Plants2013, 2(4), 769-785; doi:10.3390/plants2040769 - published online 5 December 2013 Show/Hide Abstract
Abstract: We produced transgenic Arabidopsis plants that express chimeric genes for transcription factors converted to dominant repressors, using Chimeric REpressor gene-Silencing Technology (CRES-T), and evaluated the salt tolerance of each line. The seeds of the CRES-T lines for ADA2b, Msantd, DDF1, DREB26, AtGeBP, and ATHB23 exhibited higher germination rates than Wild type (WT) and developed rosette plants under up to 200 mM NaCl or 400 mM mannitol. WT plants did not grow under these conditions. In these CRES-T lines, the expression patterns of stress-related genes such as RD29A, RD22, DREB1A, and P5CS differed from those in WT plants, suggesting the involvement of the six transcription factors identified here in the stress response pathways regulated by the products of these stress-related genes. Our results demonstrate additional proof that CRES-T is a superior tool for revealing the function of transcription factors.
Plants2013, 2(4), 750-768; doi:10.3390/plants2040750 - published online 27 November 2013 Show/Hide Abstract
Abstract: Plants have developed sophisticated recognition systems for different kinds of pathogens. Pathogen-associated molecular patterns (PAMPs) can induce various defense mechanisms, e.g., the production of reactive oxygen species (ROS) as an early event. Plant defense reactions are initiated by a signal transduction cascade involving the release of calcium ions (Ca2+) from both external and internal stores to the plant cytoplasm. This work focuses on the analysis of cytosolic Ca2+ signatures, experimentally and theoretically. Cytosolic Ca2+ signals were measured in Nicotiana tabacum plant cell cultures after elicitation with penta-N-acetylchitopentaose oligosaccharides (Ch5). In order to allow a mathematical simulation of the elicitor-triggered Ca2+ release, the Li and Rinzel model was adapted to the situation in plants. The main features of the Ca2+ response, like the specific shape of the Ca2+ transient and the dose-response relationship, could be reproduced very well. Repeated elicitation of the same cell culture revealed a refractory behavior with respect to the Ca2+ transients for this condition. Detailed analysis of the obtained data resulted in further modifications of the mathematical model, allowing a predictive simulation of Ch5-induced Ca2+ transients. The promising results may contribute to a deeper understanding of the underlying mechanisms governing plant defense.
Plants2013, 2(4), 726-749; doi:10.3390/plants2040726 - published online 27 November 2013 Show/Hide Abstract
Abstract: The clubroot disease, caused by the obligate biotrophic protist Plasmodiophora brassicae, affects cruciferous crops worldwide. It is characterized by root swellings as symptoms, which are dependent on the alteration of auxin and cytokinin metabolism. Here, we describe that two different classes of auxin receptors, the TIR family and the auxin binding protein 1 (ABP1) in Arabidopsis thaliana are transcriptionally upregulated upon gall formation. Mutations in the TIR family resulted in more susceptible reactions to the root pathogen. As target genes for the different pathways we have investigated the transcriptional regulation of selected transcriptional repressors (Aux/IAA) and transcription factors (ARF). As the TIR pathway controls auxin homeostasis via the upregulation of some auxin conjugate synthetases (GH3), the expression of selected GH3 genes was also investigated, showing in most cases upregulation. A double gh3 mutant showed also slightly higher susceptibility to P. brassicae infection, while all tested single mutants did not show any alteration in the clubroot phenotype. As targets for the ABP1-induced cell elongation the effect of potassium channel blockers on clubroot formation was investigated. Treatment with tetraethylammonium (TEA) resulted in less severe clubroot symptoms. This research provides evidence for the involvement of two auxin signaling pathways in Arabidopsis needed for the establishment of the root galls by P. brassicae.
