Biological nitrogen fixation in non-legume plants (Invited Review)
doi:10.1093/aob/mct048
Biological nitrogen fixation is accomplished by diazotrophic bacteria, some of which can develop symbiotic or endophytic associations. Santi et al. (pp. 743–767) review current knowledge concerning associations between nitrogen-fixing bacteria and non-legumes, with an emphasis on recent advances. As in legumes, the establishment of effective plant–bacteria associations in non-legumes is complex and involves recognition and transcriptional reprogramming on both sides. By identifying genes that govern associations with diazotrophs, new possibilities can be explored for transferring nitrogen-fixing ability to crops in the future.
Plant hormones in arbuscular mycorrhizal symbioses (Research in Context)
doi:10.1093/aob/mct041
Although >80 % of species form arbuscular mycorrhizal symbioses, the roles of plant hormones in the associations are still being determined. Foo et al. (pp. 769–779) critically review recent progress in our understanding of the roles of strigolactones, auxin, abscisic acid, ethylene, jasmonic acid and salicylic acid in mycorrhizal symbiosis, and present new evidence for a novel role for gibberellins in mycorrhizal development. Using gibberellin- and DELLA-deficient mutants of pea they show that bioactive gibberellins have a negative role in arbuscule formation. In contrast, a brassinosteroid-deficient pea mutant provides no indication of a role for these compounds in mycorrhizal development.
Maternal cuticle protects moss offspring from dehydration
doi:10.1093/aob/mct033
During early development, moss sporophytes are covered by the calyptra, a cap of gametophyte tissue that has a multilayered cuticle and covers the sporophyte offspring apex during early development. Budke et al. (pp. 781–789) examine calyptra function in the moss Funaria hygrometrica by removing the cuticle and exposing plants to a short-term dehydration event. They find that offspring fitness decreases significantly, suggesting that the cuticle of the maternal calyptra functions in dehydration protection of the offspring. The results provide a functional explanation for calyptra retention during the last 400 million years of moss evolution, and they conclude that the calyptra may represent the earliest occurrence of maternal protection via structural provisioning of a cuticle in green plants.
Flowering time control by miR159 in gloxinia
doi:10.1093/aob/mct034
Gloxinia (Sinningia speciosa) is a popular ornamental plant with attractive, colourful flowers, but the genetic mechanisms that regulate flowering are largely unknown. Li et al. (pp. 791–799) generate transgenic gloxinia plants that over-express or suppress miR159, and determine that this leads to late and early flowering, respectively. miR159-mediated SsGAMYB expression affects the expression levels of SsLEAFY (SsLFY) and three MADS-box genes (SsAP1, SsAP3 and SsAG), which regulate floral transition downstream of GAMYB. They conclude that transgenic manipulation of miR159 has the potential to be used to regulate flowering time in commercial ornamental plants.
Conservative shoot P content under varying supply
doi:10.1093/aob/mct035
The Australian legume Viminaria juncea forms both cluster roots and mycorrhizal associations. de Campos et al. (pp. 801–809) manipulate P supply over a range of 0 to 50 mg P kg−1 dry soil in order to investigate if these root specializations are expressed at different shoot P concentrations [P]. Remarkably, they find that shoot [P] over the entire range of supplies is constant. They conclude that to maintain stable shoot [P] values, V. juncea must down-regulate its growth rate when very little P is supplied; conversely, it down-regulates its P-uptake capacity very tightly at higher P supply, when its maximum growth rate has been reached. The persistence of cluster roots and mycorrhizal colonization up to the highest P treatments is probably a consequence of tightly controlled shoot [P].
Contribution of ERECTA to the shade-avoidance syndrome
doi:10.1093/aob/mct038
Plants growing at high densities perceive a decrease in the red to far-red ratio of incoming light, an early signal of competition between individuals that promotes shade-avoidance responses. By mapping quantitative trait loci (QTL) for hypocotyl length to end-of-day (EOD) far-red in Arabidopsis thaliana, Kasulin et al. (pp. 811–819) find that the ERECTA gene is implicated in the shade-avoidance signalling in a background-dependent manner. They also detect pleiotropic effects of ERECTA in the EOD response for petiole and lamina elongation, hyponastic growth and flowering time, suggesting that ERECTA is involved in canalization processes buffering the shade-avoidance variation against environmental light fluctuations.
