A new cell-cycle to explain how secretion is released in plants (Viewpoint)
Despite being well studied, secretory processes in plants still provide an immeasurable range of questions without accurate answers: e.g. how do secretory products leave the secretory cell and cross the cell wall? Paiva (pp. 533–540) proposes a cell-cycle model developed from different secreting systems. The article shows the mechanical action of the protoplast in successive cycles of contraction and expansion, forcing the material accumulated in the periplasmic space to cross the cell wall and the cuticle. The mechanical action of the protoplast is especially relevant for releasing lipids, resins, and highly viscous hydrophilic secretions.
Introgression, genetic and morphological variation in sympatric oaks at the edge of their range
Historical and ongoing hybridization between the oak species Quercus petraea and Q. robur leads to difficulty in identifying the taxonomic status of individual trees. Despite this, Beatty et al. (pp. 541–549) using genetic and morphometric analysis of populations from Northern Ireland, which represents the northwestern limit of the species’ ranges, identified ‘pure’ individuals of both species, as well as a range of hybrid classes. Furthermore, analysis of chloroplast DNA indicates that unidirectional introgression followed by backcrossing may have facilitated long-distance recolonization after the ice ages.
Trade-offs between tree investment in primary and secondary growth are species- and environment-dependent
Franceschini et al. (pp. 551–563) explored the trade-off between primary (e.g. height) and secondary (e.g. diameter) growth by considering simultaneously the allometric exponents relating height to diameter at breast height (γh,dbh) and stem volume to diameter at breast height (αv,dbh) obtained from an 8893 stem analysis across North-Eastern America. They give evidence that the species growth strategies differed between conifers and broadleaves with respect to the mode of development. These results have important implications for quantifying future carbon stocks in managed forests.
The origin of limes and lemons
The origin of limes and lemons has been a source of conflicting taxonomic opinions. All lime and lemon accessions are highly heterozygous with interspecific admixture of two, three and even four ancestral citrus taxa genomes. Curk et al. (pp. 565–583) describe how cytoplasmic and nuclear markers finally revealed their phylgenetic origin. Citrus medica (citron) contributed to all limes and lemons and was the direct male parent for the main subgrouEps in combination with C. micrantha or close Papeda species for ‘Mexican’ lime types of Tanaka’s taxa, C. aurantium (sour orange) for lemons (C. limon). Among triploid limes, C. latifolia accessions (‘Tahiti’ and ‘Persian’ lime types) result from the fertilization of a haploid ovule of C. limon by a diploid gamete of C. aurantifolia. As limes and lemons were vegetatively propagated (apomixis, horticultural practices) the intra-subgroup phenotypic diversity results from asexual variation.
Early Devonian rooting system provides clues to the origin of lycophyte roots
The evolution of extensive and complex rooting systems during the Devonian had significant impacts on global terrestrial ecosystems. Despite the transformative nature of roots, detailed understanding of how this fundamental organ evolved is lacking. Matsunaga and Tomescu (pp. 585–598) document the rooting system of an Early Devonian basal lycophyte, including its overall architecture, patterns of growth within the substrate, and structural homology. The morphology of this rooting system has important implications for understanding the evolution of lycophyte roots and illustrates early terrestrial plant–substrate interactions.
Kelp sieve tubes cooperate better
Sieve tubes are essential for the distribution of photoassimilates in higher plants, but are notoriously sensitive and hard to study. Kelps, large brown algae, evolved sieve tubes independently. Since kelp sieve tubes are embedded in a thick, gelatinous matrix, Knoblauch et al. (pp. 599–606) hoped that they could be more easily manipulated than their analogues in land plants. Using fluorescent dyes, they visualized transport in intact bull kelp (>10 m long). Dye injection into transporting tubes generated pressure waves that transiently reversed the flow across sieve plates, which directly demonstrated the openness of the plates for bulk flow. As similar tests proved impracticable in higher plants, kelps may be more cooperative partners in studying the physics of sieve tube translocation.
