What makes an ophiuroid? A morphological study of the problematic Ordovician stelleroid Stenaster and the palaeobiology of the earliest asteroids and ophiuroids

Extant asteroids and ophiuroids [EchinodermataJ are distinguished by differences in arm support, water vascular system structures and in details of arm and jaw structure. However, some lower Palaeozoic taxa show combinations of both asteroid-like and ophiuroid-like characters and their morphology and functional biology is poorly understood. This paper redescribes one such taxon, the middle-upper Ordovician stellate echinoderm Stenaster and clarifies its phylogenetic status. Characters in common with extant and Ordovician ophiuroids, include arm support due primarily to ambulacral ossicles, presence of extensive longitudinal arm musculature, a mobile jaw and an internalised radial water vessel with internalised podial pores. In addition, Stenaster lacks several characters which are conventionally considered to be asteroid-like, for example an axillary, madreporitc, marginal ossicles and a true ambulacral groove. However, in overall shape Stenaster is remarkably asteroid-like, showing short, broad-based arms shared podial basins and a small disc. A cladistic analysis of early asteroids, ophiuroids and somasteroid taxa consistently places Stenaster within the ophiuroids and suggests secondary convergence to asteroids. In functional terms, Stenaster is interpreted as an ophiuroid which has secondarily adopted a semi-infaunal, deposit-feeding mode of life, analogous to that of some extant paxillosid asteroids.     

Inhibition of germination of dormant barley (Hordeum vulgare L.) grains by blue light as related to oxygen and hormonal regulation

Germination of primary dormant barley grains is promoted by darkness and temperatures below 20 °C, but is strongly inhibited by blue light. Exposure under blue light at 10 °C for periods longer than five days, results in a progressive inability to germinate in the dark, considered as secondary dormancy. We demonstrate that the inhibitory effect of blue light is reinforced in hypoxia. The inhibitory effect of blue light is associated with an increase in embryo abscisic acid (ABA) content (by 3.5‐ to 3.8‐fold) and embryo sensitivity to both ABA and hypoxia. Analysis of expression of ABA metabolism genes shows that increase in ABA mainly results in a strong increase in HvNCED1 and HvNCED2 expression, and a slight decrease in HvABA8′OH‐1. Among the gibberellins (GA) metabolism genes examined, blue light decreases the expression of HvGA3ox2, involved in GA synthesis, increases that of GA2ox3 and GA2ox5, involved in GA catabolism, and reduces the GA signalling evaluated by the HvExpA11 expression. Expression of secondary dormancy is associated with maintenance of high embryo ABA content and a low HvExpA11 expression. The partial reversion of the inhibitory effect of blue light by green light also suggests that cryptochrome might be involved in this hormonal regulation.     

Ecological factors regulating brood attendance patterns of the Western Sandpiper Calidris mauri

Parental brood attendance patterns vary greatly among shorebird species. For monogamous calidridine species, biparental care with female-first brood departure is most common. It is believed that adult sandpipers balance potential individual survival costs associated with extended parental care against the benefit gained by their brood of prolonged parental care. These costs and benefits are difficult to quantify and factors affecting the termination of parental brood attendance are unclear. We compared clutch size, nesting phenology, and parental attendance patterns of Western Sandpipers Calidris mauri at Nome and Kanaryarmiut, Alaska, sites separated by three degrees of latitude. The sites differed in breeding density and duration of breeding season, but the distribution of clutch sizes did not differ between sites or between nesting attempts. Parental attendance patterns were similar between sites, suggesting that parental attendance is a highly conserved life-history trait in Western Sandpipers. Male Western Sandpipers attended broods longer than females, and the duration of parental attendance decreased at a similar rate for both sexes as the season progressed. Male and female Western Sandpipers undertake differential migrations to their non-breeding grounds, with males typically settling at more northerly locations and females at more southerly sites, a migration pattern shared by certain other monogamous calidridine species. These same species exhibit similar parental brood attendance patterns, suggesting the strong role of overall migration distance in shaping the expression of parental attendance behaviours. A contrast of more geographically disjunct sites coupled with a better understanding of the migratory connectivity between Western Sandpiper breeding and non-breeding populations would elucidate the role of cross-seasonal effects on parental brood attendance decisions.     

