California perennial grasses are physiologically distinct from both Mediterranean annual and perennial grasses

In the Central Valley of California, native perennial grass species have been largely replaced by Eurasian annual species, while in many parts of the Mediterranean Basin native perennial grasses continue to dominate, even on disturbed or degraded sites. We assessed whether differences in summer rainfall patterns have lead to the development of different plant-water strategies between grasses from these two regions. We compared six measures of plant-water physiology for three guilds of grasses: California perennial grasses, Mediterranean perennial grasses, and Mediterranean annual grasses. Discriminant analysis distinguished between the three guilds; Mediterranean perennial grasses were characterized by a more conservative water-relations physiology than Mediterranean annual grasses, whereas California perennial grasses were in some ways intermediate between the two Mediterranean grass guilds. For individual traits, California perennial grasses were either intermediate or more like Mediterranean annuals than Mediterranean perennials. Our results suggest California perennials are more drought tolerant than Mediterranean annuals but less drought tolerant than Mediterranean perennials, despite the fact that California??s Central Valley has a more intense summer drought than the Mediterranean Basin. These patterns may help explain why Mediterranean annuals, but not Mediterranean perennials, have been more successful invaders of interior California grasslands.     

Exotic herbivores directly facilitate the exotic grasses they graze: mechanisms for an unexpected positive feedback between invaders

The ability of an exotic species to establish in a system may depend not only on the invasibility of the native community, but also on its interactions with other exotic species. Though examples of mutually beneficial interactions between exotic species are known, few studies have quantified these effects or identified specific mechanisms. We used the co-invasion of an endangered island ecosystem by exotic Canada geese (Branta canadensis) and nine exotic annual grasses to study the effects of an invading herbivore on the success of invading grasses. On our study islands in southwestern Canada, we found that geese fed selectively on the exotic grasses and avoided native forbs. Counter to current theory suggesting that the grasses should be limited by a selective enemy, however, the grasses increased in proportional abundance under grazing whereas forbs showed declining abundance. Testing potential mechanisms for the effects of grazing on grasses, we found that the grasses produced more stems per area when grazing reduced vegetation height and prevented litter accumulation. Forming dense mats of short stems appeared to be an efficient reproductive and competitive strategy that the Eurasian grasses have evolved in the presence of grazers, conferring a competitive advantage in a system where the native species pool has very few annual grasses and no grazers. Germination trials further demonstrated that selective herbivory by geese enables their dispersal of exotic grass seed between heavily invaded feeding areas and the small islands used for nesting. In summary, the exotic geese facilitated both the local increase and the spatial spread of exotic grasses, which in turn provided the majority of their diet. This unexpected case of positive feedback between exotic species suggests that invasion success may depend on the overall differences between the evolutionary histories of the invaders and the evolutionary history of the native community they enter. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Changes in plant form and function across altitudinal and wetness gradients in the wetlands of the Maloti-Drakensberg,South Africa

A survey of 93 wetlands in six catchments across the Maloti-Drakensberg is used to assess the distribution of plant functional types across altitudinal and wetness gradients. Altitudes range from 1,000 to 3,200 m a.s.l. Within each catchment, the wetlands were selected to cover the complete range in altitude and wetland types. In each of the selected wetlands, vegetation was sampled in 3 by 3 m quadrats covering the entire range of wetness represented in the wetland, from temporarily wet to permanently inundated soils. Plant species were allocated to one of 11 different functional types (examples are C₃ grasses, C₄ sedges, rosette plants, and shrubs), and the proportion of the vegetation in each sample occupied by each functional type was calculated from the species’ abundances. Canonical Correspondence Analysis shows that “wetness” clearly has the highest impact on the distribution of functional types, followed by altitude. The most important plant functional types in wetlands are grasses and sedges, however, at higher altitudes, forbs (especially rosette plants) and bulbous plants become a more prominent feature in the wetlands. The total amount of graminoids gradually decreases with altitude. The general trend is that sedges tend to increase with increasing wetness and C₃ plants (grasses and sedges) increase with increasing altitude, but these effects are not independent. The distributions of C₄ sedges and C₄ grasses along an altitudinal gradient are quite different, and C₄ grasses grow abundantly at much higher altitudes than C₄ sedges. C₄ sedges are very scarce at the altitudes represented in the Maloti-Drakensberg area, whereas C₃ grasses occur in the permanently wet parts of the wetlands, especially at higher altitudes (normally mostly occupied by sedges). Shrubs are rare in wetlands and tend to be an indication of disturbance. This study complements previous studies on the distribution of grasses and sedges at the lower altitudes within KwaZulu-Natal, which found that at altitudes below 1,000 m a.s.l. C₄ sedges were much more prominent, while forbs and rosette plants were largely absent. This confirms that C₄ as an adaptation to hotter and warmer climates is sometimes a less favorable metabolism in wet high altitude areas. At high altitudes, rosette plants and bulbous plants become more competitive in wetlands, probably because grasses and sedges present at these altitudes generally grow smaller than they do in low altitude wetlands.     

