Variation in the regulatory region of FZP causes increases in secondary inflorescence branching and grain yield in rice domestication

Realizing the yield potential of Narrow Leaf 1 (NAL1) in rice: The way forward

Yield, a key parameter targeted by breeders to increase rice productivity is a complex trait, governed by source sink interactions and also subject to genotype x environmental effects. Over the last two decades, QTL mapping and map-based cloning have identified several loci and genes related to yield in rice. Among them, a variant of Narrow Leaf 1 (NAL1), a gene conferring pleiotropic effects in rice, has been inadvertently selected during domestication to enhance yield in japonica rice. In this review, we synthesize recent literature on NAL1 in rice, including molecular function, association with auxin transport, associated interactome, regulation at transcriptional and post-transcriptional levels that impact the narrow leaf phenotype. Causes of NAL1 pleiotropic effects are also examined, in addition to trade-offs between yield and photosynthesis conferred by distinct NAL1 variants. Finally, we suggest that the distinct allelic variants of NAL1, leading to partial or full functionality, found in indica and japonica rice backgrounds respectively integrate source-sink interactions to optimize rice yield in a given eco-physiological context. To realize the benefits of the fully functional NAL1 in conferring yield benefits under field conditions, genotype background is crucial and a systems approach is essential to elucidate the causes for such differences. The way forward to enhancing yield in japonica rice (with fully functional NAL1) further by introgression of additional sink and source traits from indica rice is outlined.

Harnessing neo-domestication of wild pigmented rice for enhanced nutrition and sustainable agriculture

Advances in precision gene editing have enabled the rapid domestication of wild crop relatives, a process known as neo-domestication. During domestication, breeding rice for maximum productivity under optimal growth conditions reduced genetic diversity, eliminating variants for stress tolerance and grain nutrients. Wild rice varieties have rich genetic diversity, including variants for disease resistance, stress tolerance, and grain nutritional quality. For example, the grain of pigmented wild rice has abundant antioxidants (anthocyanins, proanthocyanidins, and flavonoids), but low yield, poor plant architecture, and long life cycle limit its cultivation. In this review, we address the neo-domestication of wild pigmented rice, focusing on recent progress, CRISPR-Cas editing toolboxes, selection of key candidate genes for domestication, identifying species with superior potential via generating genomic and multi-omics resources, efficient crop transformation methods and highlight strategies for the promotion and application pigmented rice. We also address critical outstanding questions and potential solutions to enable efficient neo-domestication of wild pigmented rice and thus enhance food security and nutrition.

Reverse engineering high-level resistance to Bt Cry1Ac toxin in Plutella xylostella reveals a hormonal regulatory feedback pathway

Decoding the molecular mechanisms of insect resistance to Bacillus thuringiensis (Bt) toxins is crucial for the sustainable utilization of Bt-based bioinsecticides and transgenic crops. Our previous studies showed that a hormone-responsive transcription factor FOXO binds to an inserted short interspersed nuclear element (SINE, named SE2), causing MAP4K4 overexpression and resistance to Bt Cry1Ac toxin in Plutella xylostella. Furthermore, titers of two upstream signaling hormones (20-hydroxyecdysone and juvenile hormone) were also found to be elevated in the resistant strain, but it was unclear whether this was due to natural variation or a feedback pathway. Here, we established a homozygous knock-in strain (SE2-KI) using a reverse genetic approach to insert the SE2 retrotransposon into the MAP4K4 promoter of a Cry1Ac-susceptible strain. The SE2 insertion induced MAP4K4 overexpression, which in turn caused a downregulation of midgut receptors and an identical resistance phenotype to that seen in the evolved resistant strain. Moreover, SE2 insertion significantly increased the levels of two insect hormones providing definitive evidence for a positive feedback regulatory pathway. This study unveils an as yet uncharacterized hormonal regulatory feedback pathway orchestrating Cry1Ac resistance in P. xylostella, providing new insights into the molecular basis of Bt resistance and informing suitable field resistance management strategies.

A pangenome reference of wild and cultivated rice

Oryza rufipogon, the wild progenitor of Asian cultivated rice Oryza sativa, is an important resource for rice breeding1. Here we present a wild-cultivated rice pangenome based on 145 chromosome-level assemblies, comprising 129 genetically diverse O. rufipogon accessions and 16 diverse varieties of O. sativa. This pangenome contains 3.87 Gb of sequences that are absent from the O. sativa ssp. japonica cv. Nipponbare reference genome. We captured alternate assemblies that include heterozygous information missing in the primary assemblies, and identified a total of 69,531 pan-genes, with 28,907 core genes and 13,728 wild-rice-specific genes. We observed a higher abundance and a significantly greater diversity of resistance-gene analogues in wild rice than in cultivars. Our analysis indicates that two cultivated subpopulations, intro-indica and basmati, were generated through gene flows among cultivars in South Asia. We also provide strong evidence to support the theory that the initial domestication of all Asian cultivated rice occurred only once. Furthermore, we captured 855,122 differentiated single-nucleotide polymorphisms and 13,853 differentiated presence-absence variations between indica and japonica, which could be traced to the divergence of their respective ancestors and the existence of a larger genetic bottleneck in japonica. This study provides reference resources for enhancing rice breeding, and enriches our understanding of the origins and domestication process of rice.

NAL1 forms a molecular cage to regulate FZP phase separation

NARROW LEAF 1 (NAL1), originally identified for its role in shaping leaf morphology, plant architecture, and various agronomic traits in rice, has remained enigmatic in terms of the molecular mechanisms governing its multifaceted functions. In this study, we present a comprehensive structural analysis of NAL1 proteins, shedding light on how NAL1 regulates the phase separation of its physiological substrate, FRIZZY PANICLE (FZP), a transcription factor. We determined that NAL1 assembles as a hexamer and forms a molecular cage with a wide central channel and three narrower lateral channels, which could discriminate its different substrates into the catalytic sites. Most notably, our investigation unveils that FZP readily forms molecular condensates via phase separation both in vitro and in vivo. NAL1 fine-tunes FZP condensation, maintaining optimal concentrations to enhance transcriptional activity. While phase separation roles include sequestration and suppression of transcriptional or enzymatic activity, our study highlights its context-dependent contribution to transcriptional regulation. NAL1 assumes a pivotal role in regulating the states of these molecular condensates through its proteolytic activity, subsequently enhancing transcriptional cascades. Our findings offer insights into comprehending the molecular mechanisms underpinning NAL1's diverse functions, with far-reaching implications for the field of plant biology. Additionally, these insights provide valuable guidance for the development of rational breeding strategies aimed at enhancing crop productivity.

