sterility and its relation to rice production. The mechanisms of regulation of embryogenesis and endosperm development are essential for rice reproduction, but remain largely unknown. Here, we report a functional analysis of the rice gene OsGCD1, which encodes a highly conserved homolog of Arabidopsis GCD1 (GAMETE CELLS DEFECTIVE1). OsGCD1 mutants were generated using the CRISPR/Cas9 system and subjected to functional analysis. The homozygote mutants cannot be obtained, whereas heterozygotes showed altered phenotypes. In the majority of aborted seeds, the endosperm nucleus divided a limited number of times. The free nuclei were distributed only at the micropylar end of embryo sacs, and their oriented positioning was blocked. In addition, aleurone differentiation was interrupted. The embryo developed slowly, and pattern formation, particularly the dorsal-ventral pattern and symmetry establishment, of embryos was disturbed. Thus, the embryos showed various morphological and structural dysplasias. Our findings reveal that OsGCD1 is essential for rice fertility and is required for dorsal-ventral pattern formation and endosperm free nucleus positioning, suggesting a critical role in sexual reproduction of both monocotyledon and dicotyledon plants.

Genetic Enhancer Analysis Reveals that FLORAL ORGAN NUMBER2 and OsMADS3 Co-operatively Regulate Maintenance and Determinacy of the Flower Meristem in Rice

Meristems such as the shoot apical meristem and flower meristem (FM) act as a reservoir of stem cells, which reproduce themselves and supply daughter cells for the differentiation of lateral organs. In Oryza sativa (rice), the FLORAL ORGAN NUMBER2 (FON2) gene, which is similar to Arabidopsis CLAVATA3, is involved in meristem maintenance. In fon2 mutants, the numbers of floral organs are increased due to an enlargement of the FM. To identify new factors regulating meristem maintenance in rice, we performed a genetic screening of mutants that enhanced the fon2 mutation, and found a mutant line (2B-424) in which pistil number was dramatically increased. By using a map-based approach and next-generation sequencing, we found that the line 2B-424 had a complete loss-of-function mutation (a large deletion) in OsMADS3, a class C MADS-box gene that is known to be involved in stamen specification. Disruption of OsMADS3 in the fon2 mutant by CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats-CRISPR-associated protein 9) technology caused a flower phenotype similar to that of 2B-424, confirming that the gene responsible for enhancement of fon2 was OsMADS3. Morphological analysis showed that the fon2 and osmads3 mutations synergistically affected pistil development and FM determinacy. We also found that whorl 3 was duplicated in mature flowers and the FM was enlarged at an early developmental stage in severe osmads3 single mutants. These findings suggest that OsMADS3 is involved not only in FM determinacy in late flower development but also in FM activity in early flower development.

KNOX Protein OSH15 Induces Grain Shattering by Repressing Lignin Biosynthesis Genes

Seed shattering is an agronomically important trait. Two major domestication factors are responsible for this: qSH1 and SH5. Whereas qSH1 functions in cell differentiation in the abscission zone (AZ), a major role of SH5 is the repression of lignin deposition. We have determined that a KNOX protein, OSH15, also controls seed shattering. Knockdown mutations of OSH15 showed reduced seed-shattering phenotypes. Coimmunoprecipitation experiments revealed that OSH15 interacts with SH5 and qSH1, two proteins in the BELL homeobox family. In transgenic plants carrying the OSH15 promoter-GUS reporter construct, the reporter gene was preferentially expressed in the AZ during young spikelet development. The RNA in situ hybridization experiment also showed that OSH15 messenger RNAs were abundant in the AZ during spikelet development. Analyses of osh15 SH5-D double mutants showed that SH5 could not increase the degree of seed shattering when OSH15 was absent, indicating that SH5 functions together with OSH15. In addition to the seed-shattering phenotype, osh15 mutants displayed dwarfism and accumulated a higher amount of lignin in internodes due to increased expression of the genes involved in lignin biosynthesis. Knockout mutations of CAD2, which encodes an enzyme for the last step in the monolignol biosynthesis pathway, caused an easy seed-shattering phenotype by reducing lignin deposition in the AZ This indicated that the lignin level is an important determinant of seed shattering in rice (Oryza sativa). Chromatin immunoprecipitation assays demonstrated that both OSH15 and SH5 interact directly with CAD2 chromatin. We conclude that OSH15 and SH5 form a dimer that enhances seed shattering by directly inhibiting lignin biosynthesis genes.