喀斯特地区植物钙含量特征与高钙适应方式分析

喀斯特地区土壤的高钙含量是影响该地区植物生理特征的最重要环境因素之一。高钙影响植物的光合作用、生长速率及磷代谢, 从而限制了许多物种在该地区的分布。选取贵州4个石漠化程度不同的地区, 测定采集地内45种优势种或常见种的地上部分和地下部分的全钙含量以及土壤的交换性钙含量。通过分析喀斯特地区植物与土壤钙含量的特征发现: 喀斯特地区植物具有较高的钙含量平均值; 土壤交换性钙含量对植物地上部分钙含量的影响总体上不显著, 对植物地下部分钙含量的影响显著; 不同类别植物的钙含量存在显著差异, 蕨类植物地上部分钙含量平均值明显低于被子植物; 不同类别植物钙的分布部位也存在显著差异, 在蕨类植物和单子叶植物中地上部分和地下部分的钙含量相近, 而双子叶植物的地上部分钙含量明显高于地下部分。分析了喀斯特地区14种优势灌木和草本植物地上部分与地下部分钙含量的差异性以及与土壤交换性钙含量的相关关系, 以此为根据将14种优势植物对土壤高钙的适应方式分为3种类型: 随遇型、高钙型和低钙型。随遇型植物的钙含量主要受土壤交换性钙含量影响, 其地上部分和地下部分的钙含量均与土壤交换性钙含量成显著正相关关系; 高钙型植物具有较强的钙富集能力, 其地上部分即使在低钙含量的土壤中也可维持较高的钙含量; 低钙型植物的地上部分即使在高钙含量的土壤中亦可维持较低的钙含量。对植物适应钙的不同方式的研究可用于筛选退化生态系统恢复所需的植物资源。     

Responses of savanna lawn and bunch grasses to water limitation

The grass layer of African savannas consists of two main vegetation types: grazing lawns, dominated by short, mostly clonally reproducing grasses, and bunch grasslands, dominated by tall bunch grasses. This patchy distribution of vegetation types is mostly created by large herbivores, which selectively feed on the more nutritious lawn grass species. Besides grazing, herbivores trample the soil, thereby causing soil compaction, with possible consequences for water infiltration. This raises two questions: (i) is water more limiting in grazing lawns than in bunch grasslands and (ii) are lawn grasses more drought tolerant than bunch grasses? To study these questions, we compared drought conditions in both lawn and bunch grasslands in a South African savanna. Additionally, in a climate room, we compared the performance of three lawn and three bunch grass species under a control and a water limitation treatment. Thirdly, we investigated whether there are differences between lawn and bunch grasses in traits related to drought tolerance. Our results show that despite large differences in water availability in the field, lawn and bunch grasses did not differ in their growth response to drought. Drought reduced growth of both growth forms equally. However, we found strong intrinsic trait differences between growth forms, with lawn grasses having higher specific root length and relative growth rate and bunch grasses having a higher root:shoot ratio. These results suggest that after drought-induced plant death, lawn grasses might be more capable of recolonizing patches of bare soil.     

