Awn Development Research Articles

Redefining awn development in rice through the breeding history of Japanese awn reduction.

The study challenges the conventional understanding of awn loss as a domestication syndrome, showing instead that many awned varieties continued to be widely grown in Japan until the early twentieth century and that selection for awn reduction was active at that time, demonstrating that awn loss is not a domestication syndrome but "a trait that emerged during crop improvement". Although selection for awnless mutants was carried out independently using different types of awned cultivars in the early twentieth century in Japan, awn loss was caused by the mutation in OsEPFL1. This suggests that a single mutant haplotype of OsEPFL1 was conserved in the genomes of different cultivars and subsequently selected within each line to meet the demand for awnless varieties. The study also conducts phylogenetic analyses of EPFL1 in 48 grass plants, revealing its unique involvement in awn formation in rice while potentially playing a different role in the domestication of other grass plants. Finally, an attempt is made to isolate an awn-forming gene that has not been identified from the awned rice cultivar "Omachi", which is still cultivated in Japan. The results presented in this paper provide a new perspective on domestication against the conventional understanding of awn development, shedding light on its potential as a useful organ for breeding to mitigate environmental stress.

A delicate balance: transcriptional control of awn development and yield in barley

In a recent study, Zhang et al. identified that MADS1-regulated lemma and awn development can positively regulate barley yield. This finding, alongside the demonstration that the function of MADS1 is conserved in wheat, suggests it is an important target for the improvement of Triticeae crops.

OsRR26, a type-B response regulator, modulates salinity tolerance in rice via phytohormone-mediated ROS accumulation in roots and influencing reproductive development.

OsRR26 is a cytokinin-responsive response regulator that promotes phytohormone-mediated ROS accumulation in rice roots, regulates seedling growth, spikelet fertility, awn development, represses NADPH oxidases, and negatively affects salinity tolerance. Plant two-component systems (TCS) play a pivotal role in phytohormone signaling, stress responses, and circadian rhythm. However, a significant knowledge gap exists regarding TCS in rice. In this study, we utilized a functional genomics approach to elucidate the role of OsRR26, a type-B response regulator in rice. Our results demonstrate that OsRR26 is responsive to cytokinin, ABA, and salinity stress, serving as the ortholog of Arabidopsis ARR11. OsRR26 primarily localizes to the nucleus and plays a crucial role in seedling growth, spikelet fertility, and the suppression of awn development. Exogenous application of cytokinin led to distinct patterns of reactive oxygen species (ROS) accumulation in the roots of both WT and transgenic plants (OsRR26OE and OsRR26KD), indicating the potential involvement of OsRR26 in cytokinin-mediated ROS signaling in roots. The application of exogenous ABA resulted in varied cellular compartmentalization of ROS between the WT and transgenic lines. Stress tolerance assays of these plants revealed that OsRR26 functions as a negative regulator of salinity stress tolerance across different developmental stages in rice. Physiological and biochemical analyses unveiled that the knockdown of OsRR26 enhances salinity tolerance, characterized by improved chlorophyll retention and the accumulation of soluble sugars, K+ content, and amino acids, particularly proline.

Blocking of awn development-related gene OsGAD1 coordinately boosts yield and quality of Kam Sweet Rice.

Kam Sweet Rice is a high-quality local variety of Guizhou province in China, but most varieties have awns on lemma. In this study, we aimed to obtain awnless varieties of Kam Sweet Rice by blocking the awn development-related gene OsGAD1 using CRISPR/Cas9 technology. We determined that natural variations of the OsGAD1 triggered different lengths of awns of Kam Sweet Rice. We found that the awning rate of the CRISPR lines of OsGAD1 in Guxiangnuo, Goujingao and Gouhuanggang decreased by over 65%, and the number of grains per panicle and yield per plant increased by more than 17% and 20% compared to the wild-types. Furthermore, we indicated that blocking OsGAD1 resulted in an increase of over 2% in the brown rice rate and milled rice rate in these varieties. In addition, the analysis of the transcriptome revealed that the regulation of awn development and yield formation in CRISPR lines of OsGAD1 may involve genes associated with phytohormone and nitrogen pathways. These results suggest that blocking OsGAD1 in Kam Sweet Rice using CRISPR/Cas9 technology can be used for breeding programs seeking high yield and grain quality of Kam Sweet Rice.

