Dr. Qiong Liu
Cellular and molecular biology – how plant sense and respond to external stimuli
Owing to their sessile lifestyle, adaptation has become an eternal theme for plants that survive an ever-changing environment which is most of the time absolutely not amiable for them. To master the developmental flexibility required for adaptation, plants must be able to: first, sense stimuli of various natures; and second, integrate the signaling triggered by different environmental factors at a given time and initiate a balanced and appropriate response. Unlike animals that evolved specialized organs for recognizing their surroundings, plants have to figure out their own unique approach to achieve this goal evident from the apparent lack of sensing organs. In addition, they have to face this hostile environment right on, instead of running away like animals do. This lack of sensing organs in contrast to the amazing achievements plants demonstrated in conquering the most extreme climate niches makes the question how plants sense and respond to external stimuli a fascinating topic.
A rising body of evidence suggests that the cytoskeleton plays a role beyond a tool of cellular response, namely, in the sensing of abiotic and biotic stimuli. For microtubules, modulation of mechanosensitive activated calcium channels might be the mechanistic correlate for such a sensory function. Reactive oxygen species have been shown to activate these channels in plant roots and can feed through MAP kinase cascades on the formation of mitotic microtubule structures. For actin, the molecular players are only partially understood and seem to differ partially between the kingdoms. However, a direct interaction of actin filaments with membrane topology has been demonstrated in both directions. For instance, phosphorylation of a myosin light chain has been found to be necessary and sufficient for actin-dependent apoptotic membrane blebbing, and stabilisation of actin by dexamethasone-inducible expression of an actin-bundling factor stabilises plant membranes against pulsed electrical fields. Conversely, the actin-binding N-WASP/WIP complex can transmit a signal from membrane curving upon the assembly of F-actin. In plants, treatment with RGD-peptides (that cannot permeate the membrane, but disrupt the interaction of the cytoskeleton with the plasma membrane through an unknown integrin-like protein) caused the disruption of cytoplasmic architecture and inhibited deplasmolysis indicating a loss of membrane integrity. In fact, the so called Hechtian strands, attachment sites of actin filaments at the plasma membrane that become visible during plasmolysis are also disrupted by RGD-peptides. Since plant nuclei are not maintained by an integral lamine network, but by a cytoskeletal lattice that can even tunnel through the nucleus, the position and architecture of the nucleus is actively controlled during the cell cycle by both microtubules and actin, and this involves the activity of plant-specific KCH-kinesins that connect to microtubules as well as to actin filaments. Thus, the nucleus is tethered to its position and kept in shape by balanced tension between the different flanks of the cell. When the interaction of actin with the plasma membrane and the cell wall is disrupted by the generation of ROS through the membrane-bound NADPH-oxidase, this would result in contraction of actin filaments towards the nucleus, and a loss of nuclear shape interfering with chromatin modelling, which has been shown to be a central element in the induction of programmed cell death in plant cells.
1. Sensing of abiotic signals: mechanical forces
Sensing and responding to mechanical forces had become a fundamental skill of terrestrial plants. In fact, all living creatures are able to sense mechanical forces regardless of whether they are imposed externally or generated from within and adjust their biological processes accordingly; no matter whether they are complex multicellular organisms like us humans or simple single cell microbes as bacteria.
Among a wide range of diverse factors of distinct nature which plants rely on to perceive their surroundings and adjust their growth and development accordingly, mechanical force is the most ancient, fundamental, and constantly encountered one. Plants have been well equipped with intricate mechanisms during the course of evolution to perceive and respond to external as well as internal mechanical forces of various amplitudes and durations. However, unlike the well understood molecular bases of sensing soluble ligands, the very different molecular mechanisms how mechanical stimuli are sensed and transduced have remained enigmatic. Molecular players participating in the mechanoresponses of plants have been identified successively, like genes, proteins, hormones as well as inorganic signaling molecules. However, the identity of genuine mechanoreceptors and the exact downstream signaling pathways remained a field which needs further investigation.
Current project (DFG): How the nucleus explores geometry? (Press release 2015-04)
2 Sensing of biotic signals: plant-pathogen interaction
Life is not easy - this is especially true for plant cells that cannot run away, but have to cope with environmental challenges directly. Pathogen defence represents a specific aspect of this general ability. The interaction of pathogens and their hosts is subject to an evolutionary race of arms, where the pathogens developed various strategies to circumvent or suppress defence responses of the host, whereas the host developed various strategies to sense and attack the invading pathogen or its effector molecules. For many years, plant immunity had been dominated by the so called gene-for-gene concept, where, on the side of the host, specific resistance genes (R genes) confer immunity to particular races of pathogens by recognition of so called avirulence factors that are essential for the life cycle of the pathogen. However, recent years brought a paradigm shift, where R-gene dependent defence is understood as specialised final stage of a multilayered system of immunity (for review see Dangl and Jones, 2001).
Envisaged project: Chemical engineering of plant immunity (cooperation with the lab of Prof. Dr. Ute Schepers, Prof. Dr. Anne Ulrich, Ph.D. student Kinfemichael Asfaw)
3. Analysis of secreted intercellular signals
Secretomics is a newly emerging field which describes the global study of proteins that are secreted to the extracellular space by a cell, a tissue or an organism at certain time under given conditions through known and unknown secretory mechanisms involving constitutive and regulated secretory organelles. A combination of biochemical, proteomics and bioinformatics approaches has been developed to isolate, identify and profile secreted proteins. Initial secretome studies in plants have considerably advanced our understanding on secretion of different types of proteins and their underlying mechanisms, and opened a door for comparative analyses of plant secretomes with those of other organisms. Secretomics has also attracted interest for its applied aspects which may help to contribute to the global food security and to the ecosystem sustainability by addressing issues in i) plant biosecurity, with the design of crops resistant to pathogenic fungi, ii) crop yield enhancement, for example driven by Arbuscular Mycorrhizal fungi helping plant hosts utilise phosphate from the soil hence increase biomass, and iii) renewable energy, through the identification of fungal enzymes able to augment the bioconversion of plant lignocellulosic materials for the production of second generation biofuels that do not compete with food production.
Currently, I address the role of secreted intercellular signals in two experimental systems:
(i) whereas signals secreted by pathogens to manipulate plant immunity (so called effectors) have been intensively studied, so far, very little is known about signals secreted by the plant host. However, especially for so called hemibiotrophic pathogens (and most pathogens fall into this class), the switch from a biotrophic lifestyle (where the invader tries secretly to sneak into the host cell and to manipulate it for its own purpose) to a necrotrophic strategy, where the host cell is killed by toxins must be initiated at the right time and this can only work, when the pathogen can sense the status of the host cell by signals. I address this aspect by using specific cell lines of grapevine that differ in their immunity responses. By comparing secreted proteins under various treatments, I aim to identify proteins which are cultivar and inducer specific.
(ii) Secreted signals regulate cell proliferation and differentiation in plant tissues and seem also to act in cells cultivated in suspensions or bioreactors. There is evidence for so called quorum sensing, whereby plant cells change their behavior depending on the presence of other cells. To identify and control these signals, is of high relevance for application, for instance, during the production of valuable secondary metabolites that is currently pursued using microfluidic chamber systems as a platform (see BMBF project “Neue Bioökonomie” – press release 2014-08).