The Enemy of My Enemy Hypothesis: Why Coexisting with Grasses May Be an Adaptive Strategy for Savanna Trees

Savannas are characterized by the coexistence of trees and flammable grasses. Yet, tree–grass coexistence has been labeled as paradoxical—how do these two functional groups coexist over such an extensive area, despite being generally predisposed to excluding each other? For instance, many trees develop dense canopies that limit grass growth, and many grasses facilitate frequent/intense fires, increasing tree mortality. This study revisits tree–grass coexistence with a model of hierarchical competition between pyrogenic grasses, “forest trees” adapted to closed-canopy competition, and “savanna trees” that are inferior competitors in closed-canopy communities, but more resistant to fire. The assumptions of this model are supported by empirical observations, including a systematic review of savanna and forest tree community composition reported here. In general, the model simulations show that when savanna trees exert weaker competitive effects on grasses, a self-reinforcing grass community is maintained, which limits forest tree expansion while still allowing savanna trees to persist (albeit as a subdominant to grasses). When savanna trees exert strong competitive effects on grasses, savanna trees cover increases initially, but as grasses decline their inhibitory effect on forest trees weakens, allowing forest trees to expand and exclude grasses and savanna trees. Rather than paradoxical, these results suggest that having weaker competitive effects on grasses may be advantageous for savanna trees, leading to greater long-term abundance and stability. We label this the “enemy of my enemy hypothesis,” which might apply to species coexistence in communities defined by hierarchical competition or with species capable of generating strong ecological feedbacks.     

Effects of extreme drought on specific leaf area of grassland species: A meta‐analysis of experimental studies in temperate and sub‐Mediterranean systems

Here, we conducted a meta‐analysis of experimental drought manipulation studies using rainout shelters in five sites of natural grassland ecosystems of Europe. The single studies assess the effects of extreme drought on the intraspecific variation of the specific leaf area (SLA), a proxy of plant growth. We evaluate and compare the effect size of the SLA response for the functional groups of forbs and grasses in temperate and sub‐Mediterranean systems. We hypothesized that the functional groups of grasses and forbs from temperate grassland systems have different strategies in short‐term drought response, measured as adjustment of SLA, with SLA‐reduction in grasses and SLA‐maintenance in forbs. Second, we hypothesized that grasses and forbs from sub‐Mediterranean systems do not differ in their drought response as both groups maintain their SLA. We found a significant decrease of SLA in grasses of the temperate systems in response to drought while SLA of forbs showed no significant response. Lower SLA is associated with enhanced water‐use efficiency under water stress and thus can be seen as a strategy of phenotypic adjustment. By contrast, in the sub‐Mediterranean systems, grasses significantly increased their SLA in the drought treatment. This result points towards a better growth performance of these grasses, which is most likely related to their strategy to allocate resources to belowground parts. The observed SLA reduction of forbs is most likely a direct drought response given that competitive effect of grasses is unlikely due to the scanty vegetation cover. We point out that phenotypic adjustment is an important driver of short‐term functional plant response to climatic extremes such as drought. Differential reactions of functional groups have to be interpreted against the background of the group's evolutionary configuration that can differ between climatic zones.     

Adsorption of a hydrophobic mutagen to five contrasting dietary fiber preparations

The ability of five plant cell wall (dietary fiber) preparations with contrasting compositions to adsorb in vitro the hydrophobic, environmental mutagen, 1,8-dinitropyrene (DNP), was investigated. Many of the fruits and vegetables in Western diets are from dicotyledonous (broad leaved) plants and the dietary fiber from these consists mainly of unlignified cell walls. A representative of this wall type, prepared from immature cabbagE leaves, showed little ability to adsorb DNP. Two other cell-wall preparations, representing lignified walls of dicotyledons and unlignified walls of vegetative parts of grasses and cereals (monocotyledons belonging to the family Poaceae), adsorbed DNP much more effectively. However, two further preparations, representing suberized walls of cork cells and lignified walls of vegetative parts of grasses and cereals, were the most effective in adsorbing DNP. Extrapolation of these data to the in vivo situation would indicate that increased consumption of the vegetative parts of grasses or cereals and plant material containing cork cells, for example potato skins, could be effective in removing hydrophobic mutagens from potential contact with colonic mucosal cells.     

