On the Relevance and Control of Leaf Angle

Plants can have constitutive leaf angles that are fixed and do not vary much among different growth environments. Several species, however, have the ability to actively adjust their leaf angles. Active leaf repositioning can be functional in avoiding detrimental environmental conditions, such as avoidance of heat stress and complete submergence, or can be, for example, utilized to maximize carbon gain by positioning the leaves relative to the incoming radiation. In recent years, major advances have been made in the understanding of the molecular mechanisms, and the underlying hormonal regulation of a particular type of leaf movement: hyponastic growth. This differential petiole growth-driven upward leaf movement is now relatively well understood in model systems such as Rumex palustris and Arabidopsis thaliana. In the first part of this review we will discuss the functional consequences of leaf orientation for plant performance. Next, we will consider hyponastic growth and describe how exploitation of natural (genetic) variation can be instrumental in studying the relevance and control of leaf positioning.     

Automated analysis of hypocotyl growth dynamics during shade avoidance in Arabidopsis

Plants that are adapted to environments where light is abundant are especially sensitive to competition for light from neighboring vegetation. As a result, these plants initiate a series of changes known as the shade avoidance syndrome, during which plants elongate their stems and petioles at the expense of leaf development. Although the developmental outcomes of exposure to prolonged shade are known, the signaling dynamics during the initial exposure of seedlings to shade is less well studied. Here, we report the development of a new software-based tool, called HyDE (Hypocotyl Determining Engine) to measure hypocotyl lengths of time-resolved image stacks of Arabidopsis wild-type and mutant seedlings. We show that Arabidopsis grows rapidly in response to the shade stimulus, with measurable growth after just 45 min shade exposure. Similar to other mustard species, this growth response occurs in multiple distinct phases, including two phases of rapid growth and one phase of slower growth. Using mutants affected in shade avoidance phenotypes, we demonstrate that most of this early growth requires new auxin biosynthesis via the indole-3-pyruvate pathway. When activity of this pathway is reduced, the first phase of elongation growth is absent, and this is correlated with reduced activity of auxin-regulated genes. Finally, we show that varying shade intensity and duration can affect the shape and magnitude of the growth response, indicating a broad range of the elongation response to shade.     

Sensitivity to ethylene: the key factor in submergence-induced shoot elongation of Rumex

Rosettes of flooding-resistant Rumex palustris plants show a submergence-induced stimulation of elongation, which is confined to the petioles of young leaves. This response increases the probability of survival. It is induced by ethylene that accumulates in submerged tissues. Flooding-intolerant Rumex acetosella plants do not show this response. We investigated whether differences in shoot elongation between the species, between old and young leaves and between the petiole and leaf blade of a R. palustris plant result from differences in internal ethylene concentration or in sensitivity to the gas. Concentrations of free and conjugated ACC in petioles and leaf blades of R. palustris indicated that ethylene is synthesized throughout the submerged shoot, although production rates varied locally. Nevertheless, no differences in ethylene concentration were found between submerged leaves of various ages. In contrast, dose-response curves showed that only elongation of young petioles of R. palustris was sensitive to ethylene. In R. acetosella, elongation of all leaves was insensitive to ethylene. We conclude that variation in ethylene sensitivity rather than content explains the differences in submergence-induced shoot elongation between the two Rumex species and between leaves of R. palustris.     

Differential genetic variation in adaptive strategies to a common environmental signal in Arabidopsis accessions: phytochrome-mediated shade avoidance

Shade avoidance is a syndrome of plastic responses to light signals encountered in crowded plant communities and is a crucial component of competitive strategy in higher plants. The responses are mediated via signal perception by specific members of the phytochrome family of photoreceptors, which detect the relative proportions of red (R) and far‐red (FR) radiation within dense communities. We analysed two aspects of shade avoidance, the acceleration of flowering and the enhancement of elongation growth, displayed by more than 100 accessions of Arabidopsis thaliana (Heyn.) in response to FR‐proximity signals. Both traits showed wide variation between accessions, which was unrelated to the latitude of the location of original collection. Flowering acceleration is a major feature of shade avoidance in rosette plants such as Arabidopsis, and most accessions showed dramatic responses, but several were identified as being recalcitrant to the proximity signal. These accessions are likely to be informative in the analysis of quantitative variation in shade avoidance. Hypocotyl elongation, treated here as an indicator of elongation growth responses, also varied widely amongst accessions. The variations in flowering acceleration and elongation were not correlated, indicating that microevolution in the downstream pathways from signal perception has occurred separately.     

