Rice Domestication Research Articles

Super graph-based pan-genome: Bringing rice functional genomic study into a new dawn

Rice is one of the earliest domesticated crops that has been cultivated for thousands of years. Interestingly, unlike other crops that are predominantly domesticated by a singleton event in one place, the domestication of rice is quite complicated; it independently occurred in Asia and Africa to form the Asian cultivated rice Oryza sativa and the African cultivated rice O. glaberrima, respectively. O. sativa was mainly domesticated from the Asian wild rice O. rufipogon in East/South Asia (Huang et al., 2012).

Rice OsUBR7 modulates plant height by regulating histone H2B monoubiquitination and cell proliferation

Plant height is an important agronomic trait for lodging resistance and yield. Here, we report a new plant-height-related gene, OsUBR7 in rice (Oryza sativa L.); knockout of OsUBR7 caused fewer cells in internodes, resulting in a semi-dwarf phenotype. OsUBR7 encodes a putative E3 ligase containing a plant homeodomain finger and a ubiquitin protein ligase E3 component N-recognin 7 (UBR7) domain. OsUBR7 interacts with histones and monoubiquitinates H2B (H2Bub1) at lysine148 in coordination with the E2 conjugase OsUBC18. OsUBR7 mediates H2Bub1 at a number of chromatin loci for the normal expression of target genes, including cell-cycle-related and pleiotropic genes, consistent with the observation that cell-cycle progression was suppressed in the osubr7 mutant owing to reductions in H2Bub1 and expression levels at these loci. The genetic divergence of OsUBR7 alleles among japonica and indica cultivars affects their transcriptional activity, and these alleles may have undergone selection during rice domestication. Overall, our results reveal a novel mechanism that mediates H2Bub1 in plants, and UBR7 orthologs could be utilized as an untapped epigenetic resource for crop improvement.

Early millet cultivation, subsistence diversity, and wild plant use at Neolithic Anle, Lower Yangtze, China

This study examines the archeological seed assemblage of Anle, a middle Neolithic site located in the Lower Yangtze region, China. The Lower Yangtze is thought to be the origin of domesticated rice and most studies of this region to date have focused on rice domestication and cultivation within its paleoenvironmental setting. In contrast, we highlight here diverse uses of non-rice plant resources. In addition to large quantities of rice remains (carbonized grains and spikelet bases), we identify both foxtail and broomcorn millet, both AMS radiocarbon dated earlier than 5750 cal BP, demonstrating the dispersal of millet cultivation to the Lower Yangtze in the middle Neolithic, earlier than previously securely documented. While most wild species identified in macrobotanical assemblages are traditionally categorized as weeds or incidental intrusions among food residues, many can be exploited for food and medicinal purposes. By analyzing the ecological and functional implications of identified plants, we infer ecological niches of cultivation, gathering, and possible propagation of wild plants as food and medicine. Analyses of diversity and seasonality of plant resources identified show that residents of Anle created a complex seasonal sequence of temporally compatible crops, constructing niches for two crops (rice and millet) and actively structuring opportunities to exploit available wild plant resources in their immediate environment.

A super pan-genomic landscape of rice

Pan-genomes from large natural populations can capture genetic diversity and reveal genomic complexity. Using de novo long-read assembly, we generated a graph-based super pan-genome of rice consisting of a 251-accession panel comprising both cultivated and wild species of Asian and African rice. Our pan-genome reveals extensive structural variations (SVs) and gene presence/absence variations. Additionally, our pan-genome enables the accurate identification of nucleotide-binding leucine-rich repeat genes and characterization of their inter- and intraspecific diversity. Moreover, we uncovered grain weight-associated SVs which specify traits by affecting the expression of their nearby genes. We characterized genetic variants associated with submergence tolerance, seed shattering and plant architecture and found independent selection for a common set of genes that drove adaptation and domestication in Asian and African rice. This super pan-genome facilitates pinpointing of lineage-specific haplotypes for trait-associated genes and provides insights into the evolutionary events that have shaped the genomic architecture of various rice species.

