Predicting the future
What the future could hold for plant science
As with other life forms in biology, plant life can be studied from different perspectives, from the molecular, genetic and biochemical level through organelles, cells, tissues, organs, individuals, plant populations, and communities of plants. At each of these levels a botanist might be concerned with the classification, structure, or function of plant life.
Historically all living things were grouped as animals or plants, and botany covered all organisms not considered animals. Some organisms once included in the field of botany are no longer considered to belong to the plant kingdom – these include fungi, lichens (lichenology), bacteria (bacteriology), viruses (virology) and single-celled algae, which are now grouped as part of the Protista. However, attention is still given to these groups by botanists, and fungi, lichens, bacteria and photosynthetic protists are usually covered in introductory botany courses.
The study of plants is vital because they are a fundamental part of life on Earth, which generates the oxygen, food, fibres, fuel and medicine that allow humans and other life forms to exist. Through photosynthesis, plants absorb carbon dioxide, a greenhouse gas that in large amounts can affect global climate. Additionally, they prevent soil erosion and are influential in the hydro-cycle. A good understanding of plants is crucial to the future of human societies as it allows us to:
Produce food to feed an expanding population
Understand fundamental life processes
Produce medicine and materials to treat diseases and other ailments
Understand environmental changes more clearly
Paleobotanists study ancient plants in the fossil record. It is believed that early in the Earth's history, the evolution of photosynthetic plants altered the global atmosphere of the earth, changing the ancient atmosphere by oxidation.
What we know
Florigen (or flowering hormone) is the term used to describe the hypothesised hormone-like molecules responsible for controlling and/or triggering flowering in plants. Florigen is produced in the leaves and acts in the shoot apical meristem of buds and growing tips. It is known to be graft-transmissible and even functions between species. However, despite having been sought since the 1930s, the exact nature of florigen is still a mystery.
Central to the hunt for florigen is an understanding of how plants use seasonal changes in day length to mediate flowering, a mechanism known as photoperiodism. Plants which exhibit photoperiodism may be either “short day” or “long day” plants, which in order to flower require short days or long days respectively. Although plants in fact determine day length from night length.
The current model suggests the involvement of multiple different factors. Research into florigen is predominately centred around the model organism and long day plant, Arabidopsis thaliana. Whilst much of the florigen pathways appear to be well conserved in other studied species, variations do exist. The mechanism may be broken down into three stages: photoperiod-regulated Initiation, signal Translocation via the phloem, and induction of Flowering at the shoot apical meristem.
In Arabidopsis, the signal is initiated by the production of messenger RNA (mRNA) coding a transcription factor called CONSTANS (CO). CO mRNA is produced approximately 12 hours after dawn, a cycle regulated by the plant's biological clock. This mRNA is then translated into CO protein. However CO protein is stable only in light, so levels stay low throughout short days and are only able to peak at dusk during long days when there is still a little light. CO protein promotes transcription of another gene called Flowering Locus T (FT). By this mechanism, CO protein may only reach levels capable of promoting FT transcription when exposed to long days. Hence the transmission of florigen, and so the induction of flowering, relies on a comparison between the plant's perception of day/night and its own internal biological clock.
The FT protein resulting from the short period of CO transcription factor activity is then transported via the phloem to the shoot apical meristem.
At the shoot apical meristem the FT protein is thought to interact with another transcription factor, FD protein, to activate floral identity genes, thus inducing flowering. Specifically, arrival of FT at the shoot apical meristem and formation of this FT/FD heterodimer is followed by the increased expression of: suppressor of over expression of constant 1 (SOC1), leafy, apetala 1, sepallata 3 and fruitiful.
Florigen was first described by Russian plant physiologist Mikhail Chailakhyan in 1937, who demonstrated that floral induction can be transmitted through a graft from an induced plant to one that has not been induced to flower. Anton Lang showed that several long-day plants and biennials could be made to flower by treatment with gibberellin, when grown under a non-flower-inducing photoperiod. This led to the suggestion that florigen may be made up of two classes of flowering hormones: Gibberellins and Anthesins. It was later postulated that during non-inducing photoperiods, long-day plants produce anthesin, but no gibberellin while short-day plants produce gibberellin but no anthesin. However, these findings did not account for the fact that short-day plants grown under non-inducing conditions will not cause flowering of grafted long-day plants that are also under no inductive conditions.
