N-source preference in plants (Viewpoint)
doi:10.1093/aob/mct157
Plants can utilize two major forms of inorganic nitrogen, nitrate (NO3−) and ammonium (NH4+), with some species appearing to ‘prefer’ one form over another, under certain conditions. Soil-N speciation has been shown to be an important determinant of species distribution, but no ecophysiologically realistic, mathematically sound model has yet emerged to describe and predict this phenomenon. Britto and Kronzucker (pp. 957–963) consider that this is because assignment of such preferences is not straightforward, and must take into account a wide array of complex physiological and environmental features, which interact in ways that are still not well understood.
Selenium toxicity in plants (Botanical Briefing)
doi:10.1093/aob/mct163
Despite its toxicity in plants at higher levels, crops supply most of the essential dietary selenium in humans. Selenium toxicity in plants has been attributed to the formation of non-specific selenoproteins; however, this paradigm can be challenged now that there is increasingly abundant evidence suggesting that selenium-induced oxidative stress also contributes to toxicity in plants. Van Hoewyk (pp. 965–972) provides an updated framework on plant selenium toxicity by indicating that both malformed selenoproteins and oxidative stress drive selenium-induced stress. Although challenging, distinguishing between these two distinct types of stress may be useful in the future development of crops with fortified levels of selenium.
Basal root whorl number and P acquisition
doi:10.1093/aob/mct164
Root architectural phenes enhancing topsoil foraging are important for phosphorus acquisition. Miguel et al. (pp. 973–982) describe the utility of a novel phene, basal root whorl number (BRWN), that has significant effects on P acquisition in common bean (Phaseolus vulgaris). They define whorls as distinct tiers of basal roots that emerge in a tetrarch fashion along the base of the hypocotyl, and screen wild and domesticated bean taxa as well as a recombinant inbred line populations for BRWN and basal root number. The results indicate that BRWN is associated with increased phosphorus acquisition and that this trait may have value for selection of genotypes with better performance in low-phosphorus soils.
Phylogenetics, biogeography and trait evolution in tribe Arabideae
doi:10.1093/aob/mct165
Tribe Arabideae is the most species-rich monophyletic lineage in Brassicaceae, with more than 500 species distributed in the majority of mountain and alpine regions worldwide. Karl and Koch (pp. 983–1001) provide a comprehensive phylogenetic analysis for the species assemblage and test for association of trait and characters. They demonstrate the paraphyly of the genus Arabis and consider that the centre of origin of tribe Arabideae is most likely the Irano-Turanian region. Mid-Miocene early diversification started with increased speciation rates due to the emergence of various annual lineages. Subsequent radiations were mostly driven by diversification within perennial species during the Pliocene, but increased speciation rates also occurred during that epoch.
The tannosome: a new chloroplast-derived organelle
doi:10.1093/aob/mct168
Condensed tannins (proanthocyanidins) are widely distributed amongst the vascular plants (Tracheophyta). Brillouet et al. (pp. 1003–1014) use a variety of techniques to examine tannin-rich samples from various organs across a wide range of vascular plants and find that tannins are polymerized in a new thylakoid-derived chlorophyllous organelle, which they name the tannosome. These are formed by pearling of the thylakoids into 30-nm spheres, which are then encapsulated in a tannosome shuttle formed by budding from the chloroplast and bound by a membrane resulting from the fusion of both chloroplast envelopes. The shuttle conveys tannosomes through the cytoplasm to the vacuole, into which they are incorporated by invagination of the tonoplast. Finally, shuttles bound by a portion of tonoplast aggregate into tannin accretions, which are stored in the vacuole.
Evolution and biogeography of Anthoxanthum
doi:10.1093/aob/mct170
Repeated hybridizations and/or polyploidizations confound taxonomic classification and phylogenetic inference, and multiple colonizations at different time scales complicate biogeographic reconstructions. Pimentel et al. (pp. 1015–1030) sequence three plastid and two nuclear DNA regions in 17 Anthoxanthum taxa in order to unravel the role of these processes in shaping the current structure and diversity of the genus. Variation of floral morphology in Anthoxanthum (sections Anthoxanthum and Ataxia) can be explained by a Miocenic hybridization event between lineages with one and three fertile florets. All diversification events in the genus except one are dated back to between the Late Pliocene and the Late Pleistocene. Africa was apparently colonized twice from two different sources, namely Europe and East Asia.