The role of oxygen in submergence-induced petiole elongation in Rumex palustris: in situ measurements of oxygen in petioles of intact plants using micro-electrodes

In a study on the mechanism of stimulated petiole elongation in submerged plants, oxygen concentrations in petioles of the flood-tolerant plant Rumex palustris were measured with micro-electrodes. Short-term submergence lowered petiole partial oxygen pressure to c . 19 kPa whereas prolonged submergence under continuous illumination depressed oxygen levels to c . 8–12 kPa after 24 h. Oxygen levels in petioles depended on the presence of the lamina, even in submerged conditions, and on available light. In darkness, petiole oxygen levels in submerged plants dropped quickly to values as low as 0.5–4 kPa. It is hypothesized that prolonged submergence in the light is accompanied by a decrease in carbon dioxide in the petiole. Submergence-enhanced petiolar elongation rate was compared with emergent plants. Peak daily elongation rates occurred at the end of the dark period in emergent plants, but in the middle of the light period in submerged plants. We suggest that this shift in daily elongation pattern is induced by dependence of growth on photosynthetically derived oxygen in submerged plants. Implications of reduced oxygen for ethylene production are raised. Levels of 1- aminocyclopropane-1-carboxylic acid synthase and 1-aminocyclopropane-1-carboxylic acid oxidase and ethylene sensitivity are cited as potential factors in hypoxia-induced ethylene release.     

Plasticity to simulated shade is associated with altitude in structured populations of Arabidopsis thaliana

Plants compete for photosynthesis light and induce a shade avoidance syndrome (SAS) that confers an important advantage in asymmetric competition for light at high canopy densities. Shade plasticity was studied in a greenhouse experiment cultivating Arabidopsis thaliana plants from 15 populations spread across an altitudinal gradient in the northeast area of Spain that contain a high genetic variation into a reduced geographical range. Plants were exposed to sunlight or simulated shade to identify the range of shade plasticity. Fourteen vegetative, flowering and reproductive traits were measured throughout the life cycle. Shade plasticity in flowering time and dry mass was significantly associated with the altitude of population origin. Plants from coastal populations showed higher shade plasticity indexes than those from mountains. The altitudinal variation in flowering leaf plasticity adjusted negatively with average and minimum temperatures, whereas dry mass plasticity was better explained by negative regressions with the average, maximum and minimum temperatures, and by a positive regression with average precipitation of the population origin. The lack of an altitudinal gradient for the widest number of traits suggests that shade light could be a driver explaining the distribution pattern of individuals in smaller geographical scales than those explored here.     

A single haplotype hyposensitive to light and requiring strong vernalization dominates Arabidopsis thaliana populations in Patagonia,Argentina

The growing collection of sequenced or genotyped Arabidopsis thaliana accessions includes mostly individuals from the native Eurasian and N. African range and introduced North American populations. Here, we describe the genetic and phenotypic diversity, along with habitats and life history, of A. thaliana plants collected at the southernmost end of its worldwide distribution. Seed samples were harvested from plants growing in four sites within a ~3500‐km²‐area in Patagonia, Argentina, and represent the first germplasm to be collected in South America for this species. Whole‐genome resequencing revealed that plants from the four sites and a Patagonia herbarium specimen collected in 1967 formed a single haplogroup (Pat), indicating that the phenotypic variation observed in the field reflected plastic responses to the environment. admixture and principal components analyses suggest that the ancestor of the Pat haplogroup either came from Italy or the Balkan/Caucasus regions of Eurasia. In the laboratory, plants from the Pat haplogroup were hyposensitive to continuous red (Rc) and shade light, with corresponding changes in the expression of phytochrome signalling genes. Pat had higher PIF3 and PIF5 and lower HY5 expression under Rc light; and lower expression of PIL1, ATHB2 and HFR1 under shade compared to Col‐0. In addition, Pat plants had a strong vernalization requirement associated with high levels of FLC expression. We conclude that including Pat in studies of natural variation and in comparison with other introduced populations will provide additional information for association studies and allow for a more detailed assessment of the demographic events following colonization.     

