Plant Cell Biology 2023
Gravity is one of the most important guiding factors for land plants that are actively adjusting the axis of their growth with respect to the force vector (gravitropism). Shoots show negative, roots show positive gravitropism. Leaves, twigs and lateral roots keep a certain angle with gravity (called plagiotropism). The direction of gravity is sensed by sedimenting amyloplasts that are pulling at microtubules, such that the minute forces produced by a single cell, are focussed and able to cross the energy of thermal noise, such that the cell can sense this as stimulus. Microtubules focus the force upon calcium channels such that a chemical signal chain is deployed - by the way, also we use microtubules for sensing of gravity. The calcium influx is then leading to phytohormonal changes, for instance, auxin is pumped asymmetrically, such that a curvature results, leading to readjustment of the plant. In parallel, jasmonic acid is generated - which not only sharpens the auxin gradient, but also activates stress-related metabolism, for instance the accumulation of secondary compounds. Between 1990 and 2005, our lab has dissected the cellular events that help to sense and process gravity. Recently, this knowledge has led to surprising applications. In cooperation with Vertical Farm Tech in Waldbronn we investigate the possibility to stimulate the accumulation of valuable plant compounds by gravity stimulation, in order to increase the quality of pharmaceutical plant products. The company has developed a first prototype that has been installed in our greenhouse. Using Mint as paradigm, we test out, what happens. Here, we know already quite a bit about the molecular mechanisms behind Mint scent. In the long term, we want to enable by this new technology the cultivation of rare medicinal plants such as Arnica, thus avoiding the need to collect those endangered species in the wild. The project has just obtained funding through the Baden-Württemberg Investment Programme. In order to render the process efficient, we need to find out, how the stimulation is best applied - this requires a deeper understanding into the cellular aspects of gravitropism in Mint. Where is the stimulus sensed? Where is the response to this stimulus? How does the information travel from the site of perception to the site of response? What is the role of jasmonates in the process? A similar study in rice coleoptiles can be used as template to design this research. Gutjahr et al. (2005).
What is the topic?
When cells generate an organism their mutual relationship changes dramatically: the competition of inidividual cells for resources and survival has to change into cooperation of all cells. The clearest hallmark of completed self organisation is the sacrifice of the invidual cell for the sake of the entire population. This so called programmed cell death is observed in all multicellular organisms, precursors even in colonies of unicellular organisms. When a cell recognises that it is damaged, it will arrest proliferation and instead initiate programmed cell death. This ensures genetic integrity of the organism and thus its survival. However, for the individual cell this is disadvantageous, of course - the dead cell will not get profit from its own sacrifice. When cells return to the egoism of individual life and proliferate, although they are damaged, this will lead to cancer. Cancer in plants seems to be non-existing, but plant cells often undergo programmed cell death, when they are attacked by microbes. This allows to contain the intruder and to seal the damaged site. But how can plant cells actually sense their integrity?
In the context of our research on chemical engineering we search for ways to smuggle molecular tools through the membranes of plant cells. During this research, we discovered that many of these Trojan Horses can penetrate into the cell without making it leak. However, frequently after some time the actin cytoskeleton is organised into bundles and several hours later, the cell initiates programmed cell death. Apparently, even a transient perturbation of membrane integrity is sufficient to cause programmed cell death. In addition to chemical Trojan Horses we use meanwhile electrical pulses of high energy, but extremely short durations, so called nanosecond Pulsed Electrical Fields (nsPEFs) to evoke short interruptions of membrane integrity and to understand at the same time, how a cell is actually able to sense its integrity. In addition to the actin skeleton we could identify a membrane-located enzyme, the NADPH Oxidase, as important component of integrity sensing. This enzyme generates superoxide anions, a specific form of reactive oxygen species. These superoxide anions act as signal for a G-protein, which in turn activates a phospholipase D, a central signalling hub. This hub is linked upstream with auxin, the central growth regulator of plants, downstream with actin filament and microtubules. We are far from really understanding this Actin-Membrane-Sensor, but we have found that integrity is measured through a flow of signals through this sensor system. As long as signals are flowing, this system signals that everything is alright, when the flow is interrupted, this will arrest cell division. In unicellular organisms (we work with the Green Alga Chlamydomonas) this will cause encapsulation of the cell and formation of a silent, but persistent palmella stage. In multicellular plants, the same condition will initiate programmed cell death. To understand this integrity sensor requires more than the identification of its molecular components, though. We need to understand, how the dynamics of signal flow are sensed and transduced into different cellular responses.