Anatomical and Physiological Characteristics of Awn Development in Elymus nutans, an Important Forage Grass in Qinghai-Tibet Plateau

Awns are the important structures of inflorescence in many crops that belong to the Poaceae family. In addition, they actively participate in photosynthesis, transpiration, seed dispersal, and self-planting. The Elymus nutans Griseb. is an important, self-pollinated, allohexaploid (2n = 6x = 42) and perennial native forage grass in Qinghai-Tibet Plateau that shows variation in awns length. However, the changes in the anatomical structure, physiological traits, and biochemical characteristics during awn development remain unclear in E. nutans. Therefore, this study investigated the changes in anatomical structures, enzymatic activities, and hormonal regulations of awns at four developmental stages, i.e., booting, heading, flowering, and maturity stages of three E. nutans accessions having different awn lengths. The results showed that the cross-sections of E. nutans awns had an acute triangular shape and structural similarities to wheat awns. In addition, the growth of long awns was recorded faster than short awns at the heading stage, but no significant differences in awns lengths were found at the heading, flowering, and maturity stages. The differences in the sizes of barbs and stomata of three accessions were statistically non-significant; however, the accession with long awns had more stomata than the accession with shorter awns at all developmental stages. In addition, the content of cytokinin (CTK), abscisic acid (ABA), and ethylene (ETH), and activities of superoxide dismutase (SOD) and peroxidase (POD) were significantly related to the development of awn. At the flowering stage, the content of CTK, and activities of SOD and POD of long awn accession were significantly higher than the short awn accession. Therefore, the obtained results provide a sound basis for future research on the molecular mechanisms of awn development and their potential role in E. nutans.

The TaTCP4/10–B1 cascade regulates awn elongation in wheat (Triticum aestivum L.)

Awns are important morphological markers for wheat and exert a strong physiological effect on wheat yield. The awn elongation suppressor B1 has recently been cloned through association and linkage analysis in wheat. However, the mechanism of awn inhibition centered around B1 remains to be clarified. Here, we identified an allelic variant in the coding region of B1 through analysis of re-sequencing data; this variant causes an amino acid substitution and premature termination, resulting in a long-awn phenotype. Transcriptome analysis indicated that B1 inhibited awn elongation by impeding cytokinin- and auxin-promoted cell division. Moreover, B1 directly repressed the expression of TaRAE2 and TaLks2, whose orthologs have been reported to promote awn development in rice or barley. More importantly, we found that TaTCP4 and TaTCP10 synergistically inhibited the expression of B1, and a G-to-A mutation in the B1 promoter attenuated its inhibition by TaTCP4/10. Taken together, our results reveal novel mechanisms of awn development and provide genetic resources for trait improvement in wheat.

Analysis of Domestication Loci in Wild Rice Populations.

The domestication syndrome is defined as a collection of domestication-related traits that have undergone permanent genetic changes during the domestication of cereals. Australian wild rice populations have not been exposed to gene flow from domesticated rice populations. A high level of natural variation of the sequences at domestication loci (e.g., seed shattering, awn development, and grain size) was found in Australian AA genome wild rice from the primary gene pool of rice. This natural variation is much higher than that found in Asian cultivated rice and wild Asian rice populations. The Australian Oryza meridionalis populations exhibit a high level of homozygous polymorphisms relative to domesticated rice, inferring the fixation of distinct wild and domesticated alleles. Alleles of the seed shattering genes (SH4/SHA1 and OsSh1/SH1) present in the shattering-prone O. meridionalis populations are likely to be functional, while the dysfunctional alleles of these seed shattering genes are found in domesticated rice. This confirms that unlike Asian wild rice populations, Australian wild rice populations have remained genetically isolated from domesticated rice, retaining pre-domestication alleles in their wild populations that uniquely allow the impact of domestication on the rice genome to be characterized. This study also provides key information about the domestication loci in Australian wild rice populations that will be valuable in the utilization of these genetic resources in crop improvement and de novo domestication.