FZP modulates tillering via OsMADS57 in rice

The number of tillers in rice directly determines the number of panicles, which is crucial for enhancing plant architecture and achieving high yields. Some important genes regulating rice tillering have been identified, but their underlying mechanisms remain unclear. FRIZZY PANICLE (FZP) encodes an AP2/ERF transcription factor. Beyond its well-established role in promoting spikelet formation during the reproductive phase, here we demonstrate that FZP also inhibits axillary buds outgrowth in the vegetative phase by suppressing the expression of a MADS-box gene (OsMADS57) that functions as a growth promoter. Consequently, genome editing of the FZP-bound cis-motif in the promoter of OsMADS57 releases its expression, leading to more tillers. Furthermore, domestication analysis shows that FZP has undergone strong selection in cultivated rice, while the downstream gene OsMADS57 has been differentiated between indica and japonica subspecies. Four functional SNPs in the promoter of OsMADS57 can increase rice tillering in most indica accessions by enhancing its expression. Our findings expose hidden pleiotropy of classic spikelet identity genes that are redeployed to control stem form, potentially enriching the gene resources for rice genetic improvement.

Cis-regulatory dynamics in plant domestication

Cis-regulatory elements (CREs) are critical sequence determinants for spatiotemporal control of gene expression. Genetic variants within CREs have driven phenotypic transitions from wild to cultivated plants during domestication. This review summarizes our current understanding of genetic variants within CREs involved in plant domestication. We also propose avenues for studies to expand our understanding of both CRE biology and domestication processes, such as examining primary mechanisms that generate CRE genetic variants during plant domestication and investigating the roles of CREs in domestication syndrome. Additionally, we discuss existing challenges and highlight future opportunities for exploring CREs in plant domestication, emphasizing the potential of modifying CREs to contribute to crop improvement.

Identification of quantitative trait loci for yield traits and fine-mapping of qGW4 using the chromosome segment substitution line-Z708 and dissected single-segment substitution lines

Identifying quantitative trait loci (QTL) for yield traits using single-segment substitution lines (SSSL) is essential for both targeted breeding and functional analysis of key genes. Here, a wide-grain rice chromosome segment substitution line (CSSL), Z708, carrying four substitution segments from Jinhui35 in the genetic background of Xihui18, was used to identify the QTL associated with grain size. Seven QTL for yield-related traits (qGW4, qRLW4, qGWT4, qGW5, qRLW5, qGWT5, and qGPP5) were identified on the substitution segments of the fourth and fifth chromosomes of Z708. Subsequently, four SSSLs (S1-S4), which harbored 16 QTL for yield traits, were constructed using molecular marker-assisted selection. These lines (S1-S4) exhibited a significant increase in yield per plant compared to that of Xihui18. Among them, qGW4, which controls wide grains, belongs to a single dominant gene action in S1 based on the frequency distribution of grain width and chi-square test analysis. Finally, qGW4 was fine-mapped to the interval of 80-kb (minimum) and 310-kb (maximum) using both traditional fine mapping and overlapping substitution mapping of the newly constructed secondary SSSLs (S5-S8). Within this interval, four previously unreported candidate genes were predicted.

The SUMO-conjugating enzyme OsSCE1a from wild rice regulates the functional stay-green trait in rice

The functional stay-green trait is a major goal of rice breeding. Here, we cloned OsSCE1a encoding SUMO-conjugating enzyme from Yuanjiang common wild rice, which simultaneously regulates the functional stay-green trait and growth duration. Low expression or knocking out OsSCE1a corresponded to increased chlorophyll content, photosynthetic competence, N use efficiency and a shortened growth period without affecting yield. A natural MITE-transposon insertion/deletion in the OsSCE1a promoter is the functional variation that regulates these traits. OsSCE1a was selected during evolution and shows significant variation between indica and japonica rice. OsNAC2 interacts with the MITE to enhance OsSCE1a expression. Genetic manipulation of OsSCE1a revealed its potential for rice improvement. OsSCE1a-mediated SUMOylation of OsGS2 suppresses GS (involved in N assimilation) enzyme activity. OsSCE1a also regulates growth duration by SUMOylating the transcription factor such as OsGBP1, which regulates the expression of the key heading gene Ghd7. Our findings shed light on the role of SUMOylation in crops and provide a strategy for increasing agricultural productivity.

Regulation of tillering and panicle branching in rice and wheat

Branching is a critical aspect of plant architecture that significantly impacts the yield and adaptability of staple cereal crops like rice and wheat. Cereal crops develop tillers during the vegetative stage and panicle or spike branches during the reproductive stage, respectively, both of which are significantly impacted by hormones and genetic factors. Tillering and panicle branching are closely interconnected and exhibit high environmental plasticity. Here, we summarize the recent progress in genetic, hormonal, and environmental factors regulation in the branching of rice and wheat. This review not only provides a comprehensive overview of the current knowledge on branching mechanisms in rice and wheat, but also explores the prospects for future research aimed at optimizing crop architecture for enhanced productivity.

LATA1, a RING E3 ligase, modulates the tiller angle by affecting auxin asymmetric distribution and content in rice

The tiller angle is an important agronomic trait that determines plant architecture and grain yield in rice (Oryza sativa L.). However, the molecular regulation mechanism of the rice tiller angle remains unclear. Here, we identified a rice tiller angle gene, LARGE TILLER ANGLE 1 (LATA1), using the MutMap approach. LATA1 encodes a C3H2C3-type RING zinc finger E3 ligase and the conserved region of the RING zinc finger is essential for its E3 activity. LATA1 was highly expressed in the root and tiller base and LATA1-GFP fusion protein was specifically localized to the nucleus. The mutation of LATA1 significantly reduced indole-3-acetic acid content and attenuated lateral auxin transport, thereby resulting in defective shoot gravitropism and spreading plant architecture in rice. Further investigations found that LATA1 may indirectly affect gravity perception by modulating the sedimentation rate of gravity-sensing amyloplasts upon gravistimulation. Our findings provide new insights into the molecular mechanism underlying the rice tiller angle and new genetic resource for the improvement of plant architecture in rice.

The pan-tandem repeat map highlights multiallelic variants underlying gene expression and agronomic traits in rice

Tandem repeats (TRs) are genomic regions that tandemly change in repeat number, which are often multiallelic. Their characteristics and contributions to gene expression and quantitative traits in rice are largely unknown. Here, we survey rice TR variations based on 231 genome assemblies and the rice pan-genome graph. We identify 227,391 multiallelic TR loci, including 54,416 TR variations that are absent from the Nipponbare reference genome. Only 1/3 TR variations show strong linkage with nearby bi-allelic variants (SNPs, Indels and PAVs). Using 193 panicle and 202 leaf transcriptomic data, we reveal 485 and 511 TRs act as QTLs independently of other bi-allelic variations to nearby gene expression, respectively. Using plant height and grain width as examples, we identify and validate TRs contributions to rice agronomic trait variations. These findings would enhance our understanding of the functions of multiallelic variants and facilitate rice molecular breeding.

The Birth and Death of Floral Organs in Cereal Crops

Florets of cereal crops are the basic reproductive organs that produce grains for food or feed. The birth of a floret progresses through meristem initiation and floral organ identity specification and maintenance. During these processes, both endogenous and external cues can trigger a premature floral organ death, leading to reproductive failure. Recent advances in different cereal crops have identified both conserved and distinct regulators governing the birth of a floret. However, the molecular underpinnings of floral death are just beginning to be understood. In this review, we first provide a general overview of the current findings in the field of floral development in major cereals and outline different forms of floral deaths, particularly in the Triticeae crops. We then highlight the importance of vascular patterning and photosynthesis in floral development and reproductive success and argue for an expanded knowledge of floral birth-death balance in the context of agroecology.

