Effects of cell number and cell size on petiole length variation in a stoloniferous herb

Patterns of Leaf and Fruit Morphological Variation in Marginal Populations of Acer tataricum L. subsp. tataricum

Marginal populations are usually smaller and more isolated and grow in less favourable conditions than those at the distribution centre. The variability of these populations is of high importance, as it can support the adaptations needed for the conditions that they grow in. In this research, the morphological variability of eight Tatar maple (Acer tataricum L. subsp. tataricum) populations was analysed. Tatar maple is an insect-pollinated and wind-dispersed shrub/tree, whose northwestern distribution edge is in southeastern Europe. Morphometric methods were used to analyse the variability of the populations using leaf and fruit morphology. The research revealed significant differences between and within populations. Furthermore, differences in the distribution of the total variability were noted, which suggest that different evolutionarily factors affect different plant traits. Correlation analysis confirmed a weak dependency between the vegetative and generative traits. In addition, no evidence was found for the presence of isolation by environment (IBE). However, the Mantel test for isolation by distance (IBD) was significant for the leaf morphometric traits and non-significant for the fruit morphometric traits. Being the marginal leading-edge populations, they are younger and were less likely to have had time for adaptation to local environments, which would have resulted in the development of IBE. Overall, edge populations of Tatar maple were characterised by great morphological variability, which helps these populations in their response to the intensive selective pressures they face in their environment.

Developmental regulation of stolon and rhizome

Stolons and rhizomes are modified stems for vegetative reproduction. While stolons grow above the ground, rhizomes grow beneath the ground. Stolons and rhizomes maintain the genotypes of hybrids and hence are invaluable for agricultural propagation. Diploid strawberry is a model for studying stolon development. At the axillary meristems, gibberellins and MADS box gene SOC1 promote stolon formation, while the DELLA repressor inhibits stolon development. Photoperiod regulates stolon formation through regulating GA biosynthesis or balancing asexual with sexual mode of reproduction in the axillary meristems. In rhizomatous wild rice, the BLADE-ON-PETIOLE gene promotes sheath-to-blade ratio to confer rhizome tip stiffness and support underground growth. Together, this review aims to encourage further investigations into stolon and rhizome to benefit agriculture and environment.

Celery and Celeriac: A Critical View on Present and Future Breeding

Cultivated for the crispy petioles and round, fleshy, and flavored hypocotyl celery and celeriac have over two centuries of breeding history in Europe. In this review paper we summarized the most recent advances touching when necessary the historical context of celery and celeriac breeding. In the post genomic era of research, the genome sequence of celery is only partially available. We comprised however in this paper the most important aspects of celery genetics that are available today and have applicability in celery modern cultivars development. We discussed the problems and traits that drive the main celery and celeriac breeding goals, like hybrid seed production, disease resistance, and interesting enlarged hypocotyl and petiole characteristics. Besides the classical breeding traits we covered the potential of integration of existing cultivars as sources for consumer oriented traits like nutraceuticals and health promoting substances. Sustainability is a subject that is continuously growing in popularity and we looked at the genetic base of celery and celeriac that makes them sources for abiotic stress resistance and candidates for phytoremediation. We explored the fundamental concepts gained in various fields of celery and related species research, as resources for future improvement of celery and celeriac germplasm. We forecast what the next years will bring to Apium breeding.

Cell length instead of cell number becomes the predominant factor contributing to hypocotyl length genotypic differences under abiotic stress in Medicago truncatula

Hypocotyl elongation in the dark is a crucial process to ensure seedling emergence. It relies both on the cell number and cell length. The contribution of these two factors to the maximal hypocotyl length and the impact of environmental conditions on this contribution are not known. This is surprising considering the agronomic and economical importance of seedling emergence in crop establishment. Using 14 genotypes from a nested core collection representing Medicago truncatula (barrel medic) natural variation, we investigated how epidermal cell number and cell length contribute to hypocotyl length under optimal, low temperature (8°C) and water deficit (-0.50 MPa) conditions. Both cell number and length vary according to genotypes and contribute to maximal hypocotyl length differences between genotypes. This contribution, however, depends on growth conditions. Cell number is the major contributor under optimal conditions (60%) whereas cell length becomes the major determinant under stress. Maximal hypocotyl length is correlated with hypocotyl elongation rate under both stresses but not under optimal condition, revealing contrasted genotypes for cell elongation capacity under stress. To identify the genetic regulators determining cell number and cell length, quantitative trait loci (QTLs) were detected using a recombinant inbred lines population exhibiting segregation in maximal hypocotyl length. Two QTLs controlling cell number and three QTLs controlling cell length at low temperature were detected. One QTL for cell number and two for cell length were found to be associated with hypocotyl length under low temperature. This study provides new information to improve seedling emergence under abiotic stress.

Anatomic Characteristics Associated with Head Splitting in Cabbage (Brassica oleracea var. capitata L.)