Role of growth regulators in the senescence of Arabidopsis thaliana leaves

A homozygous, dominant, C₂H₄-resistant line of Arabidopsis thaliana (L.) Heynh (cv. Columbia; er ) was selected from ethylmethylsulfonate-mutagenized seed, and used to test the role of C₂H₄ and other growth regulators in senescence of mature leaves. Chlorophyll (Chl) loss from disks excised from leaves of er was much slower than that from wild-type (WT) disks, whether they were held in the light or in the dark. C₂H₄ accelerated Che loss from WT disks but had no effect on the yellowing of mutant disks. C₂H₄ biosynthesis was higher in disks from the mutant plants, particularly in the light. In the dark, treatment with the cytokinin, 6-benzyladenine (BA), reduced Chl loss from wild-type disks, but had no effect on mutant disks. In the light, BA treatment stimulated chlorophyll breakdown in both wild type and mutant disks. Treatment with abscisic acid (ABA) stimulated chlorophyll loss in wild-type and mutant disks, whether they were held in the light or the dark. C₂H₄ production was stimulated in ABA-treated disks, but they still yellowed even when C₂H₄ production was inhibited by application of aminooxyacetic acid (AOA). These data indicate that C₂H₄ is only one of the factors involved in leaf senescence, and that the promotion of senescence by ABA is not mediated through its stimulation of C₂H₄ production.     

QTL architecture of resistance and tolerance traits in Arabidopsis thaliana in natural environments

Quantitative-genetic approaches have offered significant insights into phenotypic evolution. However, quantitative-genetic analyses fail to provide information about the evolutionary relevance of specific loci. One complex and ecologically relevant trait for plants is their resistance to herbivory because natural enemies can impose significant damage. To illustrate the insights of combined molecular and ecological research, we present the results of a field study mapping quantitative trait loci (QTL) for resistance and tolerance to natural rabbit herbivory in the genetic model, Arabidopsis thaliana. Replicates of the Ler x Col recombinant inbred lines were planted into field sites simulating natural autumn and spring seasonal germination cohorts. Shortly after flowering, herbivores removed the main flowering inflorescence (apical meristem). We found several main-effect QTL for resistance within each seasonal cohort and significant QTL-season interactions, demonstrating that the loci underlying resistance to a single herbivore differ across seasonal environments. The presence of QTL x environment also shows that variation at specific loci is only available to selection in some environments. Despite significant among-line variance components, no QTL for tolerance were detected. The combined results of the quantitative-genetic and QTL analyses demonstrate that many loci of small effect underlie tolerance to damage by rabbits, and counter the hypothesis of locus-specific tradeoffs between resistance and tolerance. The results also provide insights as to the locus-specific nature of evolutionary constraints, i.e. some loci influence flowering time and resistance in both seasonal cohorts. Our results show how linking molecular-genetic tools with field studies in ecologically relevant settings can clarify the role of specific loci in the evolution of quantitative traits.     