Identification of genomic regions controlling spikelet degeneration under FRIZZLE PANICLE (FZP) defect genetic background in rice

The FZP gene plays a critical role in the formation of lateral branches and spikelets in rice panicle architecture. This study investigates the qSBN7 allele, a hypomorphic variant of FZP, and its influence on panicle architectures in different genetic backgrounds. We evaluated two backcross inbred lines (BILs), BC5_TCS10sbn and BC3_TCS10sbn, each possessing the homozygous qSBN7 allele but demonstrating differing degrees of spikelet degeneration. Our analysis revealed that BC5_TCS10sbn had markedly low FZP expression, which corresponded with an increase in axillary branches and severe spikelet degeneration. Conversely, BC3_TCS10sbn exhibited significantly elevated FZP expression, leading to fewer secondary and tertiary branches, and consequently decreased spikelet degeneration. Compared to BC5_TCS10sbn, BC3_TCS10sbn carries three additional chromosomal substitution segments from its donor parent, IR65598-112-2. All three segments significantly enhance the expression of FZP and reduce the occurrence of tertiary branch and spikelet degeneration. These findings enhance our understanding of the mechanisms regulating FZP and aid rice breeding efforts.

The catalytic triad of rice NARROW LEAF1 involves H234

NARROW LEAF1 (NAL1) exerts a multifaceted influence on leaf morphology and crop yield. Recent crystal study proposed that histidine 233 (H233) is part of the catalytic triad. Here we report that unlike suggested previously, H234 instead of H233 is a component of the catalytic triad alongside residues D291 and S385 in NAL1. Remarkably, residue 233 unexpectedly plays a pivotal role in regulating NAL1's proteolytic activity. These findings establish a strong foundation for utilizing NAL1 in breeding programs aimed at improving crop yield.

Identification and Transcriptome Analysis of a Novel Allelic Mutant of NAL1 in Rice

Leaf morphology is a crucial aspect of plant architecture, yet the molecular mechanisms underlying leaf development remain incompletely understood. In this study, a narrow leaf mutant, m625, was identified in rice (Oryza sativa L.), exhibiting pleiotropic developmental defects. Pigment measurement revealed reduced levels of photochromic pigments in m625. Cytological analysis demonstrated that the m625 gene affected vascular patterns and cell division. Specifically, the narrowing of the leaf was attributed to a decrease in small vein number, shorter vein spacing, and an abnormal V-shaped arrangement of bulliform cells, while the thickening was caused by longer leaf veins, thicker mesophyll cells, and an increased number of parenchyma cell layers. The dwarf stature and thickened internode were primarily due to shortened internodes and an increase in cell layers, respectively. Positional cloning and complementation assays indicated that the m625 gene is a novel allele of NAL1. In the m625 mutant, a nucleotide deletion at position 1103 in the coding sequence of NAL1 led to premature termination of protein translation. Further RNA-Seq and qRT-PCR analyses revealed that the m625 gene significantly impacted regulatory pathways related to IAA and ABA signal transduction, photosynthesis, and lignin biosynthesis. Moreover, the m625 mutant displayed thinner sclerenchyma and cell walls in both the leaf and stem, particularly showing reduced lignified cell walls in the midrib of the leaf. In conclusion, our study suggests that NAL1, in addition to its known roles in IAA transport and leaf photosynthesis, may also participate in ABA signal transduction, as well as regulate secondary cell wall formation and sclerenchyma thickness through lignification.

Boosting Triticeae crop grain yield by manipulating molecular modules to regulate inflorescence architecture: insights and knowledge from other cereal crops

One of the challenges for global food security is to reliably and sustainably improve the grain yield of cereal crops. One solution is to modify the architecture of the grain-bearing inflorescence to optimize for grain number and size. Cereal inflorescences are complex structures, with determinacy, branching patterns, and spikelet/floret growth patterns that vary by species. Recent decades have witnessed rapid advancements in our understanding of the genetic regulation of inflorescence architecture in rice, maize, wheat, and barley. Here, we summarize current knowledge on key genetic factors underlying the different inflorescence morphologies of these crops and model plants (Arabidopsis and tomato), focusing particularly on the regulation of inflorescence meristem determinacy and spikelet meristem identity and determinacy. We also discuss strategies to identify and utilize these superior alleles to optimize inflorescence architecture and, ultimately, improve crop grain yield.

Cultivating potential: Harnessing plant stem cells for agricultural crop improvement

Meristems are stem cell-containing structures that produce all plant organs and are therefore important targets for crop improvement. Developmental regulators control the balance and rate of cell divisions within the meristem. Altering these regulators impacts meristem architecture and, as a consequence, plant form. In this review, we discuss genes involved in regulating the shoot apical meristem, inflorescence meristem, axillary meristem, root apical meristem, and vascular cambium in plants. We highlight several examples showing how crop breeders have manipulated developmental regulators to modify meristem growth and alter crop traits such as inflorescence size and branching patterns. Plant transformation techniques are another innovation related to plant meristem research because they make crop genome engineering possible. We discuss recent advances on plant transformation made possible by studying genes controlling meristem development. Finally, we conclude with discussions about how meristem research can contribute to crop improvement in the coming decades.

FRIZZLE PANICLE (FZP) regulates rice spikelets development through modulating cytokinin metabolism

BACKGROUND

The number of grains per panicle is an important factor in determining rice yield. The DST-OsCKX2 module has been demonstrated to regulate panicle development in rice by controlling cytokinin content. However, to date, how the function of DST-OsCKX2 module is regulated during panicle development remains obscure.

RESULT

In this study, the ABNORMAL PANICLE 1 (ABP1), a severely allele of FRIZZY PANICLE (FZP), exhibits abnormal spikelets morphology. We show that FZP can repress the expression of DST via directly binding to its promotor. Consistently, the expression level of OsCKX2 increased and the cytokinin content decreased in the fzp mutant, suggesting that the FZP acts upstream of the DST-OsCKX2 to maintain cytokinin homeostasis in the inflorescence meristem.

CONCLUSIONS

Our results indicate that FZP plays an important role in regulating spikelet development and grain number through mediating cytokinin metabolism.

Genomic introgressions from African rice (Oryza glaberrima) in Asian rice (O. sativa) lead to the identification of key QTLs for panicle architecture

BACKGROUND

Developing high yielding varieties is a major challenge for breeders tackling the challenges of climate change in agriculture. The panicle (inflorescence) architecture of rice is one of the key components of yield potential and displays high inter- and intra-specific variability. The genus Oryza features two different crop species: Asian rice (Oryza sativa L.) and the African rice (O. glaberrima Steud.). One of the main morphological differences between the two independently domesticated species is the structure (or complexity) of the panicle, with O. sativa displaying a highly branched panicle, which in turn produces a larger number of grains than that of O. glaberrima. The gene regulatory network that governs intra- and interspecific panicle diversity is still under-studied.