Fusion within and between whorls of floral organs in Galipeinae
doi:10.1093/aob/mct039
Most genera of neotropical Galipeinae (Rutaceae) exhibit several forms of fusion between the floral organs, but few studies have shown in detail the manner in which these occur. El Ottra et al. (pp. 821–837) examine these floral features and find that they are shared by groups of genera and species. The floral tube is formed by synorganization of stamens with petals in all species, by postgenital connection and/or congenital fusion. Although floral tubes seem to be homologous within Galipeinae, the floral tube of Correa (a species from an only distantly related clade of the family) represents a convergent structure. They conclude that the degree and diversity of fusions of floral organs in Galipeinae is unique within the order Sapindales.
Peroxidase in proanthocyanidin-deficient arabidopsis seeds
doi:10.1093/aob/mct045
It has previously been shown that proanthocyanidins (PAs) in the seed coat of Arabidopsis thaliana have the ability to scavenge superoxide radicals (O2−). Jia et al. (pp. 839–847) compare wild-type and PA-deficient mutants and find that peroxidase activity is significantly over-activated in mutant seeds. Guaiacol peroxidases (class III peroxidases) are specifically activated in the seed coat, and it is suggested that this may be an adaptive mechanism for substituting the antioxidant function of PAs that is required during seed development and later in seed germination.
Thermal quantification of physical dormancy-break in Geranium
doi:10.1093/aob/mct046
Breaking of physical dormancy (PY) in Geranium carolinianum is a temperature-controlled, two-step process. Gama-Arachchige et al. (pp. 849–858) employ the concept of thermal time to explain sensitivity induction in the first step during the PY-breaking process, and propose a mechanism for the opening of the water-gap, based on differential thermal contraction of seed-coat palisade cell layers. The results indicate the feasibility of applying the thermal time model developed here to predict or manipulate sensitivity induction in seeds with a two-step PY-breaking process.
Root elongation growth and lunisolar tidal acceleration
doi:10.1093/aob/mct052
Correlative evidence suggests a relationship between the lunisolar tidal acceleration and the elongation rate of Arabidopsis thaliana roots grown under conditions of constant low light. Barlow et al. (pp. 859–872) use high temporal- and spatial-resolution video imaging to study root elongation in conjunction with geophysical variables for seedlings grown in a constant environment. They find that elongation responds not only to variation of the lunisolar force but also seems to adjust to variations in the geomagnetic field, as evaluated from the disturbance storm time and ap indices. They conclude that both lunisolar tidal acceleration and the geomagnetic field should be considered as modulators of root growth rate alongside other, stronger and more well-known abiotic environmental regulators.
Pantropical disjunction in Paederia
doi:10.1093/aob/mct053
Paederia is a pantropical genus of 31 species of woody lianas from the family Rubiaceae, with high species' diversity in both tropical continental Asia and Africa and only two species in the tropical Americas. Nie et al. (pp. 873–886) conduct phylogenetic analyses using sequences of five plastid markers and infer an origin in the Oligocene in tropical continental Asia, probably followed by long-distance dispersal across the Indian Ocean in the early-to-middle Miocene. They infer that the two Neotropical species derived independently in the late Miocene from ancestors of Asia and East Africa, respectively, and conclude that long-distance dispersal may thus be much more common than previously thought, representing a potentially important mechanism in the assembly of modern tropical floras
Behaviour of eversporting eudicot chimeras
doi:10.1093/aob/mct054
Behaviour of eversporting gymnosperm chimeras indicates a single apical cell is present in the shoot apical meristem. Korn (pp. 887–893) studies one Mentha (Lamiaceae), one Euonymus (Celastraceae) and four Spiraea (Rosaceae) cultivars to determine the fate of the yellow (mutant)–green (wild type) chimeras in order to see if eudicot chimeras have the same behaviour. He finds that unstable chimeras became stable yellow about 80 % or more of the time and 20 % or less became stable green. The statistically significant preponderance of chimeric fates becoming all yellow suggests that a single apical cell resides in the yellow tunica. As with gymnosperms, descendent cells of the yellow replacement corpus cell eventually take over the corpus. The results provide the first chimeric set of data to support the hypothesis of a one-celled meristem in eudicots rather than the traditional view of a muticellular meristem.
