Grass Spikelet Research Articles

Rice AGL1 determines grain size and sterile lemma identity

The grass spikelet is a unique inflorescence structure that determines grain size. Although many genetic factors have been well characterized for grain size and glume development, the underlying molecular mechanisms in rice are far from established. Here, we isolated rice gene, AGL1 that controlled grain size and determines the fate of the sterile lemma. Loss of function of AGL1 produced larger grains and reduced the size of the sterile lemma. Larger grains in the agl1 mutant were caused by a larger number of cells that were longer and wider than in the wild type. The sterile lemma in the mutant spikelet was converted to a rudimentary glume-like organ. Our findings showed that the AGL1 (also named LAX1) protein positively regulated G1 expression, and negatively regulated NSG1 expression, thereby affecting the fate of the sterile lemma. Taken together, our results revealed that AGL1 played a key role in negative regulation of grain size by controlling cell proliferation and expansion, and supported the opinion that rudimentary glume and sterile lemma in rice are homologous organs.

Diverse ecological functions and the convergent evolution of grass awns.

The awn of grasses is a long, conspicuous outgrowth of the floral bracts in a grass spikelet. It is known to impact agricultural yield, but we know little about its broader ecological function, nor the selective forces that lead to its evolution. Grass awns are phenotypically diverse across the extant ~12,000 species of Poaceae. Awns have been lost and gained repeatedly over evolutionary time, between and within lineages, suggesting that they could be under selection and might provide adaptive benefit in some environments. Despite the phylogenetic context, we know of no studies that have tested whether the origin of awns correlates with putative selective forces on their form and function. Presence or absence of awns is not plastic; rather, heritability is high. The awns of grasses often are suggested as adaptations for dispersal, and most experimental work has been aimed at testing this hypothesis. Proposed dispersal functions include soil burial, epizoochory, and aerial orientation. Awns may also protect the seed from drought, herbivores, or fire by helping it become buried in soil. We do not fully understand the fitness or nutrient costs of awn production, but in some species awns function in photosynthesis, providing carbon to the seed. Here we show that awns likely provide an adaptive advantage, but argue that studies on awn function have lacked critical phylogenetic information to demonstrate adaptive convergent evolution, are taxonomically biased, and often lack clear alternative hypotheses.

A Multidisciplinary Approach to Human–Environmental Interactions at the Roman-Byzantine Ibida Fortress (Dobrogea, South-Eastern Romania)

ABSTRACT Situated in the northern part of Dobrogea which belonged to Scythia Minor province, Ibida was the largest fortification in this region during Roman-Byzantine times, between the 4th and 6th centuries. This study investigates the usefulness of multidisciplinary analysis on the territory of Ibida fortress by integrating faunal data and phytolith records in relation to soil characteristics to understand past human–environmental interactions. One of the four soil profiles excavated within the fortress territory was sampled for phytolith analysis indicated the domination of grasses. The habitation phase corresponding to the Roman-Byzantine period was identified at 1.65 m depth on the Slava Rusă floodplain. This was the richest level in artefacts and elongate dendritic phytoliths, also suggesting an important accumulation of grass spikelets in the cultural layer. This could be explained either by the presence of cereals in the dwelling level or by the presence of non-cereal grasses used for animal fodder. The faunal remains were collected from various archaeological contexts (i.e. intramuros, wall and extramuros). The identification of numerous cattle and sheep/goat remains and the soil characteristics more likely suggest a pastoral use of the land from fortress proximity, where the cultivation of cereals seems to play a less important role.

A New Genus and Species of Grass, Eograminis balticus (Poaceae: Arundinoideae), in Baltic Amber