RESULTS

To identify genetic factors linked to panicle architecture diversity in the two species, we used a set of 60 Chromosome Segment Substitution Lines (CSSLs) issued from third generation backcross (BC3DH) and carrying genomic segments from O. glaberrima cv. MG12 in the genetic background of O. sativa Tropical Japonica cv. Caiapó. Phenotypic data were collected for rachis and primary branch length, primary, secondary and tertiary branch number and spikelet number. A total of 15 QTLs were localized on chromosomes 1, 2, 3, 7, 11 and 12, QTLs associated with enhanced secondary and tertiary branch numbers were detected in two CSSLs. Furthermore, BC4F3:5 lines carrying different combinations of substituted segments were produced to decipher the effects of the identified QTL regions on variations in panicle architecture. A detailed analysis of phenotypes versus genotypes was carried out between the two parental genomes within these regions in order to understand how O. glaberrima introgression events may lead to alterations in panicle traits.

CONCLUSION

Our analysis led to the detection of genomic variations between O. sativa cv. Caiapó and O. glaberrima cv. MG12 in regions associated with enhanced panicle traits in specific CSSLs. These regions contain a number of key genes that regulate panicle development in O. sativa and their interspecific genomic variations may explain the phenotypic effects observed.

Rice domestication-associated transcription factor PROSTRATE GROWTH 1 controls plant and panicle architecture by regulating the expression of LAZY 1 and OsGIGANTEA, respectively

Plant architecture and panicle architecture are two critical agronomic traits that greatly affect the yield of rice (Oryza sativa). PROSTRATE GROWTH 1 (PROG1) encodes a key C2H2-type zinc-finger transcription factor and has pleiotropic effects on the regulation of both plant and panicle architecture, thereby influencing the grain yield. However, the molecular mechanisms through which PROG1 controls plant and panicle architecture remain unclear. In this study, we showed that PROG1 directly binds the LAZY 1 (LA1) promoter and acts as a repressor to inhibit LA1 expression. Conversely, LA1 acts as a repressor of PROG1 by directly binding to the PROG1 promoter. These two genes play antagonistic roles in shaping plant architecture by regulating both tiller angle and tiller number. Interestingly, our data showed that PROG1 controls panicle architecture through direct binding to the intragenic regulatory regions of OsGIGANTEA (OsGI) and subsequent activation of its expression. Collectively, we have identified two crucial targets of PROG1, LA1 and OsGI, shedding light on the molecular mechanisms underlying plant and panicle architecture control by PROG1. Our study provides valuable insights into the regulation of key domestication-related traits in rice and identifies potential targets for future high-yield rice breeding.

Green revolution to genome revolution: driving better resilient crops against environmental instability

Crop improvement programmes began with traditional breeding practices since the inception of agriculture. Farmers and plant breeders continue to use these strategies for crop improvement due to their broad application in modifying crop genetic compositions. Nonetheless, conventional breeding has significant downsides in regard to effort and time. Crop productivity seems to be hitting a plateau as a consequence of environmental issues and the scarcity of agricultural land. Therefore, continuous pursuit of advancement in crop improvement is essential. Recent technical innovations have resulted in a revolutionary shift in the pattern of breeding methods, leaning further towards molecular approaches. Among the promising approaches, marker-assisted selection, QTL mapping, omics-assisted breeding, genome-wide association studies and genome editing have lately gained prominence. Several governments have progressively relaxed their restrictions relating to genome editing. The present review highlights the evolutionary and revolutionary approaches that have been utilized for crop improvement in a bid to produce climate-resilient crops observing the consequence of climate change. Additionally, it will contribute to the comprehension of plant breeding succession so far. Investing in advanced sequencing technologies and bioinformatics will deepen our understanding of genetic variations and their functional implications, contributing to breakthroughs in crop improvement and biodiversity conservation.

Serine protease NAL1 exerts pleiotropic functions through degradation of TOPLESS-related corepressor in rice

NARROW LEAF 1 (NAL1) is a breeding-valuable pleiotropic gene that affects multiple agronomic traits in rice, although the molecular mechanism is largely unclear. Here, we report that NAL1 is a serine protease and displays a novel hexameric structure consisting of two ATP-mediated doughnut-shaped trimeric complexes. Moreover, we identified TOPLESS-related corepressor OsTPR2 involved in multiple growth and development processes as the substrate of NAL1. We found that NAL1 degraded OsTPR2, thus modulating the expression of downstream genes related to hormone signalling pathways, eventually achieving its pleiotropic physiological function. An elite allele, NAL1A, which may have originated from wild rice, could increase grain yield. Furthermore, the NAL1 homologues in different crops have a similar pleiotropic function to NAL1. Our study uncovers a NAL1-OsTPR2 regulatory module and provides gene resources for the design of high-yield crops.

Advances in cloning functional genes for rice yield traits and molecular design breeding in China

Rice, a major food crop in China, contributes significantly to international food stability. Advances in rice genome sequencing, bioinformatics, and transgenic techniques have catalyzed Chinese researchers' discovery of novel genes that control rice yield. These breakthroughs in research also encompass the analysis of genetic regulatory networks and the establishment of a new framework for molecular design breeding, leading to numerous transformative findings in this field. In this review, some breakthroughs in rice yield traits and a series of achievements in molecular design breeding in China in recent years are presented; the identification and cloning of functional genes related to yield traits and the development of molecular markers of rice functional genes are summarized, with the intention of playing a reference role in the following molecular design breeding work and how to further improve rice yield.

OsMADS17 simultaneously increases grain number and grain weight in rice

During the processes of rice domestication and improvement, a trade-off effect between grain number and grain weight was a major obstacle for increasing yield. Here, we identify a critical gene COG1, encoding the transcription factor OsMADS17, with a 65-bp deletion in the 5' untranslated region (5' UTR) presented in cultivated rice increasing grain number and grain weight simultaneously through decreasing mRNA translation efficiency. OsMADS17 controls grain yield by regulating multiple genes and that the interaction with one of them, OsAP2-39, has been characterized. Besides, the expression of OsMADS17 is regulated by OsMADS1 directly. It indicates that OsMADS1-OsMADS17-OsAP2-39 participates in the regulatory network controlling grain yield, and downregulation of OsMADS17 or OsAP2-39 expression can further improve grain yield by simultaneously increasing grain number and grain weight. Our findings provide insights into understanding the molecular basis co-regulating rice yield-related traits, and offer a strategy for breeding higher-yielding rice varieties.

cis-Regulatory Elements in Plant Development, Adaptation, and Evolution

cis-Regulatory elements encode the genomic blueprints that ensure the proper spatiotemporal patterning of gene expression necessary for appropriate development and responses to the environment. Accumulating evidence implicates changes to gene expression as a major source of phenotypic novelty in eukaryotes, including acute phenotypes such as disease and cancer in mammals. Moreover, genetic and epigenetic variation affecting cis-regulatory sequences over longer evolutionary timescales has become a recurring theme in studies of morphological divergence and local adaptation. Here, we discuss the functions of and methods used to identify various classes of cis-regulatory elements, as well as their role in plant development and response to the environment. We highlight opportunities to exploit cis-regulatory variants underlying plant development and environmental responses for crop improvement efforts. Although a comprehensive understanding of cis-regulatory mechanisms in plants has lagged behind that in animals, we showcase several breakthrough findings that have profoundly influenced plant biology and shaped the overall understanding of transcriptional regulation in eukaryotes.