The response of native species to removal of invasive exotic grasses in a seasonally dry Hawaiian woodland

Abstract. Non-native perennial grasses form 30% of the live understory biomass in seasonally dry, submontane forests in Hawaii Volcanoes National Park, yet their effects on native species are unknown. We removed these grasses from plots of 20 m × 20 m in 1991 and maintained removal and control areas over the next three years. Two fast growing shrub species, Dodonaea viscosa and Osteomeles anthylidifolia, increased in size significantly more in removal areas than in controls. Individuals of the most abundant shrub species, Styphelia tameiameia showed no net growth response to grass removal. They did, however, change their architecture: many branches along the mid and upper sections of the main trunk died and a proliferation of new leaves and shoots occurred in the lower 40 cm of trunk. Basal diameter increase was very small in Metrosideros polymorpha, the dominant tree species in these sites. All species except Styphelia had significantly increased leaf tissue nitrogen in removal plots by 18 months after removal when compared to shrubs in control areas suggesting that removal plot shrubs had greater access to soil nitrogen. Available soil-N pools, which were generally higher in the removal plots, support this interpretation. Light levels near the soil surface were also higher where grasses were removed than where they were present which may have contributed to increased shrub growth. By contrast, soil moisture was consistently lower where grasses were removed than where they were still present. Shrub tissue carbon isotope values were consistent with the interpretation that shrubs in removal plots had less rather than more water available to them. Hence, the increased growth observed in removal plot shrubs could not be due to release from moisture competition. Lastly, our results showed that seedlings of all woody species except Metrosideros were significantly more abundant in removal plots at both one and three years after removal and initially high sapling mortality was balanced by high recruitment into the sapling class. We believe that over time this will result in increased densities of native shrubs if grasses are kept out. With the presence of grasses, shrub growth in these woodlands is reduced and biomass is shifting towards grasses.     

Molecular evolution of the seed storage proteins of barley, rye and wheat

The major storage proteins (prolamins) of barley, rye and wheat are characterized by the presence of two or more unrelated structural domains, one of which contains repeated sequences. Because of this repetitive structure and their restricted distribution (only in grasses), it has been suggested that the prolamins are of recent origin. Contrary to this hypothesis, we show that parts of the non-repetitive domain of one group of prolamins are homologous with sequences present in a large group of seed proteins from monocotyledonous and dicotyledonous plants; including Bowman-Birk protease inhibitors, cereal inhibitors of alpha-amylase and trypsin, and 2 S globulin storage proteins of castor bean and oil seed rape. This implies an ancient origin for these non-repetitive domains. The origins of the repetitive domains are not known but may lie within the grasses.     

Adsorption of a hydrophobic mutagen to five contrasting dietary fiber preparations.

The ability of five plant cell wall (dietary fiber) preparations with contrasting compositions to adsorb in vitro the hydrophobic, environmental mutagen, 1,8-dinitropyrene (DNP), was investigated. Many of the fruits and vegetables in Western diets are from dicotyledonous (broad leaved) plants and the dietary fiber from these consists mainly of unlignified cell walls. A representative of this wall type, prepared from immature cabbage leaves, showed little ability to adsorb DNP. Two other cell-wall preparations, representing lignified walls of dicotyledons and unlignified walls of vegetative parts of grasses and cereals (monocotyledons belonging to the family Poaceae), adsorbed DNP much more effectively. However, two further preparations, representing suberized walls of cork cells and lignified walls of vegetative parts of grasses and cereals, were the most effective in adsorbing DNP. Extrapolation of these data to the in vivo situation would indicate that increased consumption of the vegetative parts of grasses or cereals and plant material containing cork cells, for example potato skins, could be effective in removing hydrophobic mutagens from potential contact with colonic mucosal cells.     