Stomatal development of early angiosperms
doi:10.1093/aob/mct169
The detailed structure of stomata differs between taxa, suggesting contrasting morphogenetic pathways, with implications for the evolution of plant diversity. Rudall and Knowles (pp. 1031–1043) present a novel ultrastructural study of developing stomata in leaves of several representative early-divergent angiosperms, including Amborella. These phylogenetically pivotal taxa are essential as comparative yardsticks for understanding the evolution of stomatal development. They conclude that similar mature stomatal phenotypes can result from contrasting morphogenetic factors, often involving at least one asymmetric division, although waterlilies differ in this respect.
Tapetum and middle layer control of male fertility in kiwifruit
doi:10.1093/aob/mct173
Kiwifruit (Actinidia deliciosa) male-sterile anthers abort at the microspore-stage, but the role of sporophytic tissues in controlling male-sterility is unknown. Falasca et al. (pp. 1045–1055) show that the tapetum and middle-layer exhibit secretory activity and degenerate by programmed cell death (PCD), but PCD is later in male-sterile than in male-fertile anthers. Calcium accumulates in cell walls of the middle layer and tapetum and in the exine of microspores and pollen, reaching higher levels in male-sterile anthers. They conclude that male sterility in kiwifruit is induced by anther wall tissues through prolonged secretory activity caused by a delay in PCD, in the middle layer in particular. In vitro culture results support the sporophytic control of male fertility and open the way to applications to overcome dioecism and optimize kiwifruit production.
Phylogenomics and taxonomy of an isolated panicoid lineage
doi:10.1093/aob/mct174
Lecomtella madagascariensis is a monotypic grass lineage, micro-endemic to the Andringitra massif, Madagascar. Besnard et al. (pp. 1057–1066) report a phylogenomic study and a taxonomical revision of this species. They show that it belongs to a lineage sister to three panicoid tribes of major economic importance distributed worldwide. It is also the last known member of an ancient lineage that has evolved independently for more than 20 million years and thus has a high conservation value. In addition, the study shows that next generation sequencing can be applied to rapidly generate abundant phylogenetic information, opening new avenues for plant phylogenomics.
Morphogenesis and contact sites in lobed mesophyll cells
doi:10.1093/aob/mct175
The morphogenesis of lobed mesophyll cells (MCs) is highly controlled and coupled with intercellular space formation. Giannoutsou et al. (pp. 1067–1081) study morphogenesis of lobed MCs of Zea mays and find that it is characterized by early, patterned differentiation of two distinct cell wall subdomains. The first defines the sites of the future MC contacts and is characterized by (1 → 3, 1 → 4)-β-D-glucan degradation, the presence of callose and pectins that are recognized by 2F4, LM6, JIM5 and JIM7 antibodies, and the formation of secondary plasmodesmata clusterings. The second defines the positions of cell isthmi and is marked by the presence of (1 → 3, 1 → 4)-β-D-glucans and the local deposition of cellulose microfibril rings. This patterned cell wall differentiation precedes the cortical cytoskeleton reorganization, which controls MC morphogenesis.
Novel structure of placenta in a lycophyte
doi:10.1093/aob/mct178
Lycopodium obscurum has a subterranean, mycoheterotrophic gametophyte that nourishes the embryo for several years. An examination of an embryo in an underground gametophyte by Renzaglia and Whittier (pp. 1083–1088) reveals a massive foot with ultrastructural variability comparable to that across major clades. The intergenerational zone in unlobed regions shows unidirectional transport of materials toward the foot. Lobed, more mature areas contain degenerated gametophyte cells that lack wall ingrowths and sporophytic transfer cells. They conclude that placental features in Lycopodium reflect a dynamic, invasive and long-lived foot, and the unique reorientation of all embryonic regions during development. Homoplasy in transfer cell appearance and location is explained by diverse patterns of embryology across archegoniates.