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.

Natural variation of RGN1a regulates grain number per panicle in japonica rice

The grain number per panicle (GNP) is an important yield component. Identifying naturally favorable variations in GNP will benefit high-yield rice breeding. Here, we performed a genome-wide association study using a mini-core collection of 266 cultivated rice accessions with deep sequencing data and investigated the phenotype for three years. Three genes, i.e., TOTOU1 (TUT1), Grain height date 7 (Ghd7), and Days to heading 7/Grain height date 7.1/Pseudo-Response Regulator37 (DTH7/Ghd7.1/OsPRR37), which regulate GNP, were found in the quantitative trait loci (QTL) identified in this study. A stable QTL, qGNP1.3, which showed a strong correlation with variations in GNP, was repeatedly detected. After functional and transgenic phenotype analysis, we identified a novel gene, regulator of grain number 1a (RGN1a), which codes for protein kinase, controlling GNP in rice. The RGN1a mutation caused 37.2%, 27.8%, 51.2%, and 25.5% decreases in grain number, primary branch number per panicle, secondary branch number per panicle, and panicle length, respectively. Furthermore, breeding utilization analysis revealed that the additive effects of the dominant allelic variants of RGN1a and DTH7 played a significant role in increasing the grain number per panicle in japonica rice. Our findings enrich the gene pool and provide an effective strategy for the genetic improvement of grain numbers.

Identification of quantitative trait loci for important agronomic traits using chromosome segment substitution lines from a japonica × indica cross in rice

Asian cultivated rice (Oryza sativa L.) has two subspecies, indica and japonica, which display clear differences in yield-related traits and environmental adaptation. Here, we developed a set of chromosome segment substitution lines (CSSLs) from an advanced backcross between japonica variety C418, as the recipient, and indica variety IR24, as the donor. Through evaluating the genotypes and phenotypes of 181 CSSLs, a total of 85 quantitative trait loci (QTLs) for 14 yield-related traits were detected, with individual QTLs explaining from 6.2 to 42.9% of the phenotypic variation. Moreover, twenty-six of these QTLs could be detected in the two trial sites (Beijing and Hainan). Among these loci, the QTLs for flag leaf width and effective tiller number, qFLW4.2 and qETN4.2, were delimited to an approximately 256-kb interval on chromosome 4. Through a comparison of nucleotide sequences and expression levels in "C418" and the CSSL CR31 containing qFLW4.2 and qETN4.2, we found that the NAL1 (LOC_Os04g52479) gene was the candidate gene for qFLW4.2 and qETN4.2. Our results show that CSSLs are powerful tools for identifying and fine-mapping QTLs, while the novel QTLs identified in this study will also provide new genetic resources for rice improvement.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01343-3.

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.

Revisiting the origin and identity specification of the spikelet: A structural innovation in grasses (Poaceae)

Spikelets are highly specialized and short-lived branches and function as a constitutional unit of the complex grass inflorescences. A series of genetic, genomic, and developmental studies across different clades of the family have called for and permitted a synthesis on the regulation and evolution of spikelets, and hence inflorescence diversity. Here, we have revisited the identity specification of a spikelet, focusing on the diagnostic features of a spikelet from morphological, developmental, and molecular perspectives. Particularly, recent studies on a collection of barley (Hordeum vulgare L.), wheat (Triticum spp.), and rice (Oryza sativa L.) mutants have highlighted a set of transcription factors that are important in the control of spikelet identity and the patterning of floral parts of a spikelet. In addition, we have endeavored to clarify some puzzling issues on the (in)determinacy and modifications of spikelets over the course of evolution. Meanwhile, genomes of two sister taxa of the remaining grass species have again demonstrated the importance of genome duplication and subsequent gene losses on the evolution of spikelets. Accordingly, we argue that changes in the orthologs of spikelet-related genes could be critical for the development and evolution of the spikelet, an evolutionary innovation in the grass family. Likewise, the conceptual discussions on the regulation of a fundamental unit of compound inflorescences could be translated into other organismal groups where compound structures are similarly formed, permitting a comparative perspective on the control of biological complexity.

Panicle Apical Abortion 7 Regulates Panicle Development in Rice (Oryza sativa L.)

The number of grains per panicle significantly contributes to rice yield, but the regulatory mechanism remains largely unknown. Here, we reported a loss-of-function mutant, panicle apical abortion 7 (paa7), which exhibited panicle abortion and degeneration of spikelets on the apical panicles during the late stage of young panicle development in rice. High accumulations of H2O2 in paa7 caused programmed cell death (PCD) accompanied by nuclear DNA fragmentation in the apical spikelets. Map-based cloning revealed that the 3 bp "AGC" insertion and 4 bp "TCTC" deletion mutation of paa7 were located in the 3'-UTR regions of LOC_Os07g47330, which was confirmed through complementary assays and overexpressed lines. Interestingly, LOC_Os07g47330 is known as FRIZZY PANICLE (FZP). Thus, PAA7 could be a novel allele of FZP. Moreover, the severe damage for panicle phenotype in paa7/lax2 double mutant indicated that PAA7 could crosstalk with Lax Panicle 2 (LAX2). These findings suggest that PAA7 regulates the development of apical spikelets and interacts with LAX2 to regulate panicle development in rice.

Molecular, cellular, and developmental foundations of grass diversity

Humans have cultivated grasses for food, feed, beverages, and construction materials for millennia. Grasses also dominate the landscape in vast parts of the world, where they have adapted morphologically and physiologically, diversifying to form ~12,000 species. Sequences of hundreds of grass genomes show that they are essentially collinear; nonetheless, not all species have the same complement of genes. Here, we focus on the molecular, cellular, and developmental bases of grain yield and dispersal-traits that are essential for domestication. Distinct genes, networks, and pathways were selected in different crop species, reflecting underlying genomic diversity. With increasing genomic resources becoming available in nondomesticated species, we anticipate advances in coming years that illuminate the ecological and economic success of the grasses.

Genetic and molecular factors in determining grain number per panicle of rice

It was suggested that the most effective way to improve rice grain yield is to increase the grain number per panicle (GN) through the breeding practice in recent decades. GN is a representative quantitative trait affected by multiple genetic and environmental factors. Understanding the mechanisms controlling GN has become an important research field in rice biotechnology and breeding. The regulation of rice GN is coordinately controlled by panicle architecture and branch differentiation, and many GN-associated genes showed pleiotropic effect in regulating tillering, grain size, flowering time, and other domestication-related traits. It is also revealed that GN determination is closely related to vascular development and the metabolism of some phytohormones. In this review, we summarize the recent findings in rice GN determination and discuss the genetic and molecular mechanisms of GN regulators.