Regulator of Awn Elongation 3, an E3 ubiquitin ligase, is responsible for loss of awns during African rice domestication

Two species of rice have been independently domesticated from different ancestral wild species in Asia and Africa. Comparison of mutations that underlie phenotypic and physiological alterations associated with domestication traits in these species gives insights into the domestication history of rice in both regions. Asian cultivated rice, Oryza sativa, and African cultivated rice, Oryza glaberrima, have been modified and improved for common traits beneficial for humans, including erect plant architecture, nonshattering seeds, nonpigmented pericarp, and lack of awns. Independent mutations in orthologous genes associated with these traits have been documented in the two cultivated species. Contrary to this prevailing model, selection for awnlessness targeted different genes in O. sativa and O. glaberrima. We identify Regulator of Awn Elongation 3 (RAE3) a gene that encodes an E3 ubiquitin ligase and is responsible for the awnless phenotype only in O. glaberrima. A 48-bp deletion may disrupt the substrate recognition domain in RAE3 and diminish awn elongation. Sequencing analysis demonstrated low nucleotide diversity in a ~600-kb region around the derived rae3 allele on chromosome 6 in O. glaberrima compared with its wild progenitor. Identification of RAE3 sheds light on the molecular mechanism underlying awn development and provides an example of how selection on different genes can confer the same domestication phenotype in Asian and African rice.

Two awn-development-related peptides, GAD1 and OsEPFL2, promote seed dispersal and germination in rice

In nature, seed dispersal and germination are crucial for plant survival and propagation. Elaborate mechanisms have evolved that enhance the success of these events ( Peart, 1981 Peart M.H. Further experiments on the biological significance of the seed-dispersal units in grasses. J. Ecol. 1981; 69: 425-436https://doi.org/10.2307/2259677 Crossref Google Scholar ; Bewley, 1997 Bewley J.D. Seed germination and dormancy. Plant Cell. 1997; 9: 1055-1066https://doi.org/10.1105/tpc.9.7.1055 Crossref PubMed Scopus (2151) Google Scholar ), and the widespread distribution of grass species has been associated with the evolution of these traits. Many of these mechanisms are associated with characteristics that are intrinsic to the seed, such as awn development, seed shattering, and germination ( Ntakirutimana et al., 2019 Ntakirutimana F. Xiao B. Xie W. Zhang J. Zhang Z. Wang N. Yan J. Potential effects of awn length variation on seed yield and components, seed dispersal and germination performance in Siberian Wildrye. Plants. 2019; 8: 561https://doi.org/10.3390/plants8120561 Crossref Scopus (11) Google Scholar ). Awns and/or other seed appendages often help mature seeds to disperse widely and germinate under various challenging environments ( Elbaum et al., 2007 Elbaum R. Zaltzman L. Burgert I. Fratzl P. The role of wheat awns in the seed dispersal unit. Science. 2007; 316: 884-886https://doi.org/10.1126/science.1140097 Crossref PubMed Scopus (441) Google Scholar ). For example, the awns of wheat react to a change in humidity and propel the seeds into moist soil to facilitate seed germination ( Elbaum et al., 2007 Elbaum R. Zaltzman L. Burgert I. Fratzl P. The role of wheat awns in the seed dispersal unit. Science. 2007; 316: 884-886https://doi.org/10.1126/science.1140097 Crossref PubMed Scopus (441) Google Scholar ). It has also been shown that awn length has a remarkable correlation with seed germination in some grass species ( Ntakirutimana et al., 2019 Ntakirutimana F. Xiao B. Xie W. Zhang J. Zhang Z. Wang N. Yan J. Potential effects of awn length variation on seed yield and components, seed dispersal and germination performance in Siberian Wildrye. Plants. 2019; 8: 561https://doi.org/10.3390/plants8120561 Crossref Scopus (11) Google Scholar ; Liu et al., 2018 Liu W. Wang T. Zhang S. Ding L. Wei Z. Grazing influences Stipa breviflora seed germination in desert grasslands of the Inner Mongolia Plateau. PeerJ. 2018; 6: e4447https://doi.org/10.7717/peerj.4447 Crossref Scopus (9) Google Scholar ). However, it is not known whether the pathways that promote awn development and regulate seed germination rate are linked or whether rice awns contribute to successful seed germination in nature.