Interactions between FLORAL ORGAN NUMBER4 and floral homeotic genes in regulating rice flower development

The floral meristem (FM) is self-maintaining at the early stages of flower development, but it is terminated when a fixed number of floral organs are produced. The FLORAL ORGAN NUMBER4 (FON4; also known as FON2) gene, an ortholog of Arabidopsis CLAVATA3 (CLV3), is required for regulating FM size and determinacy in rice. However, its interactions with floral homeotic genes remain unknown. Here, we report the genetic interactions between FON4 and floral homeotic genes OsMADS15 (an A-class gene), OsMADS16 (also called SUPERWOMAN1, SPW1, a B-class gene), OsMADS3 and OsMADS58 (C-class genes), OsMADS13 (a D-class gene), and OsMADS1 (an E-class gene) during flower development. We observed an additive phenotype in the fon4 double mutant with the OsMADS15 mutant allele dep (degenerative palea). The effect on the organ number of whorl 2 was enhanced in fon4 spw1. Double mutant combinations of fon4 with osmads3, osmads58, osmads13, and osmads1 displayed enhanced defects in FM determinacy and identity, respectively, indicating that FON4 and these genes synergistically control FM activity. In addition, the expression patterns of all the genes besides OsMADS13 had no obvious change in the fon4 mutant. This work reveals how the meristem maintenance gene FON4 genetically interacts with C, D, and E floral homeotic genes in specifying FM activity in monocot rice.

OsMPK6 plays a critical role in cell differentiation during early embryogenesis in Oryza sativa

The formation of body axes is the basis of morphogenesis during plant embryogenesis. We identified embryo-lethal mutants of rice (Oryza sativa) in which T-DNAs were inserted in OsMPK6 Embryonic organs were absent because their development was arrested at the globular stage. Similar to observations made with gle4, shootless, and organless, the osmpk6 mutations affected the initial step of cell differentiation. Expression of an apical-basal axis marker gene, OSH1, was reduced in the mutant embryos while that of the radial axes marker genes OsSCR and OsPNH1 was not detected. The signal for ROC1, a protodermal cell marker, was weak at the globular stage and gradually disappeared. Transcript levels of auxin and gibberellin biosynthesis genes were diminished in osmpk6 embryos. In addition, phytoalexin biosynthesis genes were down-regulated in osmpk6 and a major diterpene phytoalexin, momilactone A, did not accumulate in the mutant embryos. These results indicate that OsMPK6 begins to play a critical role during early embryogenesis, especially when the L1 radial axis is being formed.

Genome-wide analysis of spatiotemporal gene expression patterns during early embryogenesis in rice

Embryogenesis in rice is different from that of most dicotolydonous plants in that it shows a non-stereotypic cell division pattern, formation of dorsal-ventral polarity, and endogenous initiation of the radicle. To reveal the transcriptional features associated with developmental events during rice early embryogenesis, we used microarray analysis coupled with laser microdissection to obtain both spatial and temporal transcription profiles. Our results allowed us to determine spatial expression foci for each expressed gene in the globular embryo, which revealed the importance of phytohormone-related genes and a suite of transcription factors to early embryogenesis. Our analysis showed the polarized expression of a small number of genes along the apical-basal and dorsal-ventral axes in the globular embryo, which tended to fluctuate in later developmental stages. We also analyzed gene expression patterns in the early globular embryo and how this relates to expression in embryonic organs at later stages. We confirmed the accuracy of the expression patterns found by microarray analysis of embryo subdomains using in situ hybridization. Our study identified homologous genes from Arabidopsis thaliana with known functions in embryogenesis in addition to unique and uncharacterized genes that show polarized expression patterns during embryogenesis. The results of this study are presented in a database to provide a framework for spatiotemporal gene expression during rice embryogenesis, to serve as a resource for future functional analysis of genes, and as a basis for comparative studies of plant embryogenesis.

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.

Axillary Meristem Formation in Rice Requires the WUSCHEL Ortholog TILLERS ABSENT1

Axillary shoot formation is a key determinant of plant architecture. Formation of the axillary shoot is regulated by initiation of the axillary meristem or outgrowth of the axillary bud. Here, we show that rice (Oryza sativa) TILLERS ABSENT1 (TAB1; also known as Os WUS), an ortholog of Arabidopsis thaliana WUS, is required to initiate axillary meristem development. We found that formation of the axillary meristem in rice proceeds via a transient state, which we term the premeristem, characterized by the expression of OSH1, a marker of indeterminate cells in the shoot apical meristem. In the tab1-1 (wus-1) mutant, however, formation of the axillary meristem is arrested at various stages of the premeristem zone, and OSH1 expression is highly reduced. TAB1/WUS is expressed in the premeristem zone, where it shows a partially overlapping pattern with OSH1. It is likely, therefore, that TAB1 plays an important role in maintaining the premeristem zone and in promoting the formation of the axillary meristem by promoting OSH1 expression. Temporal expression patterns of WUSCHEL-RELATED HOMEOBOX4 (WOX4) indicate that WOX4 is likely to regulate meristem maintenance instead of TAB1 after establishment of the axillary meristem. Lastly, we show that the prophyll, the first leaf in the secondary axis, is formed from the premeristem zone and not from the axillary meristem.