Genetic basis underlying tiller angle in rice (Oryza sativa L.) by genome-wide association study

Novel alleles of two reported tiller angle genes and eleven candidate genes for rice tiller angle were identified by combining GWAS with transcriptomic, qRT-PCR and haplotype analysis. Rice tiller angle is a key agronomic trait determining rice grain yield. Several quantitative trait loci (QTLs) affecting rice tiller angle have been mapped in the past decades. Little is known about the genetic base of tiller angle in rice, because rice tiller angle is a complex polygenic trait. In this study, we performed genome-wide association study (GWAS) on tiller angle in rice using a population of 164 japonica varieties derived from the 3 K Rice Genomes Project (3 K RGP). We detected a total of 18 QTLs using 1135519 single-nucleotide polymorphisms (SNP) based on three GWAS models (GLM, FastLMM and FarmCPU). Among them, two identified QTLs, qTA8.3 and qTA8.4, overlapped with PAY1 and TIG1, respectively, and additional 16 QTLs were identified for the first time. Combined with haplotype and expression analyses, we further revealed that PAY1 harbors one non-synonymous variation at its coding region, likely leading to variable tiller angle in the population, and that nature variations in the promoter of TIG1 significantly affect its expression, closely correlating with tiller angle phenotypes observed. Similarly, using qRT-PCR and haplotype analysis, we identified 1 and 7 candidate genes in qTA6.1 and qTA8.1 that were commonly detected by two GWAS models, respectively. In addition, we identified 3 more candidate genes in the remaining 14 novel QTLs after filtering by transcriptome analysis and qRT-PCR. In summary, this study provides new insights into the genetic architecture of rice tiller angle and candidate genes for rice breeding.

Genetic control of branching patterns in grass inflorescences

Inflorescence branching in the grasses controls the number of florets and hence the number of seeds. Recent data on the underlying genetics come primarily from rice and maize, although new data are accumulating in other systems as well. This review focuses on a window in developmental time from the production of primary branches by the inflorescence meristem through to the production of glumes, which indicate the transition to producing a spikelet. Several major developmental regulatory modules appear to be conserved among most or all grasses. Placement and development of primary branches are controlled by conserved auxin regulatory genes. Subtending bracts are repressed by a network including TASSELSHEATH4, and axillary branch meristems are regulated largely by signaling centers that are adjacent to but not within the meristems themselves. Gradients of SQUAMOSA-PROMOTER BINDING-like and APETALA2-like proteins and their microRNA regulators extend along the inflorescence axis and the branches, governing the transition from production of branches to production of spikelets. The relative speed of this transition determines the extent of secondary and higher order branching. This inflorescence regulatory network is modified within individual species, particularly as regards formation of secondary branches. Differences between species are caused both by modifications of gene expression and regulators and by presence or absence of critical genes. The unified networks described here may provide tools for investigating orphan crops and grasses other than the well-studied maize and rice.

Partially functional NARROW LEAF1 balances leaf photosynthesis and plant architecture for greater rice yield

NARROW LEAF1 (NAL1) is an elite gene in rice (Oryza sativa), given its close connection to leaf photosynthesis, hybrid vigor, and yield-related agronomic traits; however, the underlying mechanism by which this gene affects these traits remains elusive. In this study, we systematically measured leaf photosynthetic parameters, leaf anatomical parameters, architectural parameters, and agronomic traits in indica cultivar 9311, in 9311 with the native NAL1 replaced by the Nipponbare NAL1 (9311-NIL), and in 9311 with the NAL1 fully mutated (9311-nal1). Leaf length, width, and spikelet number gradually increased from lowest to highest in 9311-nal1, 9311, and 9311-NIL. In contrast, the leaf photosynthetic rate on a leaf area basis, leaf thickness, and panicle number gradually decreased from highest to lowest in 9311-nal1, 9311, and 9311-NIL. RNA-seq analysis showed that NAL1 negatively regulates the expression of photosynthesis-related genes; NAL1 also influenced expression of many genes related to phytohormone signaling, as also shown by different leaf contents of 3-Indoleacetic acid, jasmonic acid, Gibberellin A3, and isopentenyladenine among these genotypes. Furthermore, field experiments with different planting densities showed that 9311 had a larger biomass and yield advantage under low planting density compared to either 9311-NIL or 9311-nall. This study shows both direct and indirect effects of NAL1 on leaf photosynthesis; furthermore, we show that a partially functional NAL1 allele helps maintain a balanced leaf photosynthesis and plant architecture for increased biomass and grain yield in the field.

Translational repression of FZP mediated by CU-rich element/OsPTB interactions modulates panicle development in rice

Panicle development is an important determinant of the grain number in rice. A thorough characterization of the molecular mechanism underlying panicle development will lead to improved breeding of high-yielding rice varieties. Frizzy Panicle (FZP), a critical gene for panicle development, is regulated by OsBZR1 and OsARFs at the transcriptional stage. However, the translational modulation of FZP has not been reported. We reveal that the CU-rich elements (CUREs) in the 3' UTR of the FZP mRNA are crucial for efficient FZP translation. The knockout of CUREs in the FZP 3' UTR or the over-expression of the FZP 3' UTR fragment containing CUREs resulted in an increase in FZP mRNA translation efficiency. Moreover, the number of secondary branches (NSB) and the grain number per panicle (GNP) decreased in the transformed rice plants. The CUREs in the 3' UTR of FZP mRNA were verified as the targets of the polypyrimidine tract-binding proteins OsPTB1 and OsPTB2 in rice. Both OsPTB1 and OsPTB2 were highly expressed in young panicles. The knockout of OsPTB1/2 resulted in an increase in the FZP translational efficiency and a decrease in the NSB and GNP. Furthermore, the over-expression of OsPTB1/2 decreased the translation of the reporter gene fused to FZP 3' UTR in vivo and in vitro. These results suggest that OsPTB1/2 can mediate FZP translational repression by interacting with CUREs in the 3' UTR of FZP mRNA, leading to changes in the NSB and GNP. Accordingly, in addition to transcriptional regulation, FZP expression is also fine-tuned at the translational stage during rice panicle development.

Introgression Lines: Valuable Resources for Functional Genomics Research and Breeding in Rice (Oryza sativa L.)