Alterations in light-induced stomatal opening in a starch-deficient mutant of Arabidopsis thaliana L. deficient in chloroplast phosphoglucomutase activity

Stomatal responses to light of Arabidopsis thaliana wild-type plants and mutant plants deficient in starch (phosphoglucomutase deficient) were compared in gas exchange experiments. Stomatal density, size and ultrastructure were identical for the two phenotypes, but no starch was observed in guard cells of the mutant plants whatever the time of day. The overall extent of changes in stomatal conductance during 14 h light–10 h dark cycles was similar for the two phenotypes. However, the slow endogenous stomatal opening occurring in darkness in the wild type was not observed in the mutant plants. Stomata in the mutant plants responded much more slowly to blue light (70 μmol m^?2 s^?1) though the response to red light (250 μmol m^?2 s^?1) was similar to that of wild-type plants. In paradermal sections, stomatal responses to red light (300 μmol m^?2 s^?1) were weak for wild-type plants as well as for mutant plants. Stomatal opening was greater under low blue light (75 μmol m^?2 s^?1) than under red light for the two genotypes. However, in mutant plants, a high chloride concentration (50 mol m^?3) was necessary to achieve the same stomatal aperture as observed for the wild-type plants. These results suggest that starch metabolism, via the synthesis of a counter-ion to potassium (probably malate), is required for full stomatal response to blue light but is not involved in the stomatal response to red light.     

Elevated CO2 enhances stomatal responses to osmotic stress and abscisic acid in Arabidopsis thaliana

A , carbon assimilation rate
ABA, abscisic acid
Ci , intercellular space CO₂ concentration
g , leaf conductance
WUE, water use efficiency

Carbon dioxide and abscisic acid (ABA) are two major signals triggering stomatal closure. Their putative interaction in stomatal regulation was investigated in well-watered air-grown or double CO₂-grown Arabidopsis thaliana plants, using gas exchange and epidermal strip experiments. With plants grown in normal air, a doubling of the CO₂ concentration resulted in a rapid and transient drop in leaf conductance followed by recovery to the pre-treatment level after about two photoperiods. Despite the fact that plants placed in air or in double CO₂ for 2 d exhibited similar levels of leaf conductance, their stomatal responses to an osmotic stress (0·16–0·24 MPa) were different. The decrease in leaf conductance in response to the osmotic stress was strongly enhanced at elevated CO₂. Similarly, the drop in leaf conductance triggered by 1 μ M ABA applied at the root level was stronger at double CO₂. Identical experiments were performed with plants fully grown at double CO₂. Levels of leaf conductance and carbon assimilation rate measured at double CO₂ were similar for air-grown and elevated CO₂-grown plants. An enhanced response to ABA was still observed at high CO₂ in pre-conditioned plants. It is concluded that: (i) in the absence of stress, elevated CO₂ slightly affects leaf conductance in A. thaliana ; (ii) there is a strong interaction in stomatal responses to CO₂ and ABA which is not modified by growth at elevated CO₂.     

New clues for a cold case: nitric oxide response to low temperature

Low temperature is among the most frequent stresses met by plants during their lifespan, and a plant's ability to cold‐acclimate is a determinant for further growth and development. Although intensive research has provided a good picture of the molecular and metabolic changes triggered by cold, the underlying regulatory mechanisms remain elusive and are thus being actively sought. Recent studies have shed light on the importance of nitric oxide (NO), a ubiquitous signalling molecule in eukaryotes, for plant tolerance to chilling and freezing. Indeed, NO formation following cold exposure has been reported in a range of plant species, and a series of proteins targeted by NO‐based post‐translational modifications have been identified. Moreover, key cold‐regulated genes have been characterized as NO‐dependent, suggesting the crucial importance of NO signalling for cold‐responsive gene expression. This review provides a picture of our current understanding of the function of NO in the context of plant response to cold. Particular attention is dedicated to the open questions left by the fragmented data currently available concerning NO formation, transduction and biological significance for plant adaptation to low temperature.     

Arabidopsis lipocalins AtCHL and AtTIL have distinct but overlapping functions essential for lipid protection and seed longevity

Lipocalins are a group of multifunctional proteins, recognized as carriers of small lipophilic molecules, which have been characterized in bacteria and animals. Two true lipocalins have been recently identified in plants, the temperature‐induced lipocalin (TIL) and the chloroplastic lipocalin (CHL), the expression of which is induced by various abiotic stresses. Each lipocalin appeared to be specialized in the responses to specific stress conditions in Arabidopsis thaliana, with AtTIL and AtCHL playing a protective role against heat and high light, respectively. The double mutant AtCHL KO × AtTIL KO deficient in both lipocalins was more sensitive to temperature, drought and light stresses than the single mutants, exhibiting intense lipid peroxidation. AtCHL deficiency dramatically enhanced the photosensitivity of mutants (vte1, npq1) affected in lipid protection mechanisms (tocopherols, zeaxanthin), confirming the role of lipocalins in the prevention of lipid peroxidation. Seeds of the AtCHL KO × AtTIL KO double mutant were very sensitive to natural and artificial ageing, and again this phenomenon was associated with the oxidation of polyunsaturated lipids. The presented results show that the Arabidopsis lipocalins AtTIL and AtCHL have overlapping functions in lipid protection which are essential for stress resistance and survival.