Legume nitrogen fixation is inhibited by soil water deficit and under drought conditions ureidic legumes, such as common bean and soybean, accumulate ureides, which are the main products of N2 fixation in these plants. Coleto et al. study genotypes of common bean, Phaseolus vulgaris, with variable degrees of N2-fixation tolerance to water stress, and find variable accumulation of ureides in their leaves. There is no accumulation of ureides in the nodules of any of the genotypes and the rise in leaves occurs even after complete inhibition of N2-fixation, probably as the result of remobilization of nitrogen from stressed tissues. They therefore conclude that shoot ureide accumulation after prolonged exposure to drought is not a cause of feedback inhibition of nitrogen fixation.
Although plants and many algae (e.g. the Phaeophyceae, brown, and Rhodophyceae, red) are only very distantly related they are united in their possession of carbohydrate-rich cell walls, which are of integral importance being involved in many physiological processes. Furthermore, wall components have applications within food, fuel, pharmaceuticals, fibres (e.g. for textiles and paper) and building materials and have long been an active topic of research. As the major deposit of photosynthetically fixed carbon, and therefore energy investment, cell walls are of undisputed importance to the organisms that possess them, the photosynthetic eukaryotes (plants and algae). The complexities of cell wall components along with their interactions with the biotic and abiotic environment are becoming increasingly revealed.
The importance of plant and algal cell walls and their individual components to the function and survival of the organism, and for a number of industrial applications, are illustrated by the breadth of topics covered in a newly published Special Issue of Annals of Botany, containing 27 papers concentrating on various plants and algae, developmental stages, organs, cell wall components, and techniques. The papers are organized into topics under the general headings of (1) cell wall biosynthesis and remodelling, (2) cell wall diversity, and (3) application of new technologies to cell walls, and the Special Issue is available as FREE ACCESS online until 14 December.
In their preface the Editors of this Special Issue, Zoë Popper, Marie-Christine Ralet and David Domozych, consider future directions within plant cell wall research. Expansion of the industrial uses of cell walls and potentially novel uses of cell wall components are both avenues likely to direct future research activities. Fundamentally, it is the continued progression from characterization (structure, metabolism, properties and localization) of individual cell wall components through to defining their roles in almost every aspect of plant and algal physiology that will present many of the major challenges in future cell wall research.
Heartwood formation is a unique phenomenon of tree species but the mechanisms by which the substances involved accumulate are unclear. Kuroda et al. use time-of-flight secondary ion mass spectrometry (TOF-SIMS) in conjunction with quantitative analyses to study the distribution of ferruginol in a 30-year tree of Cryptomeria japonica (Taxodiaceae). They find that accumulation begins in the middle of the intermediate wood, initially in the earlywood near the annual ring boundary, then throughout the entire earlywood, and finally across to the whole annual ring in the heartwood. They conclude that the heterogeneous timing of ferruginol accumulation could be related to the distribution of ray parenchyma cells and/or water in the heartwood-forming xylem.
The continuous formation of earlywood vessels is crucial for the growth of ring-porous hardwoods. Kudo et al. study Quercus serrate seedlings and find that a combination of localized heating and disbudding of dormant stems results in earlier cambial reactivation and differentiation of first vessel elements than in non-heated seedlings. A few narrow vessel elements are formed during heating after disbudding, while many large earlywood vessel elements are formed in heated seedlings with buds. The results suggest that elevated temperature is a direct trigger for differentiation of first vessel elements, and that whilst bud growth is not essential for differentiation of first vessel elements, it might be important for the continuous formation of wide vessel elements.
Increased homozygosity caused by population fragmentation can directly affect individual plant fitness through the expression of deleterious alleles, and drought stress induced by climate change may exacerbate these effects. Vranckx et al. investigate various transpiration and growth traits of seedlings of pedunculate oak, Quercus robur, correlate them with their multilocus heterozygosity (MLH), and then study the effects of drought stress on these relationships. They find significant heterozygosity–fitness correlations for most fitness traits, and high atmospheric stress increases the strength of these correlations for the transpiration variables. They conclude that that ongoing climate change may strengthen the negative fitness responses to low MLH, highlighting the need to maximize individual multilocus heterozygosity in forest tree breeding programs.
The Lathyrus genus (Fabaceae) includes 160 species, some of which have economic importance as food, fodder and ornamental crops (mainly L. sativus, L. cicera and L. odoratus, respectively) and are cultivated in over 1.5 million ha worldwide. Vaz Patto and Rubiales review the current status and future prospects of Lathyrus diversity conservation and characterization, highlighting their use in L. sativus and L. cicera breeding. They conclude that efforts for improvement of these species should concentrate on the development of publicly available joint core collections, and on high-resolution genotyping. This should result in more efficient and faster breeding approaches, which are especially needed for these neglected, under-utilized Lathyrus species.
Using a thermodynamic flow–force interpretation of nitrate uptake isotherms, Malagoli and Le Deunff develop a functional– structural model to predict N uptake in winter oilseed rape, Brassica napus. The structural component of the model, the active root biomass, is derived from a combination of root mapping in the field, the relationship between specific root length and external nitrate concentration, and the assignment of an absorption capacity related to integrated root system age. They find that model simulations are well matched to measured data for N uptake under field conditions at three different levels of fertilizer application. Model ouputs indicate that the topsoil layers contain about 80 % of the total root system and account for 90–95 % of N taken up at harvest.
Difficulties in linking the various regulations of nitrate transport acting at different levels of time and on different spatial scales have hindered the development of models for nitrogen uptake. Le Deunff and Malagoli substitute the more usual enzyme–substrate interpretation for a ﬂow–force approach of nitrogen uptake isotherms and combine it with experimentally determined regulation in order to model nitrate in winter oilseed rape, Brassica napus. This approach avoids the use of unique nitrate uptake reference kinetics and allows root plasticity in response to environmental and in planta factors to be taken into account. Furthermore, it allows the regulation of nitrate uptake by roots to be scaled up relatively easily in time from hours to months.
Maintenance of tension in the transpiration stream requires coherence of the water molecules, but also their adherence to vessel walls (i.e. the lumen-facing surface must be wettable). As lignin is generally considered hydrophobic this presents a conundrum. McCully et al. observe wall contact with oil perfused into emptied vessels of maize (Zea mays) roots, contact angles of refilling sap, and wall substantivity to histochemical probes. Their observations reveal fine-scale heterogeneity of surface wettability, with lumen-facing bordered pits having hydrophilic aperture rims that repel oil, while oil adheres strongly to pit borders in empty vessels. As sap refills vessels through pits their borders become increasingly wettable by sap, which then spreads along the walls with low contact angles.
Oaks can provide a model to study hybridization as they often maintain species integrity and distinct environmental adaptations despite recurrent gene flow. Lind-Riehl et al. identify candidate genes potentially involved in local adaptation in the interfertile species Quercus rubra and Q. ellipsoidalis, which are characterized by contrasting adaptations to drought. They find that a CONSTANS-like (COL) gene is nearly fixed on alternative alleles in both species, as reflected by high FST values (55–80 %) between neighbouring Q. rubra and Q. ellipsoidalis populations. All other markers show low interspecific differentiation (approx. 5 %). They conclude that this COL gene may play a role in adaptive divergence and reproductive isolation (via flowering time), making it a promising candidate speciation gene.