Problems with isolating florigen and the inconsistent results acquired led to the suggestion that florigen does not exist; rather, a particular ratio of other hormones must be achieved for the plant to flower. However more recent findings indicate that florigen does exist and is produced, or at least activated, in the leaves of the plant and that this signal is then transported via the phloem to the growing tip at the shoot apical meristem where the signal acts by inducing flowering.
Cell Communication, the problem
What we know
Cell signalling is part of a complex system of communication that governs basic cellular activities and coordinates cell actions. The ability of cells to perceive and correctly respond to their microenvironment is the basis of development, tissue repair, and immunity as well as normal tissue homeostasis. Errors in cellular information processing are responsible for diseases such as cancer, autoimmunity, and diabetes. By understanding cell signalling, diseases may be treated effectively and, theoretically, artificial tissues may be yield
Traditional work in biology has focused on studying individual parts of cell signalling pathways. Systems biology research helps us to understand the underlying structure of cell signalling networks and how changes in these networks may affect the transmission and flow of information. Such networks are complex systems in their organisation and may exhibit a number of emergent properties including bi-stability and ultra sensitivity. Analysis of cell signalling networks requires a combination of experimental and theoretical approaches including the development and analysis of simulations and modelling.
Unicellular and muilticelluar organism cell signalling
Cell signalling has been most extensively studied in the context of human diseases and signalling between cells of a single organism. However, cell signalling may also occur between the cells of two different organisms. In many mammals, early embryo cells exchange signals with cells of the uterus. In the human gastrointestinal tract, bacteria exchange signals with each other and with human epithelial and immune system cells. For the yeast Saccharomyces cerevisiae during mating, some cells send a peptide signal into their environment. The mating factor peptide may bind to a cell surface receptor on other yeast cells and induce them to prepare for mating.
Types of signals
Cells communicate with each other via direct contact, over short distances, or over large distances and scales.
Some cell-to-cell communication requires direct cell-cell contact. Some cells can form gap junctions that connect their cytoplasm to the cytoplasm of adjacent cells. In cardiac muscle, gap junctions between adjacent cells allows for action potential propagation from the cardiac pacemaker region of the heart to spread and coordinately cause contraction of the heart.
The Notch signalling mechanism is an example of juxtacrine signalling in which two adjacent cells must make physical contact in order to communicate. This requirement for direct contact allows for very precise control of cell differentiation during embryonic development. In the worm Caenorhabditis elegans, two cells of the developing gonad each have an equal chance of terminally differentiating or becoming a uterine precursor cell that continues to divide. The choice of which cell continues to divide is controlled by competition of cell surface signals. One cell will happen to produce more of a cell surface protein that activates the Notch receptor on the adjacent cell. This activates a feedback loop or system that reduces Notch expression in the cell that will differentiate and increases Notch on the surface of the cell that continues as a stem cell.
Many cell signals are carried by molecules that are released by one cell and move to make contact with another cell. Endocrine signals are called hormones. Hormones are produced by endocrine cells and they travel through the blood to reach all parts of the body. Specificity of signalling can be controlled if only some cells can respond to a particular hormone. Paracrine signals target only cells in the vicinity of the emitting cell. Neurotransmitters represent an example. Some signalling molecules can function as both a hormone and a neurotransmitter. For example, epinephrine and norepinephrine can function as hormones when released from the adrenal gland and are transported to the heart by way of the blood stream. Norepinephrine can also be produced by neurons to function as a neurotransmitter within the brain. Estrogen can be released by the ovary and function as a hormone or act locally via Paracrine or autocrine signalling. Active species of oxygen and nitric oxide can also act as cellular messengers. This process is dubbed redox signalling.