Premise of research. This investigation was undertaken to describe a multifloral grass spikelet enclosed in Baltic amber. The fossil, described in the new genus Eograminis, represents the first definite grass to be described from Baltic amber as well as the first fossil member of the subfamily Arundinoideae. The discovery adds a new plant group to the extensive flora previously described from Baltic amber and provides insights into the ecology of the forest habitat. It also supplies morphological and developmental features that existed in early Cenozoic grasses and establishes a calibration point for future phylogenetic studies regarding the origin and splitting of genera in the subtribe Moliniinae. Methodology. The piece of amber containing the fossil was reshaped with a diamond blade lapidary trim saw, a belt sander, and a bench polisher until it was possible to view the grass spikelet from all angles. Because of the excellent preservation of the spikelet, observations of the glumes, paleae, and lemmas could be made under direct light with a stereoscopic and compound microscope. Pivotal results. After the features of the spikelet are characterized, it is described as Eograminis balticus gen. et sp. nov. (Poaceae: Arundinoideae). The new genus is characterized by the presence of stomata with well-developed dome-shaped subsidiary cells, parallel guard cells, and rows of crenulated and noncrenulated epidermal cells with long and short cells and silica bodies. Conclusions. The spikelet has some features of members of the extant genus Molinia Schrank in the tribe Molinieae, subtribe Moliniinae, a wetland genus with extant species concentrated around the Baltic Sea. However, the sparse scabrosity over the lemma veins, minute or absent keel hooks on the paleae, and bicellular or absent microhairs of Molinia spp. separate it from the fossil. The presence of an immature orthopteran and an “Alternaria-like fungal spore” associated with the fossil grass provides data on the microhabitat of the fossil grass.

The use of plant ontologies in comparative and evolutionary studies should be flexible

The use of plant ontologies in comparative and evolutionary studies should be flexible

Pilar Catalán.

Pilar Catalán.

High-throughput phenotyping.

High-throughput phenotyping.

Loss of function of OsMADS34 leads to large sterile lemma and low grain yield in rice (Oryza sativa L.)

Rice (Oryza sativa L.) is one of the most important food crops, especially in Asia. The spikelet is a characteristic structure of grass inflorescences that determines crop output. However, the molecular mechanism that controls spikelet development and grain yield in rice remains unclear. In this study, we isolated a new osmads34 allelic mutant (i.e., osmads34-t). The osmads34-t mutant showed more primary branch numbers, short panicles, and long sterile lemmas. The sterile lemmas were transformed into the lemmas and had the lemma identity in the osmads34-t mutant, suggesting that the sterile lemma and lemma are homologous organs. Additionally, osmads34-t displayed smaller grains on its secondary branches of panicles and a lower seed-setting rate. These results suggest that OsMADS34 plays an important role in determination of grain size and yield in rice. OsMADS34 was expressed in tested organs and tissues, and its green fluorescent protein (GFP) signal was located in the nucleus. The result of this study will be used to understand the identity of unique organs in grass spikelets and may improve grain yield in breeding practice.

Jasmonic acid regulates spikelet development in rice

The spikelet is the basal unit of inflorescence in grasses, and its formation is crucial for reproductive success and cereal yield. Here, we report a previously unknown role of the plant hormone jasmonic acid (JA) in determining rice (Oryza sativa) spikelet morphogenesis. The extra glume 1 (eg1) and eg2 mutants exhibit altered spikelet morphology with changed floral organ identity and number, as well as defective floral meristem determinacy. We show that EG1 is a plastid-targeted lipase that participates in JA biosynthesis, and EG2/OsJAZ1 is a JA signalling repressor that interacts with a putative JA receptor, OsCOI1b, to trigger OsJAZ1's degradation during spikelet development. OsJAZ1 also interacts with OsMYC2, a transcription factor in the JA signalling pathway, and represses OsMYC2's role in activating OsMADS1, an E-class gene crucial to the spikelet development. This work discovers a key regulatory mechanism of grass spikelet development and suggests that the role of JA in reproduction has diversified during the flowering plant evolution.

Duplication and diversification of the LEAFY HULL STERILE1 and Oryza sativa MADS5 SEPALLATA lineages in graminoid Poales