Characterization of Domestication Loci Associated with Awn Development in Rice

Rice (Oryza sativa L.) is a widely studied domesticated model plant. Seed awning is an unfavorable trait during rice harvesting and processing. Hence, loss of awn was one of the target characters selected during domestication. However, the genetic mechanisms underlying awn development in rice are not well understood. In this study, we analyzed and characterized the genes for awn development using a mapping population derived from a cross between the Korean indica cultivar ‘Milyang23’ and a near-isogenic line NIL4/9 derived from a cross between ‘Hwaseong’ and Oryza minuta. Two quantitative trait loci (QTLs), qAwn4 and qAwn9, mapped on chromosomes 4 and 9, respectively, increased awn length in an additive manner. Through comparative sequencing analyses of the parental lines, LABA1 was determined as the causal gene underlying qAwn4. qAwn9 was mapped to a 199-kb physical region between markers RM24663 and RM24679. Within this interval, 27 annotated genes were identified, and five genes, including a basic leucine zipper transcription factor 76 (OsbZIP76), were considered as candidate genes for qAwn9 based on their functional annotations and sequence variations. Haplotype analysis using the candidate gene revealed tropical-japonica specific sequence variants in the qAwn9 region, which partly explains the non-detection of qAwn9 in previous studies that used progenies from interspecific crosses. This provides further evidence that OsbZIP76 is possibly a causal gene for qAwn9. The O. minuta qAwn9 allele was identified as a major QTL, providing an important molecular target for understanding the genetic control of awn development in rice. Our results lay the foundation for further cloning of the awn gene underlying qAwn9.

Transcriptome and Metabolome Analysis Provides Insights into the Heterosis of Yield and Quality Traits in Two Hybrid Rice Varieties (Oryza sativa L.).

Heterosis is a common biological phenomenon that is useful for breeding superior lines. Using heterosis to increase the yield and quality of crops is one of the main achievements of modern agricultural science. In this study, we analysed the transcriptome and metabolome of two three-line hybrid rice varieties, Taiyou 871 (TY871), and Taiyou 398 (TY398) and the parental grain endosperm using RNA-seq (three biological repeats per variety) and untargeted metabolomic (six biological repeats per variety) methods. TY871 and TY398 showed specific heterosis in yield and quality. Transcriptome analysis of the hybrids revealed 638 to 4059 differentially expressed genes in the grain when compared to the parents. Metabolome analysis of the hybrids revealed 657 to 3714 differential grain metabolites when compared to the parents. The honeydew1 and grey60 module core genes Os04g0350700 and Os05g0154700 are involved in the regulation of awn development, grain size, and grain number, as well as the regulation of grain length and plant height, respectively. Rice grain length may be an important indicator for improving the quality of three-line hybrid rice. In addition, the rice quality-related metabolite NEG_M341T662 was highly connected to the module core genes Os06g0254300 and Os03g0168100. The functions of Os06g0254300 and Os03g0168100 are EF-hand calcium binding protein and late embroideries absolute protein repeat containing protein, respectively. These genes may play a role in the formation of rice quality. We constructed a gene and metabolite coexpression network, which provides a scientific basis for the utilization of heterosis in producing high-yield and high-quality hybrid rice.