Plants2013, 2(4), 712-725; doi:10.3390/plants2040712 - published online 8 November 2013 Show/Hide Abstract
Abstract: In the Serengeti ecosystem of East Africa, grazing ungulates prefer areas with elevated grass Na, suggesting that some grasses tolerate both high soil Na and defoliation. We performed a factorial Na-by-defoliation greenhouse study with five abundant Sporobolus congeners to explore whether Serengeti grasses possess traits which: (i) confer tolerance to both Na and defoliation (cross-tolerance); (ii) display a tradeoff; or (iii) act independently in their tolerances. Our expectation was that related grasses would exhibit cross-tolerance when simultaneously subjected to Na and defoliation. Instead, we found that physiological tolerances and growth responses to Na and defoliation did not correlate but instead acted independently: species characterized by intense grazing in the field showed no growth or photosynthetic compensation for combined Na and defoliation. Additionally, in all but the highest Na dosage, mortality was higher when species were exposed to both Na and defoliation together. Across species, mortality rates were greater in short-statured species which occur on sodic soils in heavily grazed areas. Mortality among species was positively correlated with specific leaf area, specific root length, and relative growth rate, suggesting that rapidly growing species which invest in low cost tissues have higher rates of mortality when exposed to multiple stressors. We speculate that the prevalence of these species in areas of high Na and disturbance is explained by alternative strategies, such as high fecundity, a wide range of germination conditions, or further dispersal, to compensate for the lack of additional tolerance mechanisms.
Plants2013, 2(4), 699-711; doi:10.3390/plants2040699 - published online 1 November 2013 Show/Hide Abstract
Abstract: Among several naturally occurring environmental factors, temperature is considered to play a predominant role in controlling proper growth and flowering in geophytes. Most of them require a “warm-cold-warm” sequence to complete their annual cycle. The temperature optima for flower meristem induction and the early stages of floral organogenesis vary between nine and 25 °C, followed, in the autumn, by a several-week period of lower temperature (4–9 °C), which enables stem elongation and anthesis. The absence of low temperature treatment leads to slow shoot growth in spring and severe flowering disorders. Numerous studies have shown that the effects of the temperature surrounding the underground organs during the autumn-winter period can lead to important physiological changes in plants, but the mechanism that underlies the relationship between cold treatment and growth is still unclear. In this mini-review, we describe experimental data concerning the temperature requirements for flower initiation and development, shoot elongation, aboveground growth and anthesis in bulbous plants. The physiological processes that occur during autumn-winter periods in bulbs (water status, hormonal balance, respiration, carbohydrate mobilization) and how these changes might provoke disorders in stem elongation and flowering are examined. A model describing the relationship between the cold requirement, auxin and gibberellin interactions and the growth response is proposed.
Plants2013, 2(4), 676-698; doi:10.3390/plants2040676 - published online 23 October 2013 Show/Hide Abstract
Abstract: Growth and survival of young European beech (Fagus sylvatica L.) is largely dependent on water availability. We quantified the influence of water stress (measured as Available Soil Water Storage Capacity or ASWSC) on vitality of young beech plants at a dry site. The study site was located in a semi-natural sessile oak (Quercus petraea (Mattuschka) Liebl.) stand adjacent to beech stands on a rocky gneiss outcrop in southwestern Germany. Plant vitality was measured as crown dieback and estimated by the percentage of dead above ground biomass. The magnitude of crown dieback was recorded in different vertical parts of the crown. Biomass was calculated from the harvested plants following allometric regression equations specifically developed for our study site. Stem discs from harvested plants were used for growth analysis. We found that soil depth up to bedrock and skeleton content significantly influenced ASWSC at the study site. A significant negative correlation between ASWSC and crown dieback was found. Highest rates of crown dieback were noticed in the middle and lower crown. The threshold of crown dieback as a function of drought stress for young beech plants was calculated for the first time in this study. This threshold of crown dieback was found to be 40% of above ground biomass. Beyond 40% crown dieback, plants eventually experienced complete mortality. In addition, we found that the extremely dry year of 2003 significantly hampered growth (basal area increment) of plants in dry plots (ASWSC < 61 mm) in the study area. Recovery in the plants’ radial growth after that drought year was significantly higher in less dry plots (ASWSC > 61 mm) than in dry plots. We concluded that a decrease in ASWSC impeded the vitality of young beech causing partial up to complete crown dieback in the study site.