AM fungi and P acquisition by hemiparasitic Pedicularis
doi:10.1093/aob/mct177
Most parasitic plants do not form mycorrhizal associations and hence the nutritional role of arbuscular mycorrhizal (AM) fungi in them has not been ascertained. Using 32P-labelling in compartmented pots, Li et al. (pp. 1089–1098) study the facultative root hemiparasitic species Pedicularis rex and P. tricolor and find that AM colonization is low, and less than 1 % of total plant P is taken up from the soil via the AM fungus. Inoculation with AM fungi strongly interferes with P acquisition by both species from their host plant, barley, and this is almost certainly because the numbers of haustoria formed by the parasite are significantly reduced in AM plants.
Nutrient requirements differ in two sympatric hemiparasitic species
doi:10.1093/aob/mct179
The reasons for marked differences in growth performance of hemiparasites when attached to different hosts are not fully understood. Li et al. (pp. 1099–1106) study the responses to inorganic solutes in the absence of a host plant of two facultative root hemiparasitic Pedicularis species, P. rex and P. tricolor, that have different host dependency and preference. They find that the two species differ in nutrient requirements and biomass allocation, and conclude that distinct interspecific traits in growth and nutrient requirements could therefore be driving forces for the differential interactions between hemiparasites and their hosts.
Auxin and H2O2 interaction in control of root growth
doi:10.1093/aob/mct181
One of the best-known effects of the plant hormone auxin on plant development is the inhibition of root elongation, yet the underlying mechanism remains poorly understood. Using tomato (Solanum lycopersicum) as a model, Ivanchenko et al. (pp. 1107–1116) demonstrate that auxin acts by increasing the level of hydrogen peroxide (H2O2) in the root tip. Increasing the auxin level triggers accumulation of H2O2, leading to inhibition of root cell elongation and root growth. The diageotropica (dgt) mutation in tomato, which confers auxin resistance, affects this pathway by reducing the auxin responsiveness of tissues and by disrupting the H2O2 homeostasis in the root tip.
Chamaecrista flexuosa: a possible case of atypical enantiostyly
doi:10.1093/aob/mct188
Enantiostyly involves reciprocity in the relative positions of the sexual elements within the flower, resulting in morphologically and functionally distinct floral forms. de Almeida et al. (pp. 1117–1123) investigate reproductive biology and the presence of deviations from the enantiostylous standard form in Chamaecrista flexuosa (Fabaceae), a species mainly pollinated by large bees. They identify morphologically right and morphologically left floral morphs, together with functionally right, functionally central and functionally left morphs. Combinations of morphologically and functionally defined morphs do not occur in equal proportions. The results indicate the occurrence of an atypical enantiostyly in C. flexuosa, which seems to improve reproductive success by increasing the efficiency of pollen deposition and capture.
Flooding tolerance in interspecific introgression lines in Zea
doi:10.1093/aob/mct160
Nicaraguan teosinte Zea nicaraguensis, a species found in frequently flooded areas, is useful germplasm for breeding flooding-tolerant maize. Mano and Omori (pp. 1125–1139) select flooding-tolerant lines using a library of introgression lines (ILs), each containing a chromosome segment from Z. nicaraguensis in the maize (Z. mays) inbred Mi29. The most flooding-tolerant line they identify contains a Z. nicaraguensis chromosome segment on the long arm of chromosome 4, suggesting the presence of a major quantitative trait locus (QTL) in that region. There is no significant relationship between the capacity to form constitutive aerenchyma and flooding tolerance in the ILs, indicating the presence of other factors related to flooding tolerance under reducing soil conditions.
