Heteromorphy in flowers has a profound effect on breeding patterns within a species, but little is known about how it affects reproductive barriers between species. Ma et al. study a natural hybrid zone between two heterostylous primrose species, Primula beesiana and P. bulleyana, and find that all hybrid derivatives examined are backcrosses of first or later generations to P. bulleyana, and have the cpDNA of that species.Viable F1s can not be produced with P. bulleyana pollen on P. beesiana mothers. They conclude that partial ecological isolation, inhibition of heterospecific pollen and possibly complete barriers to F1 formation on P. beesiana may be enough to make F1 formation very rare in these species. The hybrid zone may thus have a finite lifespan as successive generations become more similar to P. bulleyana.
Double B-box zinc finger (DBB) proteins are recently identified plant transcriptional regulators that participate in the response to NaCl-induced stress in arabidopsis. Kiełbowicz-Matuk et al.investigate variations in a Solanum DBB protein, SsBBX24, in S. tuberosum and S. sogarandinum and find evidence of trafficking from the nucleus to the cytosol during the light period. SsBBX24 gene expression exhibits circadian cycling under control conditions, with the highest and lowest abundances of both transcript and protein occurring 8 h and 18 h after dawn, respectively. Low temperature, salinity and polyethylene glycol disturb the circadian regulation of SsBBX24 gene expression at the protein level, and the time of the day modulates the magnitude of SsBBX24 expression in response to high salt concentration. They propose that stress-dependent, post-transcriptional mechanisms alter the regulation by the circadian clock of the amount of SsBBX24 in Solanum.
In the mangrove genus Avicennia successive vascular cambia are organized in patches, creating stems with non-concentric xylem tissue surrounded by internal phloem tissue. Robert et al. monitor stem variations in A. marina trees in a natural mangrove forest over the course of one year, and find that patchiness occurs in both the radial growth and the shrinkage and swelling patterns of the stems. Radial increment is affected by fresh water availability rather than tidal inundation. They conclude that the ability to develop successive cambia in a patchy way enables Avicennia trees to adapt to changes in the prevailing environmental conditions, enhancing its survival in the highly dynamic mangrove environment.
Since the advent of molecular phylogenetics, attempts have been made to infer the evolutionary trajectories of chromosome numbers on DNA phylogenies. Sousa et al.combine cytogenetics, using fluorescent in situ hybridization (FISH), and modelling of changing chromosome numbers in a maximum-likelihood framework to study Typhonium, a genus of Araceae with 2n = 24 and 2n = 8, the lowest known count in the family. A combination of a densely sampled phylogeny, ancestral state modelling and FISH reveals that the species with n = 4 is highly derived, with the FISH data pointing to a Robertsonian fusion-like chromosome rearrangement in the ancestor of this species.
MADS-box transcriptional regulators play important roles during plant development. Sun et al. study structural and functional variation of FUL-like (AP1 subfamily), SEP-like and AGL6-like genes in the basal eudicot Epimedium sagittatum and provide a description of EsFUL-like, EsAGL2-1, EsAGL2-2 and EsAGL6-like function divergence and conservation in comparison to a selection of model core eudicots. The results highlight how organization in genomic segments containing A- and E-class genes in sequenced model species has resulted in similar topologies of AP1 and SEP-like gene trees.
Fern classification has been highly unstable in the past, largely because the morphological characters selected for emphasis in higher-level classifications have been variable. Christenhusz and Chase review the history of fern classification, and consider that some classifications based on single characteristics have been useful for identifying species but are highly artificial in terms of their views on relationships, whilst classifications using a suite of characters have been considered more ‘natural’, but with the advance of molecular phylogenetics many of these ‘natural’ groups are also been shown to be highly artificial. They highlight which groups/genera have taxonomic issues that remain to be clarified, and propose the use of broader family concepts. Progress in developing a consensus fern classification is reflected in a new proposed classification.
GPT2,a glucose 6-phosphate/phosphate translocator, plays an important role in environmental sensing in mature leaves of Arabidopsis thaliana, and its expression has also been detected in arabidopsisseeds and seedlings. Dyson et al. study wild-type A. thaliana and a gpt2 T-DNA insertion knockout line, and find that plants lacking GPT2 expression are delayed in seedling establishment, specifically in the process of cotyledon greening (rather than germination). This phenotype cannot be rescued by glucose in the growth medium, with greening being hypersensitive to glucose. Germination itself is, however, hyposensitive to glucose in the gpt2 mutant. They conclude that endogenous sugar signals function in controlling germination and the transition from heterotrophic to autotrophic growth, and that the partitioning of glucose 6-phosphate, or related metabolites, between the cytosol and the plastid modulates these developmental responses.
Stereochemical variation is common in plant secondary metabolites, but its importance in mediating plant–herbivore interactions has received little attention. Ahern and Whitney use common garden experiments with common cocklebur, Xanthium strumarium (Asteraceae), to examine relationships between stereochemical variation in sesquiterpene lactones, herbivore damage and plant fitness. They find that the stereochemistry of sesquiterpene lactone ring junctions helps explain variation in plant herbivore resistance, in turn influencing plant fitness. Their results indicate that subtle differences in stereochemistry may be a major, yet under-appreciated, determinant of the protective role of secondary metabolites.
Pterostylis is an Australasian terrestrial orchid genus of more than 400 species, most of which use a motile, touch-sensitive labellum to trap dipteran pollinators. The mode of attraction, however, is uncertain. Phillips et al. find that a single species of male gnat (Mycetophilidae) visits and pollinates the rewardless flowers of P. sanguinea, and that the gnats often show probing copulatory behaviour on the labellum, leading to its triggering and the temporary entrapment of the gnat in the flower. Pollen deposition and removal occurs as the gnat escapes from the flower via the reproductive structures. The labellum is the sole source of the chemical attractant involved. It is predicted that sexual deception will be widespread in the genus, although the diversity of floral forms suggests that other mechanisms may also operate.
Plants can adapt to their environment by varying the hydraulic integration of their xylem network. While much is known about xylem organization in aerial parts, roots have been less well studied. Johnson et al. measure xylem embolism resistance and connectivity in roots of two co-occurring tree species in a semi-arid habitat, Quercus fusiformis and Sideroxylon lanuginosum.They find that Quercus vessels are primarily solitary, while Sideroxylon xylem is highly connected, leading to resistant and vulnerable xylem networks, respectively. Pit membrane thickness plays less of a role in embolism resistance than expected, suggesting that xylem organization is an important trait that has yet to be fully explored.