Some Lupinus species produce cluster roots in response to low plant phosphorus (P) status. Wang et al. measure relative growth rate (RGR) among three lupin species, L. albus, L. pilosus and L. atlanticus, with similar shoot P status and find that cluster-root formation is suppressed at high leaf P concentration, irrespective of RGR. Variation in cluster-root formation among these species cannot be explained by species-specific variation in RGR or leaf P concentration. This goes against the expectation that, even when P-uptake rates are high, plants with fast growth rates might not accumulate shoot P and hence a correlation between RGR and cluster-root formation could be anticipated.
In citrus the juvenile phase can last 5–20 years depending on the variety, which is a serious constraint for molecular and conventional breeding. Castillo et al. screen juvenile and adult Citrus sinensis and C. jambhiri to identify differentially expressed transcription factors, and incorporate data from C. reticulat × C. sinensis and C. paradisi in order to select genes with phase-specific regulation common to all four species. Some of the identified genes are MADS-box genes, whereas the others show high partial sequence similarity restricted to specific domains but negligible outside those domains, suggesting that they might be novel genes that play specific roles during the juvenile-to-adult transition in citrus.
Non-S-ribonucleases (non-S-RNases) are class III T2 RNases expressed in the styles of species exhibiting S-RNase-based self-incompatibility (SI). So far, no role has been attributed to these RNases, which are not functional in the SI system. By combining RT-PCR, immunoblot and enzymatic activity approaches, Rojas et al. demonstrate that NnSR1 (Nicotiana non-S-RNase1) is induced in roots of Nicotiana alata subjected to phosphate deprivation, resembling the functionality of phylogenetically divergent class I and class II S-like RNases. NnSR1 appears to have regained ancestral functions of class III RNases related to strategies to cope with phosphate limitation and possibly with other environmental challenges.
Isoprene is the most important volatile organic compound (VOC) emitted by plants. Morfopoulos et al. hypothesize that NADPH availability for isoprene biosynthesis depends on the balance of electron supply and Calvin cycle demand. A simple model based on this hypothesis explains many features of the observed response of isoprene emissions to environmental factors. The decoupling between carbon assimilation and isoprene emission (e.g. opposite responses to CO2) is accounted for, whereas previous models resorted to empirical corrections. This work suggests a way forward to global-scale VOC emission models that will be both simpler, and more robust, than those currently favoured.
Global annual losses in agricultural production from salt-affected land are in excess of US$12 billion and rising. Shabala identifies physiological mechanisms conferring salinity tolerance in halophytes that can be introduced into non-halophyte crop species to improve their performance under saline conditions. The specific traits that are discussed and advocated include: manipulation of trichome shape, size and density to enable their use for external Na+ sequestration; increasing the efficiency of internal Na+ sequestration in vacuoles by the orchestrated regulation of tonoplast NHX exchangers and slow and fast vacuolar channels, combined with greater cytosolic K+ retention; controlling stomata aperture and optimizing water use efficiency by reducing stomata density; and efficient control of xylem ion loading.
Extreme water stress episodes induce tree mortality, but the mechanistic relationships linking stem embolism and species drought performance remain poorly understood. Barigah et al. study potted juvenile trees of beech (Fagus sylvatica) and poplar (Populus deltoides × P. nigra) and find that the xylem pressure inducing 50 % mortality differs sharply between the species, being 1.75 and 4.5 MPa in poplar and beech, respectively. However, the relationships between tree mortality and the degree of cavitation in the stems are similar, with mortality occurring suddenly when >90 % cavitation has occurred. This is in contrast to the 50 % embolism threshold reported for conifers. The results demonstrate that massive cavitation is probably a causal factor for tree mortality under extreme water stress conditions.
Trichomes are epidermal outgrowths generally associated with protection against desiccation and herbivores. These structures are widely distributed on plant parts but evolutionary studies of trichomes are still scarce. Nogueira et al. integrate phylogenetic and morpho-anatomical data in a study on the evolutionary history of the tribe Bignonieae in order to understand current patterns of diversity of trichome types. They find considerable topological variation in the evolution of glandular and non-glandular trichomes, suggesting that the most recent common ancestor of Bignonieae probably presented different trichome types on different plant parts. The descriptions and ontology presented will greatly facilitate comparisons between morphological variants and functional studies of trichomes.
The coexistence of forest tree species has often been linked to differences in their response to light availability during the regeneration stage. Van Couwenberghe et al. study natural regenerated shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings and find that no rank reversal occurs between the two species along a light gradient, or along density, mixture or seedling-size gradients. The results thus do not support the classical assumption that spatial heterogeneity in a canopy opening would explain the coexistence of the two species studied. Instead, it is suggested that the main driver of the dynamics of these mixed stands is spatial variation in local size hierarchies among seedlings, which may be caused by differences in seedling emergence time or initial seedling performance.
Angiosperm trees generally form tension wood, a special type of secondary xylem, in response to a gravitational stimulus. Nugroho et al. find that pre-treatment with paclobutrazole and uniconazole-P, inhibitors of the synthesis of gibberellin, to inclined Acacia mangium seedlings inhibits negative gravitropism of the stems. The inhibitors suppress increases in the thickness of gelatinous layers and the elongation of gelatinous fibres in the tension wood. In contrast, pre-treatment with gibberellin stimulates the elongation of these fibres. The results suggest that gibberellin is important for the development of gelatinous fibres, and therefore in gravitropism.
The Orchidaceae have a history of recurring convergent evolution in floral function as nectar production has evolved repeatedly from an ancestral nectarless state. Hobbhahn et al. study the South African orchid genus Disa and find that independent nectary evolution has involved both repeated recapitulation of secretory epidermis, which is present in the sister genus Brownleea, and innovation of stomatal nectaries. These contrasting nectary types and positional diversity within types imply weak genetic, developmental or physiological constraints in ancestral, nectarless Disa. With its morphologically diverse solutions to the problem of nectar production, Disa is a good example illustrating the contribution of functional convergence to phenotypic diversification, which probably also underlies the extensive diversity of nectary types and positions in the orchid family.