BackgroundGene duplication and the subsequent divergence in function of the resulting paralogs via subfunctionalization and/or neofunctionalization is hypothesized to have played a major role in the evolution of plant form. The LEAFY HULL STERILE1 (LHS1) SEPALLATA (SEP) genes have been linked with the origin and diversification of the grass spikelet, but it is uncertain 1) when the duplication event that produced the LHS1 clade and its paralogous lineage Oryza sativa MADS5 (OSM5) occurred, and 2) how changes in gene structure and/or expression might have contributed to subfunctionalization and/or neofunctionalization in the two lineages.MethodsPhylogenetic relationships among 84 SEP genes were estimated using Bayesian methods. RNA expression patterns were inferred using in situ hybridization. The patterns of protein sequence and RNA expression evolution were reconstructed using maximum parsimony (MP) and maximum likelihood (ML) methods, respectively.ResultsPhylogenetic analyses mapped the LHS1/OSM5 duplication event to the base of the grass family. MP character reconstructions estimated a change from cytosine to thymine in the first codon position of the first amino acid after the Zea mays MADS3 (ZMM3) domain converted a glutamine to a stop codon in the OSM5 ancestor following the LHS1/OSM5 duplication event. RNA expression analyses of OSM5 co-orthologs in Avena sativa, Chasmanthium latifolium, Hordeum vulgare, Pennisetum glaucum, and Sorghum bicolor followed by ML reconstructions of these data and previously published analyses estimated a complex pattern of gain and loss of LHS1 and OSM5 expression in different floral organs and different flowers within the spikelet or inflorescence.ConclusionsPrevious authors have reported that rice OSM5 and LHS1 proteins have different interaction partners indicating that the truncation of OSM5 following the LHS1/OSM5 duplication event has resulted in both partitioned and potentially novel gene functions. The complex pattern of OSM5 and LHS1 expression evolution is not consistent with a simple subfunctionalization model following the gene duplication event, but there is evidence of recent partitioning of OSM5 and LHS1 expression within different floral organs of A. sativa, C. latifolium, P. glaucum and S. bicolor, and between the upper and lower florets of the two-flowered maize spikelet.

MADS‐box gene expression and implications for developmental origins of the grass spikelet

Basic questions regarding the origin and evolution of grass (Poaceae) inflorescence morphology remain unresolved, including the developmental genetic basis for evolution of the highly derived outer spikelet organs. To evaluate homologies between the outer sterile organs of grass spikelets and inflorescence structures of nongrass monocot flowers, we describe expression patterns of APETALA1/FRUITFULL-like (AP1/FUL) and LEAFY HULL STERILE-like (LHS1) MADS-box genes in an early-diverging grass (Streptochaeta angustifolia) and a nongrass outgroup (Joinvillea ascendens). AP1/FUL-like genes are expressed only in floral organs of J. ascendens, supporting the hypothesis that they mark the floral boundary in nongrass monocots, and JaLHS1/OsMADS5 is expressed in the inner and outer tepals, stamen filaments and pistil. In S. angustifolia, SaFUL2 is expressed in all 11 (or 12) bracts of the primary inflorescence branch, but not in the suppressed floral bract below the abscission zone. In contrast, SaLHS1 is only expressed in bracts 6-11 (or 12). Together, these data are consistent with the hypotheses that (1) bracts 1-5 of S. angustifolia primary inflorescence branches and glumes of grass spikelets are homologous and that (2) the outer tepals of immediate grass relatives, bracts 6-8 of S. angustifolia, and the lemma/palea are homologous, although other explanations are possible.

Reproductive morphology of the early-divergent grass Streptochaeta and its bearing on the homologies of the grass spikelet

Reproductive morphology and development are described in the Brazilian grass Streptochaeta spicata, in order to assess the homologies of the characteristic grass inflorescence, termed a spikelet, and other reproductive organs. Streptochaeta possesses some features that are commonly found in Poaceae, including a well-differentiated embryo. It also possesses some relatively unusual, presumably derived features, such as non-plumose stigmas, which indicate that it could be insect-pollinated. It shares some features with other early-divergent grasses, such as Pharus, which could represent plesiomorphic conditions for grasses. The inflorescence unit in Streptochaeta has been interpreted as a compound branching system or pseudospikelet. The present data suggest that it is a highly modified spikelet, with a modified flower borne either on a different axis to the basal bracts (glumes) or on the same axis as the basal bracts. The three bracts below the stamens are interpreted as homologous to the lodicules. The Streptochaeta spikelet could be considered as morphologically intermediate between the true spikelet of grasses and reproductive units of close grass relatives.

