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Genetic determinants of stomatal conductance

doi:10.1093/aob/mcu247

Leaf gas exchange is influenced by stomatal size, density, distribution between the leaf adaxial and abaxial sides, as well as by pore dimensions. Fanourakis et al. (pp. 555–565) introgress segments of Solanum pennelli into a S. lycopersicum cultivar to generate 54 introgression lines (ILs), which they study in order to determine which traits underlie genetic differences in operating stomatal conductance (g_s). They find a wide genetic variation in stomatal responsiveness to desiccation, a large part of which is explained by stomatal length. The pore area per stomatal area varies eight-fold among the ILs, and is the main determinant of differences in operating g_s. Differences between operating g_s based on pore dimensions and maximum g_s based on stomatal dimensions are large, suggesting that stomata utilize only a small portion of their operating capacity.

Genetic linkage maps for apomictic Hieracium species

doi:10.1093/aob/mcu249

Apomixis in plants generates clonal progeny with a maternal genotype through asexual seed formation. Hieracium subgenus Pilosella (Asteraceae) contains polyploid, highly heterozygous apomictic and sexual species, and Shirasawa et al. (pp. 567–580) develop a collection of expressed SSR markers in order to construct linkage maps for Hieracium species. Previously identified apomixis loci that are responsible for two independent components of apomixis, LOSS OF APOMEIOSIS (LOA) and LOSS OF PARTHENOGENESIS (LOP), are successfully assigned to linkage groups. These maps will support the cloning of controlling genes at LOA and LOP loci in Hieracium and should assist with identification of quantitative loci that affect apomixis expressivity.

Aphid repellence in arabidopsis over-expressing FPS2

doi:10.1093/aob/mcu250

Plant-synthesized sesquiterpenes play a pivotal role in chemotactic interactions with insects. In Arabidopsis thaliana, FPS2 is involved in the synthesis of cytosolic FDP, a precursor in the sesquiterpene biosynthetic pathway. Bhatia et al. (pp. 581–591) generate transgenic plants over-expressing FPS2 and find that they produce a number of novel sesquiterpenes including E-β-farnesene, which is an alarm pheromone of aphids. Both entrapped volatiles and hydro-distillate extracts of the leaves of transgenic plants trigger perceptible agitation in aphids, suggesting a potential for the development of resistant strains of arabidopsis.

Sporophyte desiccation tolerance in the moss Aloina

doi:10.1093/aob/mcu252

Ecological strategies of desiccation tolerance in plants vary along a continuum from highly inducible to highly constitutive. Stark and Brinda (pp. 593–603) study cultured sporophytes and vegetative shoots from a single genotype of the moss Aloina ambigua and find that the gametophytes and embryonic sporophytes employ a predominantly inducible strategy of desiccation tolerance, while the post-embryonic sporophytes employ a moderately constitutive strategy of tolerance. The transition from inducible to constitutive occurs over a period of just 2 days, and represents only the second time such a shift has been shown in a plant and the first time it has been observed in a bryophyte. Desiccation tolerance in mosses may therefore be an either/or phenomenon that varies in degree along a gradient of ecological inducibility.

Light effects on somatic and zygotic embryogenesis in larch

doi:10.1093/aob/mcu254

Phenotypes of cloned conifer embryos can be strongly influenced by a number of in vitro factors and in some instances clonal variation can exceed that found in nature. von Aderkas et al. (pp. 605–615) study embryos of hybrid larch, Larix × marschlinsii, and find that zygotic embryos, which mature in darkness inside seed cones, never accumulate phenolics whereas embryos cultured in vitro (i.e. somatic) accumulate phenolics readily. In contrast, light has a negative effect on protein accumulation. Somatic embryos that mature in light accumulate phenolics such as quercetrin in their embryonal root caps, which turn red. The results suggest that further study of abiotic factors is required to gain a better understanding of events during embryogenesis.

Control of adventitious rooting and xylogenesis in arabidopsis

doi:10.1093/aob/mcu258

The Arabidopsis thaliana transcription factors SHORT ROOT (SHR) and SCARECROW (SCR) affect primary/lateral root development, as do the auxin-influx carriers LAX3 and AUX1. Della Rovere et al. (pp. 617–628) study their involvement in adventitious root (AR) formation and xylogenesis using null-mutant seedlings grown with or without auxin-plus-cytokinin. They find that AR-formation and xylogenesis are developmental programmes that are inversely related, but which involve fine-tuning by the same proteins, namely SHR, SCR and AUX1. Pericycle activity is central for the equilibrium between xylary development and AR-formation in the hypocotyl, with a role for AUX1 in switching between, and balancing of, the two developmental programmes.