禾草-内生真菌共生体在草地农业系统中的作用

综述了国内外近年对禾草 -内生真菌 (N eotyphodium)的研究进展。全世界现已报道 14种禾草内生真菌 ,与 2 3个属的禾草形成共生体 ,我国已在 13属 2 5种天然草地禾草中发现内生真菌 ,其中发草属 (Descampsia)、大麦属 (H ord eum)和赖草属(L eymus)以往在国际文献中未曾报道。内生真菌在禾草体内产生的生物碱 ,致使采食带菌禾草的马、牛、羊、鹿等家畜产生中毒症状 ,每年给美国、新西兰等国造成的经济损失达 6.4亿美元之多 ,对其毒理研究取得了进展。已发现我国醉马草 (Achnatheruminebrians)对家畜的毒性与内生真菌的侵染有关。内生真菌侵染增加可使禾草对 42种害虫的抗性 ,并可增加对某些线虫和病害的抗性。与不带菌禾草相比 ,带菌禾草的另一特点是抗逆性强 ,牧草产量高。国际在该领域的研究主要集中在多年生黑麦草(L olium p erenne)和高羊茅 (Festuca arundinacea)。我国的研究发现 ,带菌布顿大麦草 (H ordeum bodg anii)和圆柱披碱草(Elymus cylind ricus)的牧草产量分别增加 3 3 .3 %和 2 78.8% ,分孽数分别增加 13 6.8%和 84.5%。目前 ,国际研究的重点包括大规模开展内生真菌生物学与生态学特性的研究 ,创造不含对家畜有害毒素的有益禾草 -内生真菌共生体 ,培育带内生真菌的草坪草品种 ,培育抗毒     

Timing of C4 grass expansion across sub-Saharan Africa

The emergence of C(4) grass biomes is believed to have first taken place in the upper Miocene, when a series of events modified global climate with long-lasting impacts on continental biotas. Changes included major shifts in floral composition-characterized in Africa by shrinking of forests and emergence of C(4) grasses and more open landscapes-followed by large-scale evolutionary shifts in faunal communities. The timing of the emergence of C(4) grasses, and the subsequent global expansion of C(4) grass-dominated biomes, however, is disputed, leading to contrasting views of the patterns of environmental changes and their links to faunal shifts, including those of early hominins. Here we evaluate the existing isotopic evidence available for central, eastern, and southern Africa, and review interpretations in light of these data. Pedogenic and biomineral carbonate delta(13)C data suggest that clear evidence for C(4) biomass in low latitudes exists only from 7-8 Ma. This likely postdates the emergence of C(4) plants, whose physiology is adapted to low atmospheric carbon dioxide concentrations. Biomes with C(4) grasses appeared later in mid-latitude sites. Moreover, C(4) grasses apparently remained a relatively minor component of most environments until the late Pliocene and early Pleistocene. Hence establishment of C(4) grasses, even as minor components of African biomes, precedes the very earliest evidence for bipedalism by two million years, and the more abundant and secure evidence by some three to four million years. This may suggest a protracted process of hominin adaptation to these emerging, more open landscapes.     

Prehistorical climate change increased diversification of a group of butterflies

Satyrinae butterflies (Lepidoptera: Nymphalidae) and grasses (Poaceae) are very diverse and distributed worldwide. Most Satyrinae use grasses as host plants, but the temporal scale of this tight association is not known. Here, we present a phylogenetic study of Satyrinae butterflies and related groups, based on 5.1 kilobases from six gene regions and 238 morphological characters for all major lineages in the 'satyrine clade'. Estimates of divergence times calibrated using a fossil from the Late Oligocene indicate that the species-rich tribe Satyrini diversified to its current 2200 species simultaneously with the expansion and radiation of grasses during the dramatic cooling and drying up of the Earth in the Oligocene. We suggest that the adaptive radiation of grass feeders in Satyrini has been facilitated by the ubiquitousness of grasses since 25Myr ago, which was triggered by a change in global climate.     