Floral Development and Embryology in the Early‐Divergent Grass Pharus

A detailed study of floral ontogeny, anatomy, and embryology in two (of six) species of Pharus is presented as part of a series of comparative investigations on early‐divergent grasses. Pharus is a taxonomically isolated genus belonging to the earliest‐diverging grass lineage with a true grass spikelet. It is unusual in possessing remarkably dimorphic florets: male florets possess two lodicules, six stamens, and a pistillode, whereas female florets lack lodicules entirely but possess six staminodes and a tricarpellary ovary with three stigmas. The rudimentary lodicules in male florets are initiated after the stamen whorls. There are most commonly six androecial organs, but in some florets, a five‐staminate condition was observed, resulting from suppression of the abaxial stamen from the inner whorl, or even a four‐staminate condition resulting from subsequent fusion of the two adaxial outer stamens (i.e., elements of both whorls). Thus, the pattern of floral zygomorphy in Pharus differs from that of many other grasses. Centrifixed anther attachment is reported for the first time in Pharus, resembling the condition in another early‐divergent grass, Anomochloa, though anthers are introrse in Anomochloa compared with latrorse in Pharus. Anther wall development is of the reduced type in Pharus, in contrast to most other monocots. Microsporogenesis is of the successive type, as in many other monocots. The ovary develops from three distinct primordia and is unilocular with a single ovule and a pronounced ovary beak that is highly characteristic of Pharus. There is a hollow style, in contrast to the solid styles that are common in many other grasses. The embryo is highly differentiated, as in other grasses, with a distinct epiblast and a small cleft between the scutellum and the coleorhiza.

Grass Spikelet Genetics and Duplicate Gene Comparisons

The grass spikelet contains floral organs such as stamens and carpels, as well as additional organs, glumes, lemma, palea, and lodicules that are unique to the angiosperms. Recent work is beginning to shed light onto the genes that regulate the development of spikelet organs. Many of these genes are members of multigene families. These multigene families have formed through gene duplications, some of which are specific to the grass family. I review the recent progress made in understanding the genetics of grass spikelet development. As many of these genes begin to be isolated, they will likely be compared with homologous genes from other angiosperms. Given that some of these genes will be duplicate genes, I provide a set of examples showing how orthologues and paralogues should be compared.

Reconstructing the evolutionary history of paralogous APETALA1/FRUITFULL-like genes in grasses (Poaceae).

Gene duplication is an important mechanism for the generation of evolutionary novelty. Paralogous genes that are not silenced may evolve new functions (neofunctionalization) that will alter the developmental outcome of preexisting genetic pathways, partition ancestral functions (subfunctionalization) into divergent developmental modules, or function redundantly. Functional divergence can occur by changes in the spatio-temporal patterns of gene expression and/or by changes in the activities of their protein products. We reconstructed the evolutionary history of two paralogous monocot MADS-box transcription factors, FUL1 and FUL2, and determined the evolution of sequence and gene expression in grass AP1/FUL-like genes. Monocot AP1/FUL-like genes duplicated at the base of Poaceae and codon substitutions occurred under relaxed selection mostly along the branch leading to FUL2. Following the duplication, FUL1 was apparently lost from early diverging taxa, a pattern consistent with major changes in grass floral morphology. Overlapping gene expression patterns in leaves and spikelets indicate that FUL1 and FUL2 probably share some redundant functions, but that FUL2 may have become temporally restricted under partial subfunctionalization to particular stages of floret development. These data have allowed us to reconstruct the history of AP1/FUL-like genes in Poaceae and to hypothesize a role for this gene duplication in the evolution of the grass spikelet.

Evidence for distinct roles of the SEPALLATA gene LEAFY HULL STERILE1 in Eleusine indica and Megathyrsus maximus (Poaceae)

LEAFY HULL STERILE1 (LHS1) is an MIKC-type MADS-box gene in the SEPALLATA class. Expression patterns of LHS1 homologs vary among species of grasses, and may be involved in determining palea and lemma morphology, specifying the terminal floret of the spikelet, and sex determination. Here we present LHS1 expression data from Eleusine indica (subfamily Chloridoideae) and Megathyrsus maximus (subfamily Panicoideae) to provide further insights into the hypothesized roles of the gene. E. indica has spikelets with three to eight florets that mature acropetally; E. indica LHS1 (EiLHS1) is expressed in the palea and lemma of all florets. In contrast, M. maximus has spikelets with two florets that mature basipetally; M. maximus LHS1 (MmLHS1) is expressed in the palea and lemma of the distal floret only. These data are consistent with the hypothesis that LHS1 plays a role in determining palea and lemma morphology and specifies the terminal floret of basipetally maturing grass spikelets. However, LHS1 expression does not correlate with floret sex expression; MmLHS1 is restricted to the bisexual distal floret, whereas EiLHS1 is expressed in both sterile and bisexual floret meristems. Phylogenetic analyses reconstruct a complex pattern of LHS1 expression evolution in grasses. LHS1 expression within the gynoecium has apparently been lost twice, once before diversification of a major clade within tribe Paniceae, and once in subfamily Chloridoideae. These data suggest that LHS1 has multiple roles during spikelet development and may have played a role in the diversification of spikelet morphology.