Cabbage belonging to Brassicaceae family is one of the most important vegetables cultivated worldwide. The economically important part of cabbage crop is head, formed by leaves which may be of splitting and non-splitting types. Cabbage varieties showing head splitting causes huge loss to the farmers and therefore finding the molecular and structural basis of splitting types would be helpful to breeders. To determine which anatomical characteristics were related to head-splitting in cabbage, we analyzed two contrasting cabbage lines and their offspring using a field emission scanning electron microscope. The inbred line “747” is an early head-splitting type, while the inbred line “748” is a head-splitting-resistant type. The petiole cells of “747” seems to be larger than those of “748” at maturity; however, there was no significant difference in petiole cell size at both pre-heading and maturity stages. The lower epidermis cells of “747” were larger than those of “748” at the pre-heading and maturity stages. “747” had thinner epidermis cell wall than “748” at maturity stage, however, there was no difference of the epidermis cell wall thickness in the two lines at the pre-heading stage. The head-splitting plants in the F₁ and F₂ population inherited the larger cell size and thinner cell walls of epidermis cells in the petiole. In the petiole cell walls of “747” and the F₁ and F₂ plants that formed splitting heads, the cellulose microfibrils were loose and had separated from each other. These findings verified that anomalous cellulose microfibrils, larger cell size and thinner-walled epidermis cells are important genetic factors that make cabbage heads prone to splitting.

Transgenerational effects of land use on offspring performance and growth in Trifolium repens

Central European grasslands vary widely in productivity and in mowing and grazing regimes. The resulting differences in competition and heterogeneity among grasslands might have direct effects on plants, but might also affect the growth and morphology of their offspring through maternal effects or adaptive evolution. To test for such transgenerational effects, we grew plants of the clonal herb Trifolium repens from seeds collected in 58 grassland sites differing in productivity and mowing and grazing intensities in different treatments: without competition, with homogeneous competition, and with heterogeneous competition. In the competition-free treatment, T. repens from more productive, less frequently mown, and less intensively grazed sites produced more vegetative offspring, but this was not the case in the other treatments. When grown among or in close proximity to competitors, T. repens plants did not show preferential growth towards open spaces (i.e., no horizontal foraging), but did show strong vertical foraging by petiole elongation. In the homogeneous competition treatment, petiole length increased with the productivity of the parental site, but this was not the case in the heterogeneous competition treatment. Moreover, petiole length increased with mowing frequency and grazing intensity of the parental site in all but the homogeneous competition treatment. In summary, although the expression of differences between plants from sites with different productivities and land-use intensities depended on the experimental treatment, our findings imply that there are transgenerational effects of land use on the morphology and performance of T. repens.

Shade avoidance: phytochrome signalling and other aboveground neighbour detection cues

Plants compete with neighbouring vegetation for limited resources. In competition for light, plants adjust their architecture to bring the leaves higher in the vegetation where more light is available than in the lower strata. These architectural responses include accelerated elongation of the hypocotyl, internodes and petioles, upward leaf movement (hyponasty), and reduced shoot branching and are collectively referred to as the shade avoidance syndrome. This review discusses various cues that plants use to detect the presence and proximity of neighbouring competitors and respond to with the shade avoidance syndrome. These cues include light quality and quantity signals, mechanical stimulation, and plant-emitted volatile chemicals. We will outline current knowledge about each of these signals individually and discuss their possible interactions. In conclusion, we will make a case for a whole-plant, ecophysiology approach to identify the relative importance of the various neighbour detection cues and their possible interactions in determining plant performance during competition.

More cells, bigger cells or simply reorganization? Alternative mechanisms leading to changed internode architecture under contrasting stress regimes

Shading and mechanical stress (MS) modulate plant architecture by inducing different developmental pathways. Shading results in increased stem elongation, often reducing whole-plant mechanical stability, while MS inhibits elongation, with a concomitant increase in stability. Here, we examined how these organ-level responses are related to patterns and processes at the cellular level by exposing Impatiens capensis to shading and MS. Shading led to the production of narrower cells along the vertical axis. By contrast, MS led to the production of fewer, smaller and broader cells. These responses to treatments were largely in line with genetic differences found among plants from open and closed canopy sites. Shading- and MS-induced plastic responses in cellular characteristics were negatively correlated: genotypes that were more responsive to shading were less responsive to MS and vice versa. This negative correlation, however, did not scale to mechanical and architectural traits. Our data show how environmental conditions elicit distinctly different associations between characteristics at the cellular level, plant morphology and biomechanics. The evolution of optimal response to different environmental cues may be limited by negative correlations of stress-induced responses at the cellular level.