The Fits and Starts of Indian Rice Domestication: How the Movement of Rice Across Northwest India Impacted Domestication Pathways and Agricultural Stories

Rice is currently the staple food for over 3.5 billion people and is arguably the most important crop exploited by humans. Understanding how we came to the point where a single crop dominates the lives of almost half of the Earth’s population has major significance for our future, even more so given the climatic instability we face today, as rice is a cereal that is dependent on water to an extreme degree. In this study, the nature of early rice agriculture in South Asia is explored, looking at how this critical crop may have begun to be exploited, cultivated, and then brought under agricultural regimes during the long span between c.6500 and 1500 BC. There is now clear evidence for early Holocene cultivation of rice in the Middle Gangetic plains of northern India, but there is still considerable debate about the timing of when this cultivation began and whether it involved domestication of rice. By 3200 BC, however, rice agriculture was present outside the Ganges in the Indus Civilization. The data show accelerated domestication in the Indus environment and agricultural systems that played a part in later hybridization with the arrival of Chinese rice. Understanding how this move from its place of origin to a new environment may have become entangled in the domestication pathways of South Asia rice prior to the arrival of Chinese rice c.1500 BC are important to the overall rice story, as they play into modern concerns relating to biodiversity and different ways of growing and watering rice.

Micro Plant Remains Reveal the Function of Grooved Pottery Vessels From the Late Neolithic Meishan Site in Central China

From 6,000 calendar years before the present (cal BP) onward, grooved pottery vessels occurred in the lower and middle reaches of the Yangtze River in succession. After a thousand years, it was introduced into central China and became popularized there around 4,000 years ago. It has been proposed that the function of the grooved vessels was to process plant foods, replacing the previously used stone slabs and rollers in terms of ethnographic investigation, but there is a lack of solid and direct evidence although little evidence of starches has been provided. Here we report our study on the sherds of grooved vessels from the Meishan site in central China (late Longshan and Erlitou period). The combined starch and phytolith analyses were first used together to examine the residues on the sherds. Starches from geophytes, millets (Setaria italica and Panicum miliaceum), wheat (Triticum aestivum), and phytoliths from the glumes and leaves of these crops plus rice (Oryza sativa), seem to suggest that the grooved pottery vessels were likely to be used to grind geophytes and dehusk grain seeds. But, incorporating the extremely low proportion of grooved vessels to entire pottery tools at the site, we hypothesize that the invention of grooved vessels may have been related to the success of rice domestication and may have been used as tools to pound by-products of crops, leaves, and husks somehow.

OsMATE6 gene putatively involved in host defense response toward susceptibility against Rhizoctonia solani in rice

ABSTRACT Sheath blight caused by Rhizoctonia solani AG1-IA is the second most serious disease of rice worldwide. Elucidating the role of multi-drug and toxic compound extrusion (MATE) gene family in host-pathogens interactions may uncover a new possible way to comprehend the mechanism of sheath blight resistance in rice. We foremost explored the role of OsMATE genes against R. solani resistance through comparative transcriptomics in PR114 (susceptible) and ShB-8 (moderately resistant) at 24 and 48 hpi (hours post-inoculation) of R. solani infection, respectively. Six OsMATE genes were differentially expressed and further validated through qRT-PCR. OsMATE6 gene was identified as a potential candidate for sheath blight susceptibility as it was significantly up-regulated in PR114. OsMATE6 is conserved within the wild relatives and might be translocated from Oryza nivara during the domestication of rice. Further studies are focused to verify its role by overexpression and protein interactions to understand the molecular mechanism of sheath blight resistance.

Control of awn length in rice breeding programs in Hokkaido

Understanding genetic diversity among local populations may facilitate the development of new crop varieties and is a primary goal of the molecular evolution of genes. Awn length is a well-documented phenotype among domestication traits in rice (Oryza sativa L.), from long to short awns. Short awned or awnless varieties have been selected in rice breeding programs. Awnlessness is favored by rice farmers in the current agriculture system. Here, we identified the genetic basis of awn length during rice breeding programs in Hokkaido, Japan. We found variation in awn length ranging from 0.0 to 37.6 mm among a local population consisted of breeding lines. Genetic analysis of awn length identified that RAE1 and RAE2 on chromosomes 4 and 8, respectively, accounted for awn presence. These genes are well known to be significant during Asian rice domestication. Sequence variations in these genes may clarify the molecular evolution of the genes for awn length in rice breeding programs. Firstly, a loss-of-function allele in RAE1, rae1, was selected for short awn length. Then, alleles of RAE2, RAE2-H01 to RAE2-H04, were targeted for the selection of short awns or awnlessness. The selections of an awnlessness phenotype can diversify these alleles in the genes RAE1 and RAE2 exhibiting variation in awn length.

Cold-adaptive evolution at the reproductive stage in Geng/japonica subspecies reveals the role of OsMAPK3 and OsLEA9.