Cress-seed exudate promotes hypocotyl elongation and inhibits root growth
Cress-seed (Lepidium sativum) exudate exerts an allelochemical effect, and the active principle had been reported as lepidimoic acid (LMA). Iqbal et al. (pp. 607–623) confirm that the exudate contains LMA, but show that this is not the allelochemical. LMA only slightly promoted hypocotyl elongation, and did not inhibit root growth in Amaranthus seedlings. Cress-seed exudate contained an additional acidic disaccharide, β-d-xylopyranosyl-(1→3)-d-galacturonate (Xyl→GalA), which also had little allelochemical effect. The major cress-seed allelochemical therefore remains unidentified. LMA and Xyl→GalA both probably arise by pectin degradation, but their roles await elucidation.
Endogenous BSVs illuminate Musa balbisiana diversity
Banana streak viruses (BSV) present in the B genome of banana or plantain cultivars give rise to spontaneous infections following abiotic stresses. Because cooking banana is an economically important staple fruit for several countries, knowledge about risks to banana has become essential. Duroy et al. (pp. 625–641) developed molecular tools to differentiate risk to bananas among samples of seedy BB diploids and both native areas of AAB cultivars. They propose a Musa phylogeny driven by BSV integrations, showing for the first time lineages between BB diploids and hybrids. These results pave the way to the re-introduction of safe-to-use materials into banana breeding programmes.
Leaf trait variation across the crown of isolated trees
Isolated trees subjected to high irradiance and concomitant stress may adjust leaf phenotypes across the tree crown. The plasticity in structural traits such as leaf angles or size can have profound consequences in the acquisition of the light resource. Escribano-Rocafort et al. (pp. 643–651) show that the expression of leaf traits in Olea europaea varies continuously through the crown in a gradient of leaf morphotypes and leaf angles depending on the exposure and location of individual leaves. The distribution of light-related traits within the tree crown could lead to complementary light-interception patterns at the individual scale.
Excessive sulphur accumulation and ionic storage behaviour identified in species of Acacia (Leguminosae: Mimosoideae)
Reid et al. (pp. 653–666) show a group of native Australian plants that exhibit thiophore behaviour. This behaviour is independent of substrate type, and present in all members of this related group of species. Other species in the same substrate do not show the same behaviour. The thiophores studied exhibit a tight coupling between sulphur and calcium uptake and storage, and apparently store these elements as inorganic salts within the cells in their foliage. Several of the thiophores occupy coastal or riparian habitats, suggesting the evolutionary and ecophysiological explanations for this trait do not lie solely in adaptation to arid conditions or gypsiferous soils.
Plant injury can reset the ageing clock in plants but only slightly alleviates senescence
Senescence is the process of losing fitness when growing old, and is shaped by the trade-off between maintenance and reproduction that makes reproduction more unsure with age. Injury to the plant body, however, provides an opportunity to reset the ageing clock and consequently potentially ameliorate senescence. Martínková et al. (pp. 667–679) examine the effects of injury on life span and on traits closely related to fitness. They report that injury to the plant body is able to reset the ageing clock; however it is so harmful to plant fitness that resprouting serves to only slightly alleviate senescence signs and injury does not cause effective rejuvenation.
Plastic adaptive cleistogamy in Lamium amplexicaule
Cleistogamous plants have the ability to produce closed obligately selfing flowers, and open potentially outcrossed flowers. Flower proportion is known to vary in response to reliable cues, which suggests that outcrossing rate can adapt to environmental variation. Using a common garden experimental design for the annual cleistogamous species Lamium amplexicaule, Stojanova et al. (pp. 681–691) show that the proportion of cleistogamous flowers is plastic with seasonal variation. Moreover, they show that the proportion of cleistogamous flowers realized in each season maximizes fitness. The study shows that outcrossing plasticity is adaptive through the variation of cleistogamy across seasons.