Silica nanoparticles and structural leaf coloration
doi:10.1093/aob/mct172
Plants with iridescent blue leaves are occasionally found in the understoreys of tropical rainforests, and the structural basis for this colour has been found in cellulose layering, either helicoidal or in dense electron-opaque bands, or in the thickness of thylakoid membranes in modified plastids. Strout et al. (pp. 1141–1148) show that in Mapania caudata, a sedge native to south-east Asia, the iridescence is produced by helicoidal cellulose microfibril deposition of 167 nm thickness for a 180 ° precession, coupled with the deposition of silica nanoparticles at discrete positions in the helicoid. Extraction of silica from the wall removes the blue iridescence. Thus, both the helicoidal wall and silica are necessary for colour production.
Avoidance of ozone-induced stress via stomatal closure
doi:10.1093/aob/mct166
Stomatal closure can be caused by an increase in internal CO2 concentration resulting from ozone-induced inhibition of carbon assimilation by chloroplasts; however, if stomata can close as a direct response to O3 then such inhibition could be avoided. Hoshika et al. (pp. 1149–1158) modify a coupled photosynthesis–stomatal model to test this hypothesis in Siebold's beech (Fagus crenata) exposed to ambient and elevated O3 and find that O3-induced stomatal closure during early summer reduces influx and allows the maximum photosynthetic capacity to be reached. However, in late summer and autumn the decrease in stomatal conductance is small and accompanied by an ozone-induced decline of photosynthetic capacity.
Reproductive differentiation into sexual and apomictic cytotypes
doi:10.1093/aob/mct167
Mechanisms enabling the maintenance of sexual and apomictic modes of seed formation in sympatry are poorly understood. Dobeš et al. (pp. 1159–1168) study populations of Potentilla puberula representing various combinations of five polyploid cytotypes (tetraploid–octoploid) and use a flow cytometric seed screen to calculate the male genomic contribution to the embryo and endosperm. They find a rare example of intraspecific differentiation into sexual and apomictic cytotypes at the polyploid level, with the integrity of sexual tetraploids being maintained due to reproductive isolation from the apomictic higher polyploids. The functionality of the gametophytic self-incompatibility system suggests that the tetraploids are functional diploids.
Tropical habitat mosaics and quantitative trait divergence
doi:10.1093/aob/mct176
In habitat mosaics, plant populations face environmental heterogeneity over short geographical distances, and such steep environmental gradients can induce ecological divergence. Brousseau et al. (pp. 1169–1179) study phenotypic differentiation for two congeneric, sympatric and ecologically divergent tree species, Eperua falcata and E. grandiflora (Fabaceae), within continuous populations occupying different habitats in a lowland rainforest. They find that mother trees from different habitats transmit divergent trait values to their progeny, and suggest that environmental variation selects for different trait optima even at a very local spatial scale. Traits for which differentiation within species follows the same pattern as differentiation between species indicate that the same ecological processes underlie intra- and interspecific variation.
Duration of shoot elongation in Scots pine
doi:10.1093/aob/mct180
The within-season timing of shoot growth in trees has often been considered independently of shoot growth rate. Schiestl-Aalto et al. (pp. 1181–1191) study lateral shoot growth in Scots pine (Pinus sylvestris) over 7 years and find that daily maximum growth rate correlates positively with growth duration, expressed as thermal time. Higher July–August temperature of the previous summer also prolongs the growth period. The results suggest that the thermal-time requirement for completion of lateral shoot extension in Scots pine may interact with resource availability to the shoot, both from year to year and among shoots in a crown each year. If growing season temperatures rise in the future, this will affect not only the rate of shoot growth but also its duration.
Nutrient reserves in geophytes may allow genome size increase
doi:10.1093/aob/mct185
The genome size of an organism is determined by its capacity to tolerate genome expansion. Veselý et al. (pp. 1193–1200) study 47 pairs of geophytic taxa and their closest non-geophytic relatives across the whole angiosperms. They find increased genome sizes in geophytes compared to their non-geophytic relatives, regardless of the storage organ type and regardless of whether or not vernal geophytes, polyploids or annuals are included in the analyses. The tendency of geophytes to possess higher genome size suggests the presence of a universal mechanism allowing for genome expansion. It is assumed that this is primarily due to the nutrient and energetic independence of geophytes perhaps allowing continuous synthesis of DNA, which is known to proceed in extreme cases of vernal geophytes even in dormant stages.