Study on some bioecological aspects of Eldana saccharina Walker (Lep., Pyralidae) on Zea mays L. and alternative host plants

Abstract:  In an attempt to set up habitat management strategy against Eldana saccharina Walker, an economically important graminaceous stem and cob borer, two laboratory experiments were carried out. The first experiment was designed to assess oviposition preference of E. saccharina for growth stages, plant parts and freshness of different host plants and between host plants species. Zea mays L., Sorghum arundinaceum (Desv.) Stapf, Andropogon gayanus Kunth, Panicum maximum Jacq., Pennisetum polystachion (Linn.) and Cyperus papyrus L. were assessed. Results showed that E. saccharina preferred wild grasses to maize for oviposition. More than 94% eggs were laid on wild grasses. Of all tested grasses, P. polystachion was the most attractive with more than 30% of collected eggs. For all plant species, post-tasseling stage was more attractive than the tasseling and pre-tasseling ones. More than 90% of total eggs were laid on dry leaves and sheaths and less than 10% on fresh leaves. Different characteristics relating to population dynamics of E. saccharina were assessed in the second experiment. Larval survival rate was 10% on Z. mays , 0.50% on S. arundinaceum and around 0% on the other wild grasses. Larval development length was longer on S. arundinaceum than on Z. mays . Females reared on Z. mays were three times more fecund than those reared on S. arundinaceum . Complete life table of females from larvae reared on Z. mays was studied.     

What are the effects of substrate and grass removal on recruitment of <Emphasis Type="Italic">Acacia mellifera</Emphasis> seedlings in a semi-arid environment?

Acacia mellifera is one of the most important encroaching woody plants in southern African savannas. Previous studies found that this species encroaches far more readily on rocky areas than on sandy substrates, although it grows larger on sandy substrates. Rocky substrates are known to retain more water than sandy substrates, which may be of vital importance during recruitment in semi-arid and arid environments. A number of studies have also indicated that competition with grasses may reduce the recruitment and biomass of tree seedlings. We created an experiment in a semi-arid environment (mean annual rainfall = 388 mm) that tested for the effects of rockiness on A. mellifera recruitment. We also tested the hypothesis that grasses effectively compete with A. mellifera in this environment by simulating the effect of grazing by clipping grasses from half the plots in both the rocky and sandy treatments. Significantly more A. mellifera seedlings established in plots where grasses were clipped than in control plots. A. mellifera seedlings had greater biomass on sandy substrates than on rocky substrates. No significant interaction effects were found between substrate and grass clipping treatment for either seedling number or biomass. We conclude that A. mellifera seedlings are more likely to encroach in habitats with low grass density, although they may achieve greater biomass on sandy soils. Thus, it may be the lower grass density rather than rockiness, which increases the encroachment observed in naturally rocky habitats. These results are also consistent with our observations that adult A. mellifera trees are larger on sandy soils than on rocky soils.     

Connected Text Reading and Differences in Text Reading Fluency in Adult Readers

The process of connected text reading has received very little attention in contemporary cognitive psychology. This lack of attention is in parts due to a research tradition that emphasizes the role of basic lexical constituents, which can be studied in isolated words or sentences. However, this lack of attention is in parts also due to the lack of statistical analysis techniques, which accommodate interdependent time series. In this study, we investigate text reading performance with traditional and nonlinear analysis techniques and show how outcomes from multiple analyses can used to create a more detailed picture of the process of text reading. Specifically, we investigate reading performance of groups of literate adult readers that differ in reading fluency during a self-paced text reading task. Our results indicate that classical metrics of reading (such as word frequency) do not capture text reading very well, and that classical measures of reading fluency (such as average reading time) distinguish relatively poorly between participant groups. Nonlinear analyses of distribution tails and reading time fluctuations provide more fine-grained information about the reading process and reading fluency.     