Phylogeography of invasive Acacia
doi:10.1093/aob/mct057
Understanding the introduction history of invasive plant species is important for their management. Ndlovu et al. (pp. 895–904) use nuclear and plastid DNA sequence data to reconstruct phylogeographical relationships between populations of Acacia pycnantha (golden wattle), a tree species that is native to south-eastern Australia and invasive in South Africa, Western Australia and Portugal. They find that the plastid network indicates that Australian populations are geographically structured into two informally recognized lineages, wetland and dryland forms, whereas the nuclear phylogeny shows little geographical structure between the two. Hybrid zones may explain these findings, supported by incongruent phylogenetic placement of some of these taxa between nuclear and plastid genealogies. Invasive populations in Portugal and South Africa represent wetland forms, although some South African populations resemble the dryland form.
Stomatal position and diurnal pattern of conductance
doi:10.1093/aob/mct061
The considerable distance between peripheral regions and the petiole in large leaves is expected to cause zone-specific differences in water supply and/or gas exchange. Miranda et al. (pp. 905–915) study stomatal conductance at defined lamina zones over two consecutive vegetation periods in the large-leaved liana Aristolochia macrophylla and find zone-specific diurnal patterns, with differences between apical, marginal, central and basal regions. They suggest that the spatial-temporal patterns of stomatal conductance observed may represent an adaptation for avoiding local water stress and subsequent vein embolism.
Ecology of sex-ratio variation
doi:10.1093/aob/mct040
Sex ratios in dioecious angiosperms commonly exhibit deviations from the expected 1 : 1 equilibrium. Field et al. (pp. 917–923) use studies from the literature to investigate intra-specific variation in sex ratios in relation to environmental factors and employ metapopulation simulations to explore the role of non-equilibrium conditions on sex-ratio variation. They find that male-biased ratios are associated with the amount of flowering in populations and more stressful conditions. Simulation results indicate that clonal species exhibit greater heterogeneity in sex ratios than non-clonal species, a pattern confirmed by empirical data. They conclude that sex-based differences in reproductive costs and non-equilibrium conditions both play important roles in affecting flowering sex ratios in angiosperm species.
para-Aminobenzoic acid elicits induced resistance against pathogens
doi:10.1093/aob/mct049
Systemic acquired resistance (SAR) triggered by avirulent pathogens and chemical inducers is a broad-spectrum immune response against pathogen attack in plants. Song et al. (pp. 925–934) conduct field trials using pepper plants (Capsicum annuum) treated with para-aminobenzoic acid (PABA, vitamin Bx) and find that it stimulates SAR against artificially infiltrated Xanthomonas axonopodis pv. vesicatoria and naturally occurring cucumber mosaic virus. Fruit yield is increased in PABA-treated plants, indicating that PABA-mediated SAR successfully protects pepper plants from infection by bacterial and viral pathogens without significant fitness allocation costs. PABA therefore has potential as a water-soluble and safe application for crop protection under field conditions.
Decoupled variation in floral and vegetative traits
doi:10.1093/aob/mct050
Mechanisms allowing the decoupling of the phenotypic variation between floral and vegetative traits in insect-pollinated plants remain unclear. By growing Campanula rotundifolia from two distinct populations at two different temperatures, Pélabon et al. (pp. 935–944) find that flower size is less sensitive to temperature variation than leaf size. Furthermore, these traits show temperature-induced reaction norms in opposite directions, as flower size decreases with increasing temperature, while leaf size increases. At constant temperature, however, floral and vegetative traits tend to remain positively correlated. The results confirm that the decoupling of the phenotypic variation between vegetative and floral traits can be dependent on the environment. They also underline the importance of distinguishing sources of phenotypic covariance when testing hypotheses about phenotypic integration.