Cold stress at the reproductive stage severely affects the production and geographic distribution of rice. The Geng/japonica subpopulation gradually developed stronger cold adaptation than the Xian/indica subpopulation during the long-term domestication of cultivated rice. However, the evolutionary path and natural alleles underlying the cold adaptability of intra-Geng subspecies remain largely unknown. Here, we identified MITOGEN-ACTIVATED PROTEIN KINASE 3 (OsMAPK3) and LATE EMBRYOGENESIS ABUNDANT PROTEIN 9 (OsLEA9) as two important regulators for the cold adaptation of Geng subspecies from a combination of transcriptome analysis and genome-wide association study. Transgenic validation showed that OsMAPK3 and OsLEA9 confer cold tolerance at the reproductive stage. Selection and evolution analysis suggested that the Geng version of OsMAPK3 (OsMAPK3Geng ) directly evolved from Chinese Oryza rufipogon III and was largely retained in high-latitude and high-altitude regions with low temperatures during domestication. Later, the functional nucleotide polymorphism (FNP-776) in the Kunmingxiaobaigu and Lijiangxiaoheigu version of the OsLEA9 (OsLEA9KL ) promoter originated from novel variation of intra-Geng was selected and predominantly retained in temperate Geng to improve the adaptation of Geng together with OsMAPK3Geng to colder climatic conditions in high-latitude areas. Breeding potential analysis suggested that pyramiding of OsMAPK3Geng and OsLEA9KL enhanced the cold tolerance of Geng and promotes the expansion of cultivated rice to colder regions. This study not only highlights the evolutionary path taken by the cold-adaptive differentiation of intra-Geng, but also provides new genetic resources for rice molecular breeding in low-temperature areas.

A stepwise route to domesticate rice by controlling seed shattering and panicle shape

Asian rice (Oryza sativa L.) is consumed by more than half of the world's population. Despite its global importance, the process of early rice domestication remains unclear. During domestication, wild rice (Oryza rufipogon Griff.) acquired non-seed-shattering behavior, allowing humans to increase grain yield. Previous studies argued that a reduction in seed shattering triggered by the sh4 mutation led to increased yield during rice domestication, but our experiments using wild introgression lines show that the domesticated sh4 allele alone is insufficient for shattering loss in O. rufipogon. The interruption of abscission layer formation requires both sh4 and qSH3 mutations, demonstrating that the selection of shattering loss in wild rice was not as simple as previously suggested. Here we identified a causal single-nucleotide polymorphism at qSH3 within the seed-shattering gene OsSh1, which is conserved in indica and japonica subspecies but absent in the circum-aus group of rice. Through harvest experiments, we further demonstrated that seed shattering alone did not significantly impact yield; rather, yield increases were observed with closed panicle formation controlled by SPR3 and further augmented by nonshattering, conferred by integration of sh4 and qSH3 alleles. Complementary manipulation of panicle shape and seed shattering results in a mechanically stable panicle structure. We propose a stepwise route for the earliest phase of rice domestication, wherein selection of visible SPR3-controlled closed panicle morphology was instrumental in the sequential recruitment of sh4 and qSH3, which together led to the loss of shattering.

New evidence supports the continuous development of rice cultivation and early formation of mixed farming in the Middle Han River Valley, China

Located in the transitional region between northern and southern China, the middle Han River valley is of pivotal significance for current debates on rice domestication and the formation of mixed farming in early China. Based on phytoliths and macro-plant remains from 10 sites, along with radiocarbon dating, this study has narrowed the 2000-year gap in early rice utilization by 700 years in the middle Han River valley, and demonstrated the continuous development of rice agriculture in this region. This new evidence also reveals that as early as 7000 cal. BP, mixed farming emerged in this region and facilitated the further southward dispersal of foxtail millet and broomcorn millet into southern China and beyond. In addition, a sample-by-sample comparison of our results with phytoliths and macro-plant remains highlighted the potential biases inherent in these methods. This work emphasized the importance of multi-index archaeobotanical analysis to reconstruct ancient crop assemblages.

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

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

Genome-Wide Comprehensive Survey of the Subtilisin-Like Proteases Gene Family Associated With Rice Caryopsis Development.