Gondwanan evolution of the grass alliance of families (Poales).

Phylogenetic interrelationships among all 18 families of Poales were assessed by cladistic analysis of chloroplast DNA rbcL and atpB sequences from 65 species. There are two well-supported main clades; the graminoid clade with Poaceae (grasses), Anarthriaceae, Centrolepidaceae, Ecdeiocoleaceae, Flagellariaceae, Joinvilleaceae, and Restionaceae; and the cyperoid clade with Cyperaceae, Juncaceae, and Thurniaceae. A sister group relationship between Poaceae and Ecdeiocoleaceae is identified with strong support. The sister group of this pair is Joinvilleaceae. These relationships help in elucidating the evolution of grasses and the grass spikelet. Dating of the tree was done by nonparametric rate smoothing of rbcL molecular evolution. Most Poales families date back to the Cretaceous >65 million years ago (mya). Dispersal-vicariance analysis indicates that the Poales originated in South America, the cyperoid clade in West Gondwana (South America or Africa), and the graminoid clade in East Gondwana (Australia). The Trans-Antarctic connection between South America and Australia, and its breakup about 35 mya, probably influenced the evolution of the Poales and the graminoid clade in particular, leading to vicariance between the continents, but the separation of Africa from the other Gondwanan areas, completed about 105 mya, is too old for such a relation.

Studies into Abstract Properties of Individuals. VII. Emergence in Hesperostipa comata and Three Species of Achnatherum (Poaceae)

Emergence is examined in populations of Hesperostipa comata, and this grass is compared to individuals, populations, and three additional species of Achnatherum. The Achnatherum species are closely related and more distantly related to H. comata. Emergent properties are assessed through measurements of grass spikelets and the divergence from vectors of isometry as expressed in individuals, populations, species, and species groups. Our results demonstrate that the strongest degrees of emergence are expressed at the levels of individuals, species, and groups of species, with the strongest of all being seen when H. comata is compared with the trio of Achnatherum species; populations express the least emergence. This reflects evolutionary differentiation as expressed through the ontogenetic events that led to the formation of spikelets. It can be related to concepts of local and global time; the flow of energy, matter, and information; and the driving force of the expression of novelty (entropy) in organized and historically constrained biological systems.

GONDWANAN EVOLUTION OF THE GRASS ALLIANCE OF FAMILIES (POALES)

Phylogenetic interrelationships among all 18 families of Poales were assessed by cladistic analysis of chloroplast DNA rbcL and atpB sequences from 65 species. There are two well-supported main clades; the graminoid clade with Poaceae (grasses), Anarthriaceae, Centrolepidaceae, Ecdeiocoleaceae, Flagellariaceae, Joinvilleaceae, and Restionaceae; and the cyperoid clade with Cyperaceae, Juncaceae, and Thurniaceae. A sister group relationship between Poaceae and Ecdeiocoleaceae is identified with strong support. The sister group of this pair is Joinvilleaceae. These relationships help in elucidating the evolution of grasses and the grass spikelet. Dating of the tree was done by nonparametric rate smoothing of rbcL molecular evolution. Most Poales families date back to the Cretaceous >65 million years ago (mya). Dispersal-vicariance analysis indicates that the Poales originated in South America, the cyperoid clade in West Gondwana (South America or Africa), and the graminoid clade in East Gondwana (Australia). The Trans-Antarctic connection between South America and Australia, and its breakup about 35 mya, probably influenced the evolution of the Poales and the graminoid clade in particular, leading to vicariance between the continents, but the separation of Africa from the other Gondwanan areas, completed about 105 mya, is too old for such a relation.

Grass spikelets: a thorny problem

Isolation of a maize gene with strong homologies to the Arabidopsis floral organ identity gene APETALA2 has provided new insights to understanding the generation of architecture in grass inflorescences. Grass inflorescences are built of repeated units called spikelets, which consist of a pair of glumes (bracts) enclosing a cluster of one to as many as 40 flowers, the number depending on the species. The gene indeterminate spikelet (ids), isolated by Chuck and co-workers,(1) is involved in limiting the number of floral meristems produced by the spikelet meristem in maize inflorescences. Altered regulation of activity of ids homologues may be responsible for variation in flower number among different grasses. BioEssays 20:789–793, 1998. © 1998 John Wiley & Sons, Inc.