Post-shedding seed development in Galanthus and Narcissus
doi:10.1093/aob/mct051
Seeds of the moist temperate woodland species Galanthus nivalis and Narcissus pseudonarcissus, which disperse during spring or early summer, germinate poorly in laboratory tests. Newton et al. (pp. 945–955) study seed characteristics and find a distinct typology of seed development matching their ecological niche, whereby seeds are dispersed close to the end of seed filling without substantial developmental arrest or significant maturation drying. Seeds are comparatively immature with limited desiccation tolerance at shedding, with substantial embryo elongation occurring slowly post-shedding, consistent with the idea that seed development continues naturally on woodland floors or in soil seed banks under the tree canopy after seed dispersal.
Flowering and morphology responses to high latitudes
doi:10.1093/aob/mct055
Timing of flowering is an adaptive trait regulated by environmental cues and has been intensively studied in annual plants, but in perennials it is currently not well characterized. Quilot-Turion et al. (pp. 957–968) apply cold treatments to two locally adapted populations of the perennial Arabidopsis lyrata from central Europe and Scandinavia and study responses to vernalization. They find that the population from central Europe flowers more rapidly and invests more in inflorescence growth than the Scandinavian population, which has longer leaves and higher leaf rosettes. QTL mapping in the F2 population reveals genomic regions governing these traits and some QTL × vernalization interactions. The results indicate that the two populations have diverged in their plastic responses to vernalization in traits closely connected to fitness through changes in many genomic areas.
Floral structure and systematics in Malpighiales
doi:10.1093/aob/mct056
The suprafamilial phylogenetic structure of Malpighiales, one of the largest orders of flowering plants, has been continuously improved over the past decade and the relationships of some subgroups are radically different from pre-molecular classifications. Endress et al. (pp. 969–985) provide a first attempt at a floral structural characterization of the currently resolved suprafamilial clades based on comparative studies and use of primary literature. They find that most new suprafamilial clades are well supported by floral structural features, and inner morphological structures of the gynoecium and ovules appear to be especially suitable for characterizing suprafamilial clades within Malpighiales.
Characterization of longevity in Silene seeds
doi:10.1093/aob/mct058
Seed longevity can vary considerably across species and populations from different environments, but knowledge of the eco-physiological processes underlying such variation remains limited. Donà et al. (pp. 987–998) apply artificial ageing to seeds of Silene vulgaris subsp. vulgaris and S. acaulis, which have differing longevity, and find that RAPD profiles highlight different capacities of the two species to overcome DNA damage. In addition, high-altitude S. acaulis is characterized by a lower antioxidant specific activity, and significant upregulation of MT2 and SOD genes is observed only in rehydrated seeds of low-altitude S. vulgaris. The results thus identify potentially low-cost and time-saving markers of seed deterioration.
Chilling-shock responses in Miscanthus
doi:10.1093/aob/mct059
Miscanthus is a giant grass native to Eastern Asia and is grown as an energy crop in Europe and North America, where productivity is affected by chilling temperatures. Purdy et al. (pp. 999–1013) subject four genotypes (two M. sinensis, one M. sacchariflorus and one inter-species hybrid, M. × giganteus) to short-term chilling conditions and determine changes in photosynthesis, carbohydrate composition and the expression of target transcripts. They find that M. × giganteus exhibits a superior tolerance to chilling shock compared with the other genotypes. The absence of sucrose accumulation in M. sacchariflorus during chilling-shock suggests an impairment in enzyme function. A candidate transcription factor, MsCBF3, is most highly expressed in the two genotypes that suffer the greatest decline in photosynthesis, and may thus be a suitable molecular marker for predicting chilling sensitivity.