Subtilisin-like proteases (SUBs), which are extensively distributed in three life domains, affect all aspects of the plant life cycle, from embryogenesis and organogenesis to senescence. To explore the role of SUBs in rice caryopsis development, we recharacterized the OsSUB gene family in rice (Oryza sativa ssp. japonica). In addition, investigation of the SUBs was conducted across cultivated and wild rice in seven other Oryza diploid species (O. brachyantha, O. glaberrima, O. meridionalis, O. nivara, O. punctata, O. rufipogon, and O. sativa ssp. indica). Sixty-two OsSUBs were identified in the latest O. sativa ssp. japonica genome, which was higher than that observed in wild species. The SUB gene family was classified into six evolutionary branches, and SUB1 and SUB3 possessed all tandem duplication (TD) genes. All paralogous SUBs in eight Oryza plants underwent significant purifying selection. The expansion of SUBs in cultivated rice was primarily associated with the occurrence of tandem duplication events and purifying selection and may be the result of rice domestication. Combining the expression patterns of OsSUBs in different rice tissues and qRT–PCR verification, four OsSUBs were expressed in rice caryopses. Moreover, OsSUBs expressed in rice caryopses possessed an earlier origin in Oryza, and the gene cluster formed by OsSUBs together with the surrounding gene blocks may be responsible for the specific expression of OsSUBs in caryopses. All the above insights were inseparable from the continuous evolution and domestication of Oryza. Together, our findings not only contribute to the understanding of the evolution of SUBs in cultivated and wild rice but also lay the molecular foundation of caryopsis development and engineering improvement of crop yield.

Small EPIDERMAL PATTERNING FACTOR-LIKE2 peptides regulate awn development in rice.

The EPIDERMAL PATTERNING FACTOR (EPF) and EPF-LIKE (EPFL) family of small secreted peptides act to regulate many aspects of plant growth and development; however, their functions are not widely characterized in rice (Oryza sativa). Here, we used clustered regularly interspaced short palindromic repeats (CRISPR/Cas9) technology to individually knockout each of 11 EPF/EPFL genes in the rice cultivar Kasalath. Loss of function of most OsEPF/EPFL genes generated no obvious phenotype alteration, while disruption of OsEPFL2 in Kasalath caused a short or no awn phenotype and reduced grain size. OsEPFL2 is strongly expressed in the young panicle, consistent with a role in regulating awn and grain development. Haplotype analysis indicated that OsEPFL2 can be classified into six major haplotypes. Nucleotide diversity and genetic differentiation analyses suggested that OsEPFL2 was positively selected during the domestication of rice. Our work to systematically investigate the function of EPF/EPFL peptides demonstrates that different members of the same gene family have been independently selected for their ability to regulate a similar biological function and provides perspective on rice domestication.

Potential of Genome Editing to Capture Diversity From Australian Wild Rice Relatives.

Rice, a staple food worldwide and a model crop, could benefit from the introduction of novel genetics from wild relatives. Wild rice in the AA genome group closely related to domesticated rice is found across the tropical world. Due to their locality outside the range of domesticated rice, Australian wild rice populations are a potential source of unique traits for rice breeding. These rice species provide a diverse gene pool for improvement that could be utilized for desirable traits such as stress resistance, disease tolerance, and nutritional qualities. However, they remain poorly characterized. The CRISPR/Cas system has revolutionized gene editing and has improved our understanding of gene functions. Coupled with the increasing availability of genomic information on the species, genes in Australian wild rice could be modified through genome editing technologies to produce new domesticates. Alternatively, beneficial alleles from these rice species could be incorporated into cultivated rice to improve critical traits. Here, we summarize the beneficial traits in Australian wild rice, the available genomic information and the potential of gene editing to discover and understand the functions of novel alleles. Moreover, we discuss the potential domestication of these wild rice species for health and economic benefits to rice production globally.

Can the grains offer each other helping hands? Convergent molecular mechanisms associated with domestication and crop improvement in rice and maize

The grasses, Poaceae, are an extraordinarily ecologically successful clade, with more than 10 000 described extant species. The grasses include more than 30 domesticated grain crops, ranging from maize (Zea mays), rice (Oryza sativa), and wheat (Triticum aestivum), which are collectively responsible for 50% of all calories consumed by humans around the world, to minor and orphan crops such as teff (Eragrostis tef), pearl millet (Pennisetum glaucum), proso millet (Panicum milaceum), and Job’s tears (Coix lacryma-jobi) (Glémin and Bataillon, 2009).

Reticulate Evolution in AA-Genome Wild Rice in Australia.

The wild rice gene pool, i.e., AA-genome, in Australia is geographically and genetically distinct from that in Asia. Two distinct taxa are found growing together in northern Australia, Oryza meridionalis (including annual and perennial forms) and an Oryza rufipogon like taxa that have been shown to have a chloroplast genome sequence that is closer to that of O. meridionalis than to O. rufipogon from Asia. Rare plants of intermediate morphology have been observed in the wild despite a reported reproductive barrier between these two species. We now report the resequencing of plants from 26 populations including both taxa and putative hybrids. A comparison of chloroplast and nuclear genome sequences indicated re-combinations that demonstrated hybridisation in both directions. Individuals with intermediate morphology had high nuclear genome heterozygosity consistent with a hybrid origin. An examination of specific genes (e.g., starch biosynthesis genes) revealed the presence of heterozygotes with alleles from both parents suggesting that some wild plants were early generation hybrids. These plants may have low cross-fertility preserving the continuation of the two distinct species. Repeated backcrossing of these rare hybrids to one parent would explain the plants exhibiting chloroplast capture. These observations suggest that reticulate evolution is continuing in wild Oryza populations and may have been a key process in rice evolution and domestication.