Effect of ABA on the UV-B-Induced Ethylene Evolution by the etr and ctr Mutants of Arabidopsis thaliana

The responses of 14-day-old Arabidopsis thaliana (L.) Heynh. plants to UV-B irradiation (280–320 nm) and ABA treatment were investigated. Wild-type plants as well as ethylene-insensitive etr1-1 and ctr1-1 mutants were used. Theetr1-1 mutant considerably differed from the ctr1-1 one in the fresh weight production after UV-B treatment (29.5 kJ/m²). The irradiated etr1-1 plants fell well behind the nonirradiated ones during the first two days after stress, but by the 8th day, their weight attained 70% of control plant weight. In contrast, Ctr1-1 mutant weight comprised 70% of control level after two days of stress but, by the 8th day, it was only 56% of the weight of control plants. In wild-type and ctr1-1 plants, ABA, in the 8 × 10^–6 to 2 × 10^–4 M concentration range, increased the difference between the weights of nonirradiated and irradiated plants, but in etr1-1 plants, ABA decreased this difference. The etr1-1, ctr1-1, and wild-type plants were very similar in the dynamics of ethylene evolution after UV-B treatment (7.4 kJ/m²). In wild-type, etr1-1, and ctr1-1 plants, ABA, in a concentration-dependent manner, inhibited UV-B-induced ethylene evolution to the same extent. The results obtained show that ABA exerted an opposite effect on UV-B-dependent growth in the plants with active (wild type and ctr1-1) and blocked (etr1-1) ethylene signal pathway, whereas the inhibition of ethylene synthesis by ABA was not related to ethylene signal transmission.     

Identification of genes regulated by dark adaptation and far‐red light illumination in roots of Arabidopsis thaliana*

Roots in the soil are illuminated by far‐red (FR) light passed through plant tissues in the daytime, and are in complete darkness at night. To evaluate whether gene expression of roots is affected by a dark‐FR light cycle, gene expression profiles were analysed for dark‐adapted versus light‐grown plants and for FR light‐illuminated versus dark‐adapted plants using the RIKEN Arabidopsis full‐length cDNA microarray (containing approximately 7000 independent, full‐length cDNA groups). Among candidate dark‐ and FR‐regulated genes, several were further analysed. Eleven dark‐inducible and five dark‐repressed genes were characterized. Almost all the dark‐inducible and –repressed genes were oppositely regulated by FR light illumination. The functions of dark‐ and FR‐responsive genes and the significance of FR light‐regulated gene expression in roots under ground are discussed.     

模式植物拟南芥开花时间分子调控研究进展

植物从营养生长到生殖生长的转变是开花发育的关键,在合适的时间开花对植物的生长和繁衍极为重要,植物开花时间的调控对农业生产发展意义重大。植物开花是由遗传因子和环境因子协同调节的一个复杂过程。近年来,对不同植物开花调控的研究,特别是对模式植物拟南芥（Arabidopsis thaliana（L.） Heynh.）的开花调控研究取得了显著进展,已探明开花时间分子调控的6条主要途径分别是光周期途径、春化途径、自主途径、温度途径、赤霉素途径和年龄途径。各遗传调控途径既相互独立又相互联系,构成一个复杂的开花调控网络。本文综述了模式植物拟南芥开花时间调控分子机制相关研究的最新进展,并对未来的研究进行了展望。     

Requirement of KNAT1/BP for the Development of Abscission Zones in Arabidopsis thaliana

The KNAT1 gene is a member of the Class I KNOXhomeobox gene family and is thought to play an important role in meristem development and leaf morphogenesis. Recent studies have demonstrated that KNAT1/BP regulates the architecture of the inflorescence by affecting pedicle development in Arabidopsis thaliana. Herein, we report the characterization of an Arabidopsis T-DNA insertion mutant that shares considerable phenotypic similarity to the previously identified mutant brevipedicle （bp）. Molecular and genetic analyses showed that the mutant is allelic to bp and that the T-DNA is located within the first helix of the KNAT1 homeodomain （HD）. Although the mutation causes a typical abnormality of short pedicles, propendent siliques, and semidwarfism, no obvious defects are observed in the vegetative stage. A study on cell morphology showed that asymmetrical division and inhibition of cell elongation contribute to the downward-pointing and shorter pedicle phenotype. Loss of KNAT/BPfunction results in the abnormal development of abscission zones. Mlcroarray analysis of gene expression profiling suggests that KNAT1/BP may regulate abscission zone development through hormone signaling and hormone metabolism in Arabidopsis.