DOMINANT AWN INHIBITOR Encodes the ALOG Protein Originating from Gene Duplication and Inhibits AWN Elongation by Suppressing Cell Proliferation and Elongation in Sorghum.

The awn, a needle-like structure extending from the tip of the lemma in grass species, plays a role in environmental adaptation and fitness. In some crops, awns appear to have been eliminated during domestication. Although numerous genes involved in awn development have been identified, several dominant genes that eliminate awns are also known to exist. For example, in sorghum (Sorghum bicolor), the dominant awn-inhibiting gene has been known since 1921; however, its molecular features remain uncharacterized. In this study, we conducted quantitative trait locus analysis and a genome-wide association study of awn-related traits in sorghum and identified DOMINANT AWN INHIBITOR (DAI), which encodes the ALOG family protein on chromosome 3. DAI appeared to be present in most awnless sorghum cultivars, likely because of its effectiveness. Detailed analysis of the ALOG protein family in cereals revealed that DAI originated from a duplication of its twin paralog (DAIori) on chromosome 10. Observations of immature awns in near-isogenic lines revealed that DAI inhibits awn elongation by suppressing both cell proliferation and elongation. We also found that only DAI gained a novel function to inhibit awn elongation through an awn-specific expression pattern distinct from that of DAIori. Interestingly, heterologous expression of DAI with its own promoter in rice inhibited awn elongation in the awned cultivar Kasalath. We found that DAI originated from gene duplication, providing an interesting example of gain-of-function that occurs only in sorghum but shares its functionality with rice and sorghum.

Phenotype and Genotype Interaction Underlying Distributive Characteristic for Awn Development in Rice.

As a wild ancestor of cultivated rice, Oryza rufipogon is domesticated into cultivated rice Oryza sativa, many agricultural traits are newly created or disappear. In particular, in wild rice, awn protects from predators and is easily blown by the wind and used as a means of propagation. However, awns gradually disappeared as they were breeding from wild rice to cultivated rice. Since awn development is disadvantageous to rice yield, it is important to understand the genetic basis related to awn development. In addition, characterization of the genes associated with awn development is helpful in analyzing the genetic relationships of rice from ancient times to the present for the regulatory mechanisms of awn formation. QTL analysis identified RM14330-RM218 on chromosome 3 using a 120 Cheongcheong/Nagdong double haploid population. Through screening of genes related to awn development in RM-14330-RM218, it is indicated that OsDRPq3 is a causal gene that can be involved in awn development. OsDRPq3 transcription level is maintained high in long awn and less yield populations during the panicle formation stage, the period during awn development. Moreover, the sequence of OsDRPq3 has high homology with the drooping protein leaf. This study provides a new resource for phylogenetic research of rice and exploration of awn development.

Mapping and Characterization of QTLs for Awn Morphology Using Crosses between "Double-Awn" Wheat 4045 and Awnless Wheat Zhiluowumai.

Awns play important roles in seed dispersal, protection against predators, and photosynthesis. The characterization of genes related to the formation of awns helps understand the regulation mechanisms of awn development. In the present study, the “double-awn” wheat 4045, which features super-long lemma awns and long glume awns, and an awnless wheat line, Zhiluowumai, were used to investigate QTLs or genes involved in awn development. QTL analysis identified three loci—Qawn-1D, Qawn-5A, and Qawn-7B—using a population of 101 4045 × ZLWM F2 plants. Fine mapping with a total of 9018 progenies narrowed the mapping interval of Qawn-5A to an 809-kb region, which was consistent with the B1 locus, containing five genes on chromosome 5AL. Gene structure and expression analysis indicated that TraesCS5A02G542800 was the causal gene, which was subsequently verified by overexpression of TraesCS5A02G542800 in a “double-awn” wheat, Yangmai20. The retained “double-awn” phenotype of transgenic plants suggested that B1 represses the elongation but does not influence the emergence of the awns. Moreover, 4045 harbors a new allele of B1 with a 261-bp insertion in the promoter region and a lack of the EAR2 motif in the encoding region, which influences several important agronomic traits. In this study, we identify two novel QTLs and a novel allele of B1, providing new resources for exploration of awn development.