Rain as a trigger for xylogenesis in Juniperus

doi:10.1093/aob/mcu259

It has been shown that temperature triggers the onset of xylogenesis in trees after winter dormancy, but little is known about whether this is affected by moisture availability. Ren et al. (pp. 629–639) monitor xylogenesis in Juniperus przewalskii under extreme dry conditions on the north-eastern Tibetan Plateau and find that precipitation in the early growing season can be a critical trigger of xylogenesis when the thermal conditions are favourable. Xylem growth shows a positive and significant response to precipitation but not to temperature. The delay in the initiation of xylogenesis under extremely dry conditions seems to be a stress-avoidance strategy against hydraulic failure.

Petal spur development in Centranthus

doi:10.1093/aob/mcu261

When concealed by petal spurs, nectar typically is restricted to flower-visiting animals possessing a sufficient tongue length to acquire it, and thus such spurs have evolutionary significance, often leading to speciation. Mack and Davis (pp. 641–649) microscopically examine petal spur growth in Centranthus ruber and find that cell divisions dominate very early in development but it is cell elongation that leads to attainment of the spur’s final length. This pattern corroborates recent studies in Aquilegia and Linaria, inferring the existence of a common underlying mechanism for petal spur ontogeny in distant lineages of dicotyledons.

Phenotype instability modification by auxin inhibitors and 2,4-D

doi:10.1093/aob/mcu263

Barley (Hordeum vulgare) double mutants Hv-Hd/tw2, formed by hybridization, are characterized by inherited phenotypic instability in the development of inflorescences and floral organs. Šiukšta et al. (pp. 651–663) study callus cultures and intact plants and find that the auxin inhibitors HFCA and PCIB as well as 2,4-D partially rescue several different features of single and double mutants. The results suggest that ectopic auxin maxima or deficiencies arise in various regions of the inflorescence (spike)/flower primordia. Based on the phenotypic instability observed, definite trends in the development of ectopic flower structures can be outlined, from insignificant outgrowths on awns right through to flowers with sterile organs.

Phylogenetic relationships in Epidendroideae (Orchidaceae)

doi:10.1093/aob/mcu253

Epidendroideae comprise about 80 % of the approximately 25 000 species of orchids and contain most of the epiphytic and tropical species. Freudenstein and Chase (pp. 665–681) analyse seven loci from 310 genera of epidendroids and find increased resolution and support for many groups. Analysis of diversification suggests a strong association between species richness and epiphytism, but pollination characters are also correlated with richness, as are shifts in distribution. The results therefore suggest that no single character accounts for the success of this group. Rather, it appears that a succession of key features appeared that have contributed to diversification, sometimes in parallel.

Genetic structure and gene flow patterns in Ulmus

doi:10.1093/aob/mcu256

The introduction into Europe of the exotic Ulmus pumila, highly tolerant to Dutch Elm Disease, has resulted in it widely replacing native U. minor populations. Bertolasi et al. (pp. 683–692) investigate pollen dispersal patterns in a relic U. minor stand surrounded by U. minor and U. pumila trees scattered across approximately 80 km² in an agricultural area. They find high pollen immigration rates and extremely large dispersal distances, confirming that the pollen dispersal capabilities of the two species can maintain the genetic connectivity among isolated individuals and stands. The detection of a few inter-cluster pollinations and hybrid individuals suggests that hybridization, albeit rare, has occurred, supporting previous evidence for low mating barriers between U. minor and U. pumila.

UV-B and synthesis of chiloglottone floral volatiles

doi:10.1093/aob/mcu262

Excess UV-B radiation is known to result in accumulation of phenolics, carotenoids and glucosinolates in plants, but it has not been directly linked to floral volatile production. Amarasinghe et al. (pp. 693–703) study the sexually deceptive Chiloglottis trapeziformis orchid, which attracts its specific male wasp pollinators by means of chiloglottones, a newly discovered class of volatiles with unique structures. They find that while initiation of chiloglottone biosynthesis requires only UV-B light, sustained production requires both UV-B and de novo protein synthesis. The amount of chiloglottone in flowers reflects the interplay between developmental stage, time and intensity of UV-B exposure, de novo protein synthesis, and feedback loops that cap the internal amount.

Fluid properties influence diet in Nepenthes pitcher plants

doi:10.1093/aob/mcu266

The acidic fluid present in the traps of Nepenthes pitcher plants has a primary function of digesting prey, but also plays a role in the capture of insects. Bazile et al. (pp. 705–716) collect fluid in the field from pitchers of four Nepenthes species and determine retention capacity and time-to-kill for different species of ants and flies under controlled conditions. They find that both the pH and viscoelasticity properties of the fluid influence a plant’s diet and its carnivorous status. Thus the plants may select the prey that they retain by manipulating the secretion of H⁺ ions and polysaccharides in their pitcher fluid, which has implications for potential adaptive radiation of this genus with regard to the nutrient sequestration strategies of different species in various habitats.