Plasticity as a plastic response: how submergence-induced leaf elongation in Rumex palustris depends on light and nutrient availability in its early life stage

Plants may experience different environmental cues throughout their development which interact in determining their phenotype. This paper tests the hypothesis that environmental conditions experienced early during ontogeny affect the phenotypic response to subsequent environmental cues. This hypothesis was tested by exposing different accessions of Rumex palustris to different light and nutrient conditions, followed by subsequent complete submergence. Final leaf length and submergence-induced plasticity were affected by the environmental conditions experienced at early developmental stages. In developmentally older leaves, submergence-induced elongation was lower in plants previously subjected to high-light conditions. Submergence-induced elongation of developmentally younger leaves, however, was larger when pregrown in high light. High-light and low-nutrient conditions led to an increase of nonstructural carbohydrates in the plants. There was a positive correlation between submergence-induced leaf elongation and carbohydrate concentration and content in roots and shoots, but not with root and shoot biomass before submergence. These results show that conditions experienced by young plants modulate the responses to subsequent environmental conditions, in both magnitude and direction. Internal resource status interacts with cues perceived at different developmental stages in determining plastic responses to the environment.

Testing mechanisms and context dependence of costs of plastic shade avoidance responses in Impatiens capensis (Balsaminaceae)

PREMISE OF THE STUDY

Many plant species elongate their shoots in response to neighbor proximity and neighbor height. Although these plastic responses may be beneficial in terms of enhancing light interception, they also may have costs in terms of increased risk of mechanical failure (i.e., lodging or breaking) because of thinner stems. This trade-off between light acquisition and stability may shape the evolution of plastic elongation responses to foliage shade.

METHODS

In a field experiment manipulating elongation phenotypes and densities, we tested two hypotheses. We predicted that the risks of mechanical failure depend on plastic elongation and/or on characteristics of the immediate neighborhood, such as density and neighbor height. Further, we predicted that plants that fail mechanically would have reduced fitness.

KEY RESULTS

Mechanical failure was earlier and more frequent at high density and showed a complex interaction with neighborhood characteristics such as relative height of the neighbors and the expression of early plasticity. Plants that broke earlier had shorter lifespan and lower reproductive output.

CONCLUSIONS

Our results show that depending on the height and density of the group, plastic elongation responses can remain advantageous despite costs of increased risk of mechanical failure of the taller stems, as mechanical failure was not associated with strong costs in terms of reduced lifespan or seed production. The overall benefits of elongation outweigh the costs resulting in selection for elongation at the population level.

Variation in flooding-induced morphological traits in natural populations of white clover (Trifolium repens) and their effects on plant performance during soil flooding

BACKGROUND AND AIMS

Soil flooding leads to low soil oxygen concentrations and thereby negatively affects plant growth. Differences in flooding tolerance have been explained by the variation among species in the extent to which traits related to acclimation were expressed. However, our knowledge of variation within natural species (i.e. among individual genotypes) in traits related to flooding tolerance is very limited. Such data could tell us on which traits selection might have taken place, and will take place in future. The aim of the present study was to show that variation in flooding-tolerance-related traits is present among genotypes of the same species, and that both the constitutive variation and the plastic variation in flooding-induced changes in trait expression affect the performance of genotypes during soil flooding.

METHODS

Clones of Trifolium repens originating from a river foreland were subjected to either drained, control conditions or to soil flooding. Constitutive expression of morphological traits was recorded on control plants, and flooding-induced changes in expression were compared with these constitutive expression levels. Moreover, the effect of both constitutive and flooding-induced trait expression on plant performance was determined.

KEY RESULTS

Constitutive and plastic variation of several morphological traits significantly affected plant performance. Even relatively small increases in root porosity and petiole length contributed to better performance during soil flooding. High specific leaf area, by contrast, was negatively correlated with performance during flooding.

CONCLUSIONS

The data show that different genotypes responded differently to soil flooding, which could be linked to variation in morphological trait expression. As flooded and drained conditions exerted different selection pressures on trait expression, the optimal value for constitutive and plastic traits will depend on the frequency and duration of flooding. These data will help us understanding the mechanisms affecting short- and long-term dynamics in flooding-prone ecosystems.

Genome size is a strong predictor of cell size and stomatal density in angiosperms

Across eukaryotes phenotypic correlations with genome size are thought to scale from genome size effects on cell size. However, for plants the genome/cell size link has only been thoroughly documented within ploidy series and small subsets of herbaceous species. Here, the first large-scale comparative analysis is made of the relationship between genome size and cell size across 101 species of angiosperms of varying growth forms. Guard cell length and epidermal cell area were used as two metrics of cell size and, in addition, stomatal density was measured. There was a significant positive relationship between genome size and both guard cell length and epidermal cell area and a negative relationship with stomatal density. Independent contrast analyses revealed that these traits are undergoing correlated evolution with genome size. However, the relationship was growth form dependent (nonsignificant results within trees/shrubs), although trees had the smallest genome/cell sizes and the highest stomatal density. These results confirm the generality of the genome size/cell size relationship. The results also suggest that changes in genome size, with concomitant influences on stomatal size and density, may influence physiology, and perhaps play an important genetic role in determining the ecological and life-history strategy of a species.