Morphological variations of lobate phytoliths from grasses in China and the south-eastern United States

Abstract. Phytolith analysis of grasses is a useful tool in palaeoenvironmental and archaeobotanical research. Lobate phytolith is one of the most important morphotypes of grass phytoliths. This study describes morphological variations of diagnostic lobate phytoliths and produces a tentative classification scheme based on 250 modern grass species from China and the south‐eastern U.S.A. Eighty‐five grass species were found to contain lobate phytoliths. They are derived mainly from Panicoideae, but also include the Chloridoideae, Oryzoideae and Arundinoideae subfamilies. Twenty‐five lobate morphological types were observed from different subfamilies, genera or tribes of grasses, based on two important parameters: (1) the length of the lobate shank and (2) the shape of the outer margin of the two lobes. The identification of grass tribe or even genus is possible based on the differences in lobate shape parameters or the composition of assemblages. However, not all of the lobate assemblages have a definite relationship with the genera that produce them, because grasses can only produce a limited range of lobate shapes that often overlap from one genus to another. Several C₃ grasses and Chloridoideae subfamily grasses also produce characteristic lobate phytoliths. The variations of lobate morphologies can be related to environmental factors, especially moisture. Typical hygrophytic grasses tend to yield lobate phytoliths with very short shank, whereas typical xerophytic grasses tend to produce lobate phytoliths with a very long shank. The potential link between phytolith morphology, grass taxonomy and environmental conditions opens the possibility that phytolith morphology may be used as a proxy in palaeoclimatic reconstruction.     

Comparative genomics in the grass family: molecular characterization of grass genome structure and evolution

The genomes of grasses are very different in terms of size, ploidy level and chromosome number. Despite these significant differences, it was found by comparative mapping that the linear order (colinearity) of genetic markers and genes is very well conserved between different grass genomes. The potential of such conservation has been exploited in several directions, e.g. in defining rice as a model genome for grasses and in designing better strategies for positional cloning in large genomes. Recently, the development of large insert libraries in species such as maize, rice, barley and diploid wheat has allowed the study of large stretches of DNA sequence and has provided insight into gene organization in grasses. It was found that genes are not distributed randomly along the chromosomes and that there are clusters of high gene density in species with large genomes. Comparative analysis performed at the DNA sequence level has demonstrated that colinearity between the grass genomes is retained at the molecular level (microcolinearity) in most cases. However, detailed analysis has also revealed a number of exceptions to microcolinearity, which have given insight into mechanisms that are involved in grass-genome evolution. In some cases, the use of rice as a model to support gene isolation from other grass genomes will be complicated by local rearrangements. In this Botanical Briefing, we present recent progress and future prospects of comparative genomics in grasses.     

Out of the ground: aerial and exotic habitats of the melioidosis bacterium Burkholderia pseudomallei in grasses in Australia

Melioidosis is an emerging infectious disease of humans and animals in the tropics caused by the soil bacterium Burkholderia pseudomallei. Despite high fatality rates, the ecology of B.pseudomallei remains unclear. We used a combination of field and laboratory studies to investigate B.pseudomallei colonization of native and exotic grasses in northern Australia. Multivariable and spatial analyses were performed to determine significant predictors for B.pseudomallei occurrence in plants and soil collected longitudinally from field sites. In plant inoculation experiments, the impact of B.pseudomallei upon these grasses was studied and the bacterial load semi-quantified. Fluorescence in situ hybridization and confocal laser scanning microscopy were performed to localize the bacteria in plants. Burkholderia pseudomallei was found to inhabit not only the rhizosphere and roots but also aerial parts of specific grasses. This raises questions about the potential spread of B.pseudomallei by grazing animals whose droppings were found to be positive for these bacteria. In particular, B.pseudomallei readily colonized exotic grasses introduced to Australia for pasture. The ongoing spread of these introduced grasses creates new habitats suitable for B.pseudomallei survival and may be an important factor in the evolving epidemiology of melioidosis seen both in northern Australia and elsewhere globally.