The narrow base of genetic diversity of modern rice varieties is mainly attributed to the overuse of the common backbone parents that leads to the lack of varied favorable alleles in the process of breeding new varieties. Introgression lines (ILs) developed by a backcross strategy combined with marker-assisted selection (MAS) are powerful prebreeding tools for broadening the genetic base of existing cultivars. They have high power for mapping quantitative trait loci (QTLs) either with major or minor effects, and are used for precisely evaluating the genetic effects of QTLs and detecting the gene-by-gene or gene-by-environment interactions due to their low genetic background noise. ILs developed from multiple donors in a fixed background can be used as an IL platform to identify the best alleles or allele combinations for breeding by design. In the present paper, we reviewed the recent achievements from ILs in rice functional genomics research and breeding, including the genetic dissection of complex traits, identification of elite alleles and background-independent and epistatic QTLs, analysis of genetic interaction, and genetic improvement of single and multiple target traits. We also discussed how to develop ILs for further identification of new elite alleles, and how to utilize IL platforms for rice genetic improvement.

Rice SEPALLATA genes OsMADS5 and OsMADS34 cooperate to limit inflorescence branching by repressing the TERMINAL FLOWER1-like gene RCN4

The spatiotemporal control of meristem identity is critical for determining inflorescence architecture, and thus yield, of cereal plants. However, the precise mechanisms underlying inflorescence and spikelet meristem determinacy in cereals are still largely unclear. We have generated loss-of-function and overexpression mutants of the paralogous OsMADS5 and OsMADS34 genes in rice (Oryza sativa), and analysed their panicle phenotypes. Using chromatin immunoprecipitation, electrophoretic mobility-shift and dual-luciferase assays, we have also identified RICE CENTRORADIALIS 4 (RCN4), a TFL1-like gene, as a direct downstream target of both OsMADS proteins, and have analysed RCN4 mutants. The osmads5 osmads34 mutant lines had significantly enhanced panicle branching with increased secondary, and even tertiary and quaternary, branches, compared to wild-type (WT) and osmads34 plants. The osmads34 mutant phenotype could largely be rescued by also knocking out RCN4. Moreover, transgenic panicles overexpressing RCN4 had significantly increased branching, and initiated development of c. 7× more spikelets than WT. Our results reveal a role for OsMADS5 in panicle development, and show that OsMADS5 and OsMADS34 play similar functions in limiting branching and promoting the transition to spikelet meristem identity, in part by repressing RCN4 expression. These findings provide new insights to better understand the molecular regulation of rice inflorescence architecture.

The Streptochaeta Genome and the Evolution of the Grasses

In this work, we sequenced and annotated the genome of Streptochaeta angustifolia, one of two genera in the grass subfamily Anomochlooideae, a lineage sister to all other grasses. The final assembly size is over 99% of the estimated genome size. We find good collinearity with the rice genome and have captured most of the gene space. Streptochaeta is similar to other grasses in the structure of its fruit (a caryopsis or grain) but has peculiar flowers and inflorescences that are distinct from those in the outgroups and in other grasses. To provide tools for investigations of floral structure, we analyzed two large families of transcription factors, AP2-like and R2R3 MYBs, that are known to control floral and spikelet development in rice and maize among other grasses. Many of these are also regulated by small RNAs. Structure of the gene trees showed that the well documented whole genome duplication at the origin of the grasses (ρ) occurred before the divergence of the Anomochlooideae lineage from the lineage leading to the rest of the grasses (the spikelet clade) and thus that the common ancestor of all grasses probably had two copies of the developmental genes. However, Streptochaeta (and by inference other members of Anomochlooideae) has lost one copy of many genes. The peculiar floral morphology of Streptochaeta may thus have derived from an ancestral plant that was morphologically similar to the spikelet-bearing grasses. We further identify 114 loci producing microRNAs and 89 loci generating phased, secondary siRNAs, classes of small RNAs known to be influential in transcriptional and post-transcriptional regulation of several plant functions.

FRIZZY PANICLE defines a regulatory hub for simultaneously controlling spikelet formation and awn elongation in bread wheat

Grain yield in bread wheat (Triticum aestivum L.) is largely determined by inflorescence architecture. Zang734 is an endemic Tibetan wheat variety that exhibits a rare triple spikelet (TRS) phenotype with significantly increased spikelet/floret number per spike. However, the molecular basis underlying this specific spike morphology is completely unknown. Through map-based cloning, the causal genes for TRS trait in Zang734 were isolated. Furthermore, using CRISPR/Cas9-based gene mutation, transcriptome sequencing and protein-protein interaction, the downstream signalling networks related to spikelet formation and awn elongation were defined. Results showed that the null mutation in WFZP-A together with deletion of WFZP-D led to the TRS trait in Zang734. More interestingly, WFZP plays a dual role in simultaneously repressing spikelet formation gene TaBA1 and activating awn development genes, basically through the recruitments of chromatin remodelling elements and the Mediator complex. Our findings provide insights into the molecular bases by which WFZP suppresses spikelet formation but promotes awn elongation and, more importantly, define WFZP-D as a favourable gene for high-yield crop breeding.

OsTPR boosts the superior grains through increase in upper secondary rachis branches without incurring a grain quality penalty

To address the future food security in Asia, we need to improve the genetic gain of grain yield while ensuring the consumer acceptance. This study aimed to identify novel genes influencing the number of upper secondary rachis branches (USRB) to elevate superior grains without compromising grain quality by studying the genetic variance of 310 diverse O. sativa var. indica panel using single- and multi-locus genome-wide association studies (GWAS), gene set analyses and gene regulatory network analysis. GWAS of USRB identified 230 significant (q-value < 0.05) SNPs from chromosomes 1 and 2. GWAS targets narrowed down using gene set analyses identified large effect association on an important locus LOC_Os02g50790/LOC_Os02g50799 encoding a nuclear-pore anchor protein (OsTPR). The superior haplotype derived from non-synonymous SNPs identified in OsTPR was specifically associated with increase in USRB with superior grains being low chalk. Through haplotype mining, we further demonstrated the synergy of offering added yield advantage due to superior allele of OsTPR in elite materials with low glycaemic index (GI) property. We further validated the importance of OsTPR using recombinant inbred lines (RILs) population by introgressing a superior allele of OsTPR into elite materials resulted in raise in productivity in high amylose background. This confirmed a critical role for OsTPR in influencing yield while maintaining grain and nutritional quality.

Natural Variation in Crops: Realized Understanding, Continuing Promise

Crops feed the world's population and shape human civilization. The improvement of crop productivity has been ongoing for almost 10,000 years and has evolved from an experience-based to a knowledge-driven practice over the past three decades. Natural alleles and their reshuffling are long-standing genetic changes that affect how crops respond to various environmental conditions and agricultural practices. Decoding the genetic basis of natural variation is central to understanding crop evolution and, in turn, improving crop breeding. Here, we review current advances in the approaches used to map the causal alleles of natural variation, provide refined insights into the genetics and evolution of natural variation, and outline how this knowledge promises to drive the development of sustainable agriculture under the dome of emerging technologies.