Expression pattern and subcellular localization of the ovate protein family in rice

The Arabidopsis ovate family proteins (AtOFPs) have been shown to function as transcriptional repressors and regulate multiple aspects of plant growth and development. There are 31 genes that encode the full-length OVATE-domain containing proteins in the rice genome. In this study, the gene structure analysis revealed that OsOFPs are intron poor. Phylogenetic analysis suggested that OVATE proteins from rice, Arabidopsis and tomato can be divided into 4 groups (I–IV). Real-time quantitative polymerase chain reaction (RT-qPCR) analysis identified OsOFPs with different tissue-specific expression patterns at all stages of development in the rice plant. Interestingly, nearly half of the total number of OsOFP family was more highly expressed during the seed developmental stage. In addition, seed developmental cis-elements were found in the promoter region of the OsOFPs. Subcellular localization analysis revealed that YFP-OsOFP fusion proteins predominantly localized in the nucleus. Our results suggest that OsOFPs may act as regulatory proteins and play pivotal roles in the growth and development of rice.

Down-regulation of a LBD-like gene, OsIG1, leads to occurrence of unusual double ovules and developmental abnormalities of various floral organs and megagametophyte in rice

The indeterminate gametophyte1 (ig1) mutation was first characterized to modulate female gametophyte development in maize (Zea mays). However, the function of its rice orthologue, OsIG1, remains unknown. For this, we first analysed OsIG1 localization from differential tissues in rice. Real-time quantitative PCR (qRT-PCR) and histochemical staining results demonstrated that the expression signal of OsIG1 was strongly detected in young inflorescence, moderately in mature flower and weakly in leaf. Furthermore, RNA in situ hybridization analyses exhibited that OsIG1 was strongly expressed in inflorescence meristems, floral meristems, empty-glume- and floret- primordia, especially in the primordia of stamens and immature ovules, and the micropylar side of the mature ovary. In OsIG1-RNAi lines, wrinkled blade formation was accompanied by increased leaf inclination angle. Cross-section further showed that the number of bulliform cells located between the vasculatures was significantly increased, indicating that OsIG1 is involved in division and differentiation of bulliform cell and lateral growth during leaf development. OsIG1-RNAi suppression lines showed pleiotropic phenotypes, including degenerated palea, glume-like features and open hull. In addition, a single OsIG1-RNAi floret is characterized by frequently developing double ovules with abnormal embryo sac development. Additionally, down-regulation of OsIG1 differentially affected the expression of genes associated with the floral organ development including EG1, OsMADS6 and OsMADS1. Taken together, these results demonstrate that OsIG1 plays an essential role in the regulation of empty-glume identity, floral organ number control and female gametophyte development in rice.

The BEL1-type homeobox gene SH5 induces seed shattering by enhancing abscission-zone development and inhibiting lignin biosynthesis

Seed shattering is an important trait that influences grain yield. A major controlling quantitative trait locus in rice is qSH1. Although the degree of shattering is correlated with the level of expression of qSH1, some qSH1-defective cultivars display moderate shattering while others show a non-shattering phenotype. Os05 g38120 (SH5) on chromosome 5 is highly homologous to qSH1. Although we detected SH5 transcripts in various organs, this gene was highly expressed at the abscission zone (AZ) in the pedicels. When expression of this gene was suppressed in easy-shattering 'Kasalath', development of the AZ was reduced and thereby so was seed loss. By contrast, the extent of shattering, as well as AZ development, was greatly enhanced in moderate-shattering 'Dongjin' rice when SH5 was overexpressed. Likewise, overexpression of SH5 in the non-shattering 'Ilpum' led to an increase in seed shattering because lignin levels were decreased in the basal region of spikelets in the absence of development of an AZ. We also determined that two shattering-related genes, SHAT1 and Sh4, which are necessary for proper formation of an AZ, were induced by SH5. Based on these observations, we propose that SH5 modulates seed shattering by enhancing AZ development and inhibiting lignin biosynthesis.

