Browsing by Subject "plant Biology"

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  • Wang, Fang (Helsingin yliopisto, 2020)
    Introduction Plants must maintain a balanced water budget to support fundamental processes. Light, on the other hand, is the ultimate energy source and therefore vital in the plants’ energy balance. To use light or to use water efficiently represents a trade-off that persists throughout a plant’s life, but is especially important during the establishment of a seedling. Control of gas exchange is required to attain the optimal balance between the fluxes of carbon dioxide and water vapour. This is achieved by regulating the degree of stomatal opening. Individual leaf gas-exchange measurements allow the assessment of the trade-off in stomatal conductance at a point in time while repeated measurements over an extended period can provide diurnal time-courses which are useful to evaluate the strategies that plants employ to control water use efficiency. Tree seedlings exposed to shade or sunlight require a strategy for water use that accommodates both light conditions. Under the current changing of climate, droughts in many regions increasingly threaten seedling survival, providing the context for the research in this thesis. Objectives The main objective is to improve our understanding of the effect of light quality and irradiance on water relations. To achieve this, it is necessary to link sensory and physiological mechanisms that control gas exchange to adaptation and acclimation of whole plants. Conclusions Light can affect plant water relations by regulating the rate and magnitude of stomatal movement. Blue-light photoreceptors play important roles in this movement under blue and green light, with their contributions to stomatal control changing through the photoperiod (I). When plants grow in shade, the response of stomatal conductance to long-term water-deficit may become more conservative (III). Under drought, related species within the same genus can have different water-use strategies, including the regulation of stomatal conductance (II). Differences in water use are correlated with local climate and help to explain the geographic distribution of related species (II) and variation among local populations (III). My findings broaden our understanding of the mechanisms behind maintenance of a favourable water balance in plants. I achieved this by connecting photoreceptor function to stomatal regulation at different times of the day, and stomatal regulation to the tradeoff between water use efficiency and maintenance of hydraulic conductivity. Much remains to be done before these connections can be validated over the full range environmental gradients that occur in nature. The roles of photoreceptors in diurnal regulation of water relations during a drought period are far from clear; neither are their roles in acclimation and adaptation of different species and populations to the dynamic light environments of natural habitats. Further research will be required to understand the roles played by photoreceptors in different water-use strategies and at different time scales spanning from days to seasons.
  • Siligato, Riccardo (2016)
    Plants possess the rare capability to shape the own architecture according to biotic and abiotic stimuli received from the environment. Spatially defined groups of cells, called meristems, contribute to the division and differentiation processes continuously occurring inside the organism. Meristems can be classified as primary meristems, if they are specified during embryogenesis, or secondary meristems, if they form from undifferentiated, quiescent cells outside the primary meristems. Primary meristems, like the Root Apical Meristem (RAM) and the Shoot Apical Meristem (SAM), coordinate the apical growth of the plant in opposite directions, while secondary meristems shape the radial architecture, regulating the thickness and branching of the primary root and shoot. Cambium is a secondary meristem which produces the vascular tissues xylem and phloem. Xylem transports water and minerals from the root to the photosynthetic tissues; it comprises lignified dead conducting cells called tracheary elements, living parenchyma cells, and lignified dead cells, called fibres, which confer mechanical support and strength. Phloem distributes glucose, RNA, viruses, and proteins from the photosynthetic sources to the sink cells; it consists of empty living sieve elements, supporting companion cells, and parenchyma cells. In order to investigate the regulation of primary and secondary growth, we developed a new chemically inducible system to control the timing and location of the induction of an effector or gene of interest. This enables us to avoid deleterious effects such as seed lethality or sterility when studying the role of a gene in a particular cell type. For example, the meristem cambium is difficult to access through normal techniques, since mutations affecting cambial cell divisions often inhibit the primary growth, too. We developed the inducible system by combining the Multi-Site Gateway cloning technology with the already extant XVE inducible system. This system was used to perform part of the research presented in the thesis. Phytohormones are involved in virtually every aspect of plant life, from development to stress response. They are small molecules which act cellautonomously or non-cell-autonomously to mediate the majority of developmental and environmental responses and, consequently, the activity of the meristems throughout the plant life cycle. Auxin and cytokinins, which were among the first phytohormones discovered, regulate almost every aspect of plant life, such as the division and differentiation processes occurring continuously in the RAM and SAM. The two phytohormones have long been known to interact, and recent studies have uncovered significant crosstalk on the level of biosynthesis, transport, signalling and degradation. We investigated the dynamic role of auxin in maintaining the balance between division, elongation, differentiation in the RAM of the model organism Arabidopsis thaliana. Our results confirm that an optimal level of auxin response is required for division and elongation, while differentiation mechanisms require just a minimal concentration of auxin to proceed normally. We discovered that auxin and cytokinin responses interact synergistically to specify the stem cells and to regulate the timing of divisions in the cambium of Arabidopsis thaliana. The auxin and cytokinin signalling pathways both have a positive role in triggering secondary growth, but the hierarchy of the crosstalk between them is still unclear. Finally, auxin transported via the AUX1/LAX auxin influx carriers regulates the differentiation of vessel elements in the later stages of root cambium development. In summary, we confirm that auxin and cytokinins behave as master regulators of meristematic activities throughout the root, as the signalling pathways associated with both phytohormones heavily influence primary and secondary growth.
  • Alonso-Serra, Juan (Helsingin yliopisto, 2020)
    Plant development takes place through continuous changes in the size and shape of organs. Along the organs’ morphogenic gradients, cells derived from the undifferentiated meristematic stem cells follow different regulatory pathways leading to a variety of developmental trajectories and cellular functions. In the developmental process called secondary growth, molecular factors and physical forces interact in the radial growth of stems and roots to produce their cylindrical shape. Cambium, the largest connected meristem in plants, is responsible for secondary growth. It produces vascular tissues with two essential functions: the transport of water, nutrients and photoassimilates, and the physical support of the plant. Recent years have seen an increasing number of studies focused on the regulation of cambial activity, primarily because this meristem produces a great part of the Earth’s woody biomass, thereby fixing a large quantity of carbon. The aim of this thesis is to explore aspects of radial growth which have thus far remained largely uncharacterized: the contribution of bark tissues, the role of mechanical forces, and the genetic robustness of cambial development.