Formation of awns in wheat lines with introgressions from Aegilops spp. caused by novel regulatory genes

Awns play a significant role in the plant physiology and are a well-known morphological marker in wheat. Awnedness in wheat is regulated by three inhibitors – Hd, B1, and B2, but awn promoters are still largely unknown. The present study is aimed at analysis of the expression level of awn development regulatory genes orthologs, TaDL, TaTOB1, TaETT2, and TaKNOX3, in Triticum aestivum, genome substitution amphidiploids AABBSshSsh and AABBUU, and derived lines with introgressions from Aegilops sharonensis and Ae. umbellulata.Expression of four mentioned genes was detected in the lemma of all strains studied, and the role of these genes in awn development was assumed. In awned introgression lines, expression of all studied genes differed from mid-parent value: it was present in parent genotypes and absent in derived lines. Non-additive expression of four studied genes in introgression lines is considered to be the possible reason that caused development of nonparental awned phenotype. The presence of two products resulting from TaTOB1 cDNA amplification, one of which contained fourth intron and another lacking it, is considered to be the result of two mRNA presence due to different TaTOB1 homoeoalleles expression.

Gene duplication drove the loss of awn in sorghum

Loss of the awn in some cereals, including sorghum, is a key transition during cereal domestication or improvement that has facilitated grain harvest and storage. The genetic basis of awn loss in sorghum during domestication or improvement remains unknown. Here, we identified the awn1 gene encoding a transcription factor with the ALOG domain that is responsible for awn loss during sorghum domestication or improvement. awn1 arose from a gene duplication on chromosome 10 that translocated to chromosome 3, recruiting a new promoter from the neighboring intergenic region filled with “noncoding DNA” and recreating the first exon and intron. awn1 acquired high expression after duplication and represses the elongation of awns in domesticated sorghum. Comparative mapping revealed high collinearity at the awn1 paralog locus on chromosome 10 across cereals, and awn growth and development were successfully reactivated on the rice spikelet by inactivating the rice awn1 ortholog. RNA-seq and DAP-seq revealed that as a transcriptional repressor, AWN1 bound directly to a motif in the regulatory regions of three MADS genes related to flower development and two genes, DL and LKS2, involved in awn development. AWN1 downregulates the expression of these genes, thereby repressing awn elongation. The preexistence of regulatory elements in the neighboring intergenic region of awn1 before domestication implicates that noncoding DNA may serve as a treasure trove for evolution during sorghum adaptation to a changing world. Taken together, our results suggest that gene duplication can rapidly drive the evolution of gene regulatory networks in plants.

Identification and characterization of An-4, a potential quantitative trait locus for awn development in rice