A cluster of Ankyrin and Ankyrin-TPR repeat genes is associated with panicle branching diversity in rice

The number of grains per panicle is an important yield-related trait in cereals which depends in part on panicle branching complexity. One component of this complexity is the number of secondary branches per panicle. Previously, a GWAS site associated with secondary branch and spikelet numbers per panicle in rice was identified. Here we combined gene capture, bi-parental genetic population analysis, expression profiling and transgenic approaches in order to investigate the functional significance of a cluster of 6 ANK and ANK-TPR genes within the QTL. Four of the ANK and ANK-TPR genes present a differential expression associated with panicle secondary branch number in contrasted accessions. These differential expression patterns correlate in the different alleles of these genes with specific deletions of potential cis-regulatory sequences in their promoters. Two of these genes were confirmed through functional analysis as playing a role in the control of panicle architecture. Our findings indicate that secondary branching diversity in the rice panicle is governed in part by differentially expressed genes within this cluster encoding ANK and ANK-TPR domain proteins that may act as positive or negative regulators of panicle meristem’s identity transition from indeterminate to determinate state.

Grain yield is one of the most important indexes in rice breeding, which is controlled in part by panicle branching complexity. A new QTL with co-location of spikelet number (SpN) and secondary branch number (SBN) traits was identified by genome-wide association study in a Vietnamese rice landrace panel. A set of four Ankyrin and Tetratricopeptide repeat domain-encoding genes was identified from this QTL based on their difference of expression levels between two contrasted haplotypes for the SpN and SBN traits. The differential expression is correlated with deletions in the promoter regions of these genes. Two of the genes act as negative regulators of the panicle meristem’s identity transition from indeterminate to determinate state while the other two act as positive regulators of this meristem fate transition. Based on the different phenotypes between overexpressed and mutant plants, two of these genes were confirmed as playing a role in the control of panicle architecture. These findings can be directly used to assist selection for grain yield improvement.

Wheat FRIZZY PANICLE activates VERNALIZATION1-A and HOMEOBOX4-A to regulate spike development in wheat

Kernel number per spike determined by the spike or inflorescence development is one important agricultural trait for wheat yield that is critical for global food security. While a few important genes for wheat spike development were identified, the genetic regulatory mechanism underlying supernumerary spikelets (SSs) is still unclear. Here, we cloned the wheat FRIZZY PANICLE (WFZP) gene from one local wheat cultivar. WFZP is specifically expressed at the sites where the spikelet meristem and floral meristem are initiated, which differs from the expression patterns of its homologs FZP/BD1 in rice and maize, indicative of its functional divergence during species differentiation. Moreover, WFZP directly activates VERNALIZATION1 (VRN1) and wheat HOMEOBOX4 (TaHOX4) to regulate the initiation and development of spikelet. The haplotypes analysis showed that the favourable alleles of WFZP associated with spikelet number per spike (SNS) were preferentially selected during breeding. Our findings provide insights into the molecular and genetic mechanisms underlying wheat spike development and characterize the WFZP as elite resource for wheat molecular breeding with enhanced crop yield.

Molecular Insights into Inflorescence Meristem Specification for Yield Potential in Cereal Crops

Flowering plants develop new organs throughout their life cycle. The vegetative shoot apical meristem (SAM) generates leaf whorls, branches and stems, whereas the reproductive SAM, called the inflorescence meristem (IM), forms florets arranged on a stem or an axis. In cereal crops, the inflorescence producing grains from fertilized florets makes the major yield contribution, which is determined by the numbers and structures of branches, spikelets and florets within the inflorescence. The developmental progression largely depends on the activity of IM. The proper regulations of IM size, specification and termination are outcomes of complex interactions between promoting and restricting factors/signals. Here, we focus on recent advances in molecular mechanisms underlying potential pathways of IM identification, maintenance and differentiation in cereal crops, including rice (Oryza sativa), maize (Zea mays), wheat (Triticum aestivum), and barley (Hordeum vulgare), highlighting the researches that have facilitated grain yield by, for example, modifying the number of inflorescence branches. Combinatorial functions of key regulators and crosstalk in IM determinacy and specification are summarized. This review delivers the knowledge to crop breeding applications aiming to the improvements in yield performance and productivity.

Harnessing Knowledge from Maize and Rice Domestication for New Crop Breeding

Crop domestication has fundamentally altered the course of human history, causing a shift from hunter-gatherer to agricultural societies and stimulating the rise of modern civilization. A greater understanding of crop domestication would provide a theoretical basis for how we could improve current crops and develop new crops to deal with environmental challenges in a sustainable manner. Here, we provide a comprehensive summary of the similarities and differences in the domestication processes of maize and rice, two major staple food crops that feed the world. We propose that maize and rice might have evolved distinct genetic solutions toward domestication. Maize and rice domestication appears to be associated with distinct regulatory and evolutionary mechanisms. Rice domestication tended to select de novo, loss-of-function, coding variation, while maize domestication more frequently favored standing, gain-of-function, regulatory variation. At the gene network level, distinct genetic paths were used to acquire convergent phenotypes in maize and rice domestication, during which different central genes were utilized, orthologous genes played different evolutionary roles, and unique genes or regulatory modules were acquired for establishing new traits. Finally, we discuss how the knowledge gained from past domestication processes, together with emerging technologies, could be exploited to improve modern crop breeding and domesticate new crops to meet increasing human demands.

TAC4 controls tiller angle by regulating the endogenous auxin content and distribution in rice

Tiller angle, an important component of plant architecture, greatly influences the grain yield of rice (Oryza sativa L.). Here, we identified Tiller Angle Control 4 (TAC4) as a novel regulator of rice tiller angle. TAC4 encodes a plant-specific, highly conserved nuclear protein. The loss of TAC4 function leads to a significant increase in the tiller angle. TAC4 can regulate rice shoot gravitropism by increasing the indole acetic acid content and affecting the auxin distribution. A sequence analysis revealed that TAC4 has undergone a bottleneck and become fixed in indica cultivars during domestication and improvement. Our findings facilitate an increased understanding of the regulatory mechanisms of tiller angle and also provide a potential gene resource for the improvement of rice plant architecture.

Phytohormone-Mediated Molecular Mechanisms Involving Multiple Genes and QTL Govern Grain Number in Rice

Increasing the grain number is the most direct route toward enhancing the grain yield in cereals. In rice, grain number can be amplified through increasing the shoot branching (tillering), panicle branching, panicle length, and seed set percentage. Phytohormones have been conclusively shown to control the above characteristics by regulating molecular factors and their cross-interactions. The dynamic equilibrium of cytokinin levels in both shoot and inflorescence meristems, maintained by the regulation of its biosynthesis, activation, and degradation, determines the tillering and panicle branching, respectively. Auxins and gibberellins are known broadly to repress the axillary meristems, while jasmonic acid is implicated in the determination of reproductive meristem formation. The balance of auxin, gibberellin, and cytokinin determines meristematic activities in the inflorescence. Strigolactones have been shown to repress the shoot branching but seem to regulate panicle branching positively. Ethylene, brassinosteroids, and gibberellins regulate spikelet abortion and grain filling. Further studies on the optimization of endogenous hormonal levels can help in the expansion of the grain yield potential of rice. This review focuses on the molecular machinery, involving several genes and quantitative trait loci (QTL), operational in the plant that governs hormonal control and, in turn, gets governed by the hormones to regulate grain number and yield in rice.