The genetic control of glabrous glume during African rice domestication

African cultivated rice, Oryza glaberrima, is characterized by its glabrous glumes. During domestication, the pubescent glumes of its wild ancestor, Oryza barthii, lost their trichomes, and in this study, we show that glabrous glume 5 (GLAG5), a WUSCHEL-like homeobox transcription factor gene on chromosome 5, is required for trichome development. DNA methylation associated with an hAT transposable element inserted in the promoter region of GLAG5 is found to reduce its expression, leading to the formation of glabrous glumes and leaves in African cultivated rice. Among 82 African cultivated rice varieties investigated in this study, 59 (approximately 71%) lines exhibit glabrous glumes and harbor the hAT transposon; however, the other 23 varieties (approximately 29%), which exhibit pubescent glumes, lack the hAT transposon, indicating that glag5 had undergone strong artificial selection. The πw/πc ratios also show the hAT transposon insertions influence the genetic diversity of an approximately 150-kb interval encompassing the GLAG5 locus. The identification of the GLAG5 gene provides new insights into the domestication of cultivated rice in Africa. We speculate that the selection of varieties with mutations in their promoter regions is an important aspect of crop domestication.

Dynamic Interaction Between Deforestation and Rice Cultivation During the Holocene in the Lower Yangtze River, China

Human activity has transformed the Earth’s landscapes pervasively for thousands of years, and the most important anthropogenic alteration was the clearing of forests and the establishment of agriculture. As a center of rice domestication and early population growth, the lower Yangtze River has been extensively transformed in the Holocene. However, the timing, extent, and process of deforestation and its relationship with the intensification of rice cultivation remain controversial. Here, four representative archaeological sites ranging from 8,100 to 4,300 cal a BP, that is, Jingtoushan, Hemudu, Yushan, and Xiawangdu sites, were selected for detailed palynological analysis, and evidence of anthropogenic deforestation and subsistence strategy were also synthesized to investigate dynamic human–forest interaction. Although natural vegetation had already been altered at the Jingtoushan site around 8,000 cal a BP, it was more likely to be the management of acorns by limiting burning to open habitats and increasing yield. As the subsistence shifted from acorn exploitation toward rice cultivation after 6,000 cal a BP, real deforestation for agriculture may occur at the Yushan and Xiawangdu sites due to conflict on labor input and land use. However, these deforestations were just confined to the archaeological sites at local scale, and no consistent vegetation change occurred at regional scales induced by human activities until the last 3,000 years.

Degradome comparison between wild and cultivated rice identifies differential targeting by miRNAs

BackgroundSmall non-coding (s)RNAs are involved in the negative regulation of gene expression, playing critical roles in genome integrity, development and metabolic pathways. Targeting of RNAs by ribonucleoprotein complexes of sRNAs bound to Argonaute (AGO) proteins results in cleaved RNAs having precise and predictable 5` ends. While tools to study sliced bits of RNAs to confirm the efficiency of sRNA-mediated regulation are available, they are sub-optimal. In this study, we provide an improvised version of a tool with better efficiency to accurately validate sRNA targets.ResultsHere, we improvised the CleaveLand tool to identify additional micro (mi)RNA targets that belong to the same family and also other targets within a specified free energy cut-off. These additional targets were otherwise excluded during the default run. We employed these tools to understand the sRNA targeting efficiency in wild and cultivated rice, sequenced degradome from two rice lines, O. nivara and O. sativa indica Pusa Basmati-1 and analyzed variations in sRNA targeting. Our results indicate the existence of multiple miRNA-mediated targeting differences between domesticated and wild species. For example, Os5NG4 was targeted only in wild rice that might be responsible for the poor secondary wall formation when compared to cultivated rice. We also identified differential mRNA targets of secondary sRNAs that were generated after miRNA-mediated cleavage of primary targets.ConclusionsWe identified many differentially targeted mRNAs between wild and domesticated rice lines. In addition to providing a step-wise guide to generate and analyze degradome datasets, we showed how domestication altered sRNA-mediated cascade silencing during the evolution of indica rice.