The rice WUSCHEL-related homeobox genes are involved in reproductive organ development, hormone signaling and abiotic stress response

The WUSCHEL-related homeobox (WOX) genes are important transcription regulators participated in plant development processes. Rice (Oryza sativa L.) genome encodes at least 13 WOX members. In this study, a systematic microarray-based gene expression profiling of eleven WOX genes was performed for the whole life cycle of rice at 16 different tissues/organs of MH63 (rice indica cultivar), which included eight reproductive organs and eight vegetative tissues. The results demonstrated that four genes (OsWUS, OsNS1/OsNS2, OsWOX3 and OsWOX9A) were specifically expressed in panicle and endosperm development, and six genes (OsWOX5, OsWOX9B, OsWOX9D, OsWOX11, OsWOX12A and OsWOX12B) were preferentially expressed in seeds (72h after imbibitions) during root emergence or growth. In situ hybridization analysis revealed differential transcript levels of OsWOX4, OsWOX5, OsWOX9A and OsWOX12B during panicle development and embryogenesis. Results of qRT-PCR showed that expression of four rice WOX genes (OsWOX5, OsWOX11, OsWOX12B and OsWOX12A) was up- or down-regulated by plant hormones (auxin, cytokinin and gibberellin). More interestingly, most WOX genes were responsive to abiotic stress stimuli of drought, salt and cold. The molecular studies presented here will further provide insight in understanding the functions of rice WOX gene family in rice development, hormone signaling, and abiotic stress response.

An increase in melatonin in transgenic rice causes pleiotropic phenotypes, including enhanced seedling growth, delayed flowering, and low grain yield

No previous reports have described the effects of an increase in endogenous melatonin levels on plant yield and reproduction. Here, the phenotypes of melatonin-rich transgenic rice plants overexpressing sheep serotonin N-acetyltransferase were investigated under field conditions. Early seedling growth of melatonin-rich transgenic rice was greatly accelerated, with enhanced biomass relative to the wild type (WT). However, flowering was delayed by 1 wk in the transgenic lines compared with the WT. Grain yields of the melatonin-rich transgenic lines were reduced by 33% on average. Other phenotypes also varied among the transgenic lines. For example, the transgenic line S1 exhibited greater height and biomass than the WT, while the S10 transgenic line showed diminished height and an increase in panicle numbers per plant. The expression levels of Oryza sativa homeobox1 (OSH1) and TEOSINTE BRANCHED1 (TB1) genes, two key regulators of meristem initiation and maintenance, were not altered in the transgenic lines. These data demonstrate that an alteration of endogenous melatonin levels leads to pleiotropic effects such as height, biomass, panicle number, flowering time, and grain yield, indicating that melatonin behaves as a signaling molecule in plant growth and reproduction.

Control of rice embryo development, shoot apical meristem maintenance, and grain yield by a novel cytochrome p450

Angiosperm seeds usually consist of two major parts: the embryo and the endosperm. However, the molecular mechanism(s) underlying embryo and endosperm development remains largely unknown, particularly in rice, the model cereal. Here, we report the identification and functional characterization of the rice GIANT EMBRYO (GE) gene. Mutation of GE resulted in a large embryo in the seed, which was caused by excessive expansion of scutellum cells. Post-embryonic growth of ge seedling was severely inhibited due to defective shoot apical meristem (SAM) maintenance. Map-based cloning revealed that GE encodes a CYP78A subfamily P450 monooxygenase that is localized to the endoplasmic reticulum. GE is expressed predominantly in the scutellar epithelium, the interface region between embryo and endosperm. Overexpression of GE promoted cell proliferation and enhanced rice plant growth and grain yield, but reduced embryo size, suggesting that GE is critical for coordinating rice embryo and endosperm development. Moreover, transgenic Arabidopsis plants overexpressing AtCYP78A10, a GE homolog, also produced bigger seeds, implying a conserved role for the CYP78A subfamily of P450s in regulating seed development. Taken together, our results indicate that GE plays critical roles in regulating embryo development and SAM maintenance.

An-1 encodes a basic helix-loop-helix protein that regulates awn development, grain size, and grain number in rice

Long awns are important for seed dispersal in wild rice (Oryza rufipogon), but are absent in cultivated rice (Oryza sativa). The genetic mechanism involved in loss-of-awn in cultivated rice remains unknown. We report here the molecular cloning of a major quantitative trait locus, An-1, which regulates long awn formation in O. rufipogon. An-1 encodes a basic helix-loop-helix protein, which regulates cell division. The nearly-isogenic line (NIL-An-1) carrying a wild allele An-1 in the genetic background of the awnless indica Guangluai4 produces long awns and longer grains, but significantly fewer grains per panicle compared with Guangluai4. Transgenic studies confirmed that An-1 positively regulates awn elongation, but negatively regulates grain number per panicle. Genetic variations in the An-1 locus were found to be associated with awn loss in cultivated rice. Population genetic analysis of wild and cultivated rice showed a significant reduction in nucleotide diversity of the An-1 locus in rice cultivars, suggesting that the An-1 locus was a major target for artificial selection. Thus, we propose that awn loss was favored and strongly selected by humans, as genetic variations at the An-1 locus that cause awn loss would increase grain numbers and subsequently improve grain yield in cultivated rice.