Special Issue on Halophytes and Saline Adaptations
Most of the water on Earth is seawater, each kiolgram of which contains about 35 g of salt, and yet most plants cannot grow in this solution; less than 0.2 % of species can develop and reproduce with repeated exposure to seawater. These ‘extremophiles’ are called halophytes.
Improved knowledge of halophytes is of importance to understanding our natural world and to enable the use of some of these fascinating plants in land re-vegetation, as forages for livestock, and to develop salt-tolerant crops. Annals of Botany has recently published a Special Issue on Halophytes and Saline Adaptations, in which the evolution of salt tolerance in halophytes, their life-history traits and progress in understanding the molecular, biochemical and physiological mechanisms contributing to salt tolerance are summarized in 16 papers by leading researchers in the subject. In particular, cellular processes that underpin the ability of halophytes to tolerate high tissue concentrations of Na+ and Cl–, including regulation of membrane transport, their ability to synthesize compatible solutes and to deal with reactive oxygen species, are highlighted. Interacting stress factors in addition to salinity, such as heavy metals and flooding, are also topics gaining increased attention in the search to understand the biology of halophytes, and are addressed in this Special Issue.
Genetic linkage maps for apomictic Hieracium species
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. 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.
Genetic determinants of stomatal conductance
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. 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 (gs). 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 gs. Differences between operating gs based on pore dimensions and maximum gs based on stomatal dimensions are large, suggesting that stomata utilize only a small portion of their operating capacity.
Sustainable intensification in agricultural systems (Invited Review)
Sustainable agroecosystems tend to have a positive impact on natural, social and human capital, while unsustainable ones deplete these assets, leaving fewer for the future. In this review, Pretty and Bharucha define sustainable intensification (SI) as a process or system where agricultural yields are increased without adverse environmental impact and without the conversion of additional non-agricultural land. They analyse recent evidence of the impacts of SI in both developing and industrialized countries, and demonstrate that both yield and natural capital dividends can occur. They conclude with observations on policies and incentives necessary for the wider adoption of SI, and indicate how SI could both promote transitions towards greener economies as well as benefit from progress in other sectors.
Flow–Force theory and modelling nitrogen uptake (Viewpoint)
Recent molecular and histochemical analyses of nitrate transporters have cast doubt on the ability of the Enzyme–Substrate interpretation of analysis of nitrate influx isotherms to improve modelling of N uptake in agronomic models. Le Deunff and Malagoli advocate the use an alternative formalism, the Flow–Force theory, to describe ion isotherms based upon biophysical ‘flows and forces’ relationships of non-equilibrium thermodynamics. This formalism can be combined easily with changes in the nitrate influx rate induced by climatic and in planta factors formalized by polynomial curves. They argue that application of the Flow–Force formalism allows nitrate uptake to be modelled in a more realistic manner, and allows scaling-up in time and space of the regulation of nitrate uptake across the plant growth cycle.
Multi-imaging approach to study the root–soil interface
Dynamic processes occurring at the soil–root interface crucially influence soil physical, chemical and biological properties at a local scale around the roots, and are technically challenging to capture in situ. Rudolph-Mohr et al. introduce a novel combination of neutron and fluorescence imaging to study the spatiotemporal patterns of oxygen, pH and water content distribution in the rhizosphere of living roots of lupin (Lupinus albus) grown in rhizotrons. The combined imaging set-up enables them to relate observed changes in soil pH and oxygen levels to root structure and soil water content non-destructively.
Increased competitive ability in an invasive plant
A common hypothesis explaining plants’ invasive success is the evolution of increased competitive ability (EICA). Joshi et al. study the invasive plant Lythrum salicaria (purple loosestrife) and carefully distinguish between competitive effects and responses of invasive and native plants, under both intra- and interspecific competition. They find that while intraspecific competition results in no differences in competitive effects or responses between native and invasive plants, interspecific competition produces greater competitive effects and responses of invasive plants in terms of both biomass and seed production. They conclude that under interspecific competition there is strong support for the EICA hypothesis, with an evolutionarily increased competitive ability in invasive populations of L. salicaria.
Biogeography of circumboreal taxa of Pyrola
Arctic-Alpine species in the northern hemisphere are occasionally derived from Tertiary relict groups, but the genus Pyrola (Ericaceae) appears to be exceptional and lineages may have arisen multiple times. Liu et al. reconstruct the biogeographic history of Pyrola based on a phylogenetic analysis, and find that three circumboreal lineages have arisen independently, and at least two of these appear to have originated in Asia. The cool, high-altitude habitats of many Pyrola species together with the fact that diversification in the genus coincided with global cooling from the late Miocene onwards fits a hypothesis of pre-adaptation to become circumboreal within this group.
Cryptic refugia of Cryptomeria japonica in last glacial maximum
Distribution shifts and natural selection through past climatic changes are important factors in determining the genetic structure of forest species. Kimura et al. use a combination of phylogeography and species distribution modelling methods to identify the responses of Cryptomeria japonica, endemic to the Japanese Archipelago, to past climatic changes. They identify four gene pools, one of which is likely to have been established by cryptic refugia. An ambiguous genetic diversity cline along elevations would be predicted by the distribution pattern and accessibility of refugial populations after the last glacial maximum, suggesting multiple refugia and/or admixture events affected the overall structure of genetic diversity.
Evolution of genome size and chromosome numbers in Genlisea
Some species of the carnivorous plant genus Genlisea (Lentibulariaceae) possess ultrasmall nuclear genomes. Fleischmann et al. measure nuclear genome sizes from cultivated plant material for a comprehensive taxon sampling that includes nearly half of all species, and perform chromosome counts for the majority of these taxa. They find a high degree of congruence between genome size distribution and the major phylogenetic lineages. Ultrasmall genomes with 1C-values of less than 100 Mbp are almost exclusively found in a derived lineage of South American species, and they determine the smallest plant genomes known to date in G. tuberosa (1C = approx. 61 Mbp) and some strains of G. aurea (1C = approx. 64 Mbp).