BackgroundAwn of rice is an important domestication trait closely associated with yield traits. Therefore, the identification of genes for awn development is of great significance for the elucidation of molecular mechanism of awn development and the genetic improvement of yield traits in rice.ResultsIn this study, using chromosome segment substitution lines (CSSLs) derived from a long-awned Guangxi common wild rice (GXCWR, Oryza rufipogon Griff.) and a short-awned indica cultivar 9311, we identified An-4, a potential quantitative trait locus (QTL) for awn development. Then, An-4 was fine mapped into a 56-kb region of chromosome 2, which contained four annotated genes. Among these four annotated genes, Os02g0594800 was concluded to be the potential candidate gene for An-4. An-4 exhibited pleiotropic effects on awn development and several yield traits. Scanning electron microscopy (SEM) analysis showed that An-4 significantly promoted awn development at Sp7 and Sp8 stage of spikelet development. Transcriptome analysis suggested that An-4 might influence the development of awn by regulating the expression of genes related to growth, developmental process, channel regulation and extracellular region. By contrast to those of 9311, the expression level of OsRR5 in CSSL128 was significantly down-regulated, whereas the expression levels of OsCKX2 and OsGA2ox5 in CSSL128 were significantly up-regulated. In addition, our study showed that An-4 had additive effects with other genes for awn development, such as An-1, An-2/LABA1 and An-3/GAD1/RAE2.ConclusionsThe identification of An-4 lays a foundation for cloning of An-4 and further elucidation of the molecular mechanism of awn development. Moreover, the identification of favorable allelic variation of An-4 from 9311 will be useful to improve rice yield traits.

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.

Exploring the Loci Responsible for Awn Development in Rice through Comparative Analysis of All AA Genome Species

Wild rice species have long awns at their seed tips, but this trait has been lost through rice domestication. Awn loss mitigates harvest and seed storage; further, awnlessness increases the grain number and, subsequently, improves grain yield in Asian cultivated rice, highlighting the contribution of the loss of awn to modern rice agriculture. Therefore, identifying the genes regulating awn development would facilitate the elucidation of a part of the domestication process in rice and increase our understanding of the complex mechanism in awn morphogenesis. To identify the novel loci regulating awn development and understand the conservation of genes in other wild rice relatives belonging to the AA genome group, we analyzed the chromosome segment substitution lines (CSSL). In this study, we compared a number of CSSL sets derived by crossing wild rice species in the AA genome group with the cultivated species Oryza sativa ssp. japonica. Two loci on chromosomes 7 and 11 were newly discovered to be responsible for awn development. We also found wild relatives that were used as donor parents of the CSSLs carrying the functional alleles responsible for awn elongation, REGULATOR OF AWN ELONGATION 1 (RAE1) and RAE2. To understand the conserveness of RAE1 and RAE2 in wild rice relatives, we analyzed RAE1 and RAE2 sequences of 175 accessions among diverse AA genome species retrieved from the sequence read archive (SRA) database. Comparative sequence analysis demonstrated that most wild rice AA genome species maintained functional RAE1 and RAE2, whereas most Asian rice cultivars have lost either or both functions. In addition, some different loss-of-function alleles of RAE1 and RAE2 were found in Asian cultivated species. These findings suggest that different combinations of dysfunctional alleles of RAE1 and RAE2 were selected after the speciation of O. sativa, and that two-step loss of function in RAE1 and RAE2 contributed to awnlessness in Asian cultivated rice.

Is a long hygroscopic awn an advantage for Themeda triandra in drier areas?

Themeda triandra has bigeniculate hygroscopic lemma seed awns that twist when wet and drying, thereby transporting the caryopsis across the soil surface into suitable germination microsites. The prediction that awns would be longer in drier grassland and have greater motility to enable them to move quickly and farther to find scarce germination sites was tested in KwaZulu-Natal. Awns (n = 100) were collected from 16 sites across a mean annual precipitation gradient (575–1 223 mm), ranging from 271−1 097 m asl. The daily movement of hydrated long and short awns (n = 10) across blotting paper was tracked for five days, and the rotational speed of anchored awns was measured. Awn length varied considerably (mean: 41.4–63.2 mm; SD: 3.44–8.99), but tended to increase (r = 0.426, p = 0.099) not decline, with increasing MAP. Awn length was unrelated to elevation, temperature and aridity indices. Long awns rotated at the same rate (2 min 48 sec), but moved twice as fast (46.3 vs 22.1 mm day−1) and much farther (maximum: 82.1 vs 38.6 mm day−1) than short awns. Whether moisture limits awn development, the benefit of longer awns to negotiate densely tufted mesic grassland, and the multifunctionality of awns require investigation.