Lukas Hoermayer’s research while affiliated with Gregor Mendel Institute of Molecular Plant Biology (GMI) and other places

The puzzle-shaped cells found in the shoot epidermis of many plant species are a fascinating example of complex cell shapes. Because biological form often follows function, the unique shape of these cells suggests that they must serve some adaptive purpose for the plant. We previously proposed that these intricate shapes provide an effective strategy for reducing mechanical stress on the cell wall when epidermal cells undergo growth in more than one direction. Here we analyze a large selection of living and paleo plant species and find that the ability to make puzzle cells is a shared feature across all plant species, although their presence can be hidden as it varies depending on the organ, developmental stage, and environmental conditions. Computational modeling of Arabidopsis and maize epidermal cells revealed that presence and patterning of lobes is a dynamic process that is intricately linked to the growth history and environmental context of the plant organ. Conversely, disrupted lobeyness in mutants or with drug treatments affects plant development and leads to compensatory strategies. We propose that the mechanism underlying the formation of puzzle-shaped cells is likely conserved among higher plants and is a response to a developmental constraint driven by growth and mechanical stress.

Plant morphogenesis relies exclusively on oriented cell expansion and division. Nonetheless, the mechanism(s) determining division plane orientation remain elusive. Here we studied tissue healing after laser-assisted wounding in roots and uncovered how mechanical forces of cell expansion stabilize and reorient microtubule cytoskeleton for orientation of cell division. We revealed that root tissue functions as interconnected cell matrix with a radial gradient of tissue extendibility causing a predictable tissue deformation after wounding. This causes instant redirection of expansion in the surrounding cells and reorientation of microtubule arrays ultimately predicting cell division orientation. Microtubules are destabilized under low tension, whereas stretching of cells, either through wounding or external aspiration immediately induce their polymerisation. The higher microtubule abundance in the stretched cell parts leads to reorientation microtubule arrays and ultimately cell division planes. This provides a long-sought mechanism for flexible re-arrangement of cell divisions by mechanical forces for tissue reconstruction and plant architecture.

The plant hormone auxin and its directional transport across tissues mediate much of the remarkably adaptive and plastic development of higher plants. Positive feedback between auxin signalling and transport is a key prerequisite for self-organizing development including flexible vascular tissue formation and directional growth responses, such as gravitropism. Here we identified a mechanistic link between cell surface ABP1-based auxin perception, the associated TMK1 kinase and PIN auxin transporters. abp1 and tmk1 mutants are defective in auxin-triggered phosphorylation of PIN proteins and in PIN-mediated directional auxin transport. Auxin, via ABP1, induces activation and stabilization of TMK1, thus promoting direct interaction with and phosphorylation of PIN2. Following gravistimulation, the auxin-activated TMK1 acts along the lower root side to reinforce asymmetry in PIN2-mediated auxin fluxes for gravitropic root bending. This ABP1-TMK1-dependent positive feedback on PIN-mediated directional auxin transport is fundamental for robust root gravitropism and presumably also for other self-organizing developmental processes.

The phytohormone auxin is the major coordinative signal in plant development1, mediating transcriptional reprogramming by a well-established canonical signalling pathway. TRANSPORT INHIBITOR RESPONSE 1 (TIR1)/AUXIN-SIGNALING F-BOX (AFB) auxin receptors are F-box subunits of ubiquitin ligase complexes. In response to auxin, they associate with Aux/IAA transcriptional repressors and target them for degradation via ubiquitination2,3. Here we identify adenylate cyclase (AC) activity as an additional function of TIR1/AFB receptors across land plants. Auxin, together with Aux/IAAs, stimulates cAMP production. Three separate mutations in the AC motif of the TIR1 C-terminal region, all of which abolish the AC activity, each render TIR1 ineffective in mediating gravitropism and sustained auxin-induced root growth inhibition, and also affect auxin-induced transcriptional regulation. These results highlight the importance of TIR1/AFB AC activity in canonical auxin signalling. They also identify a unique phytohormone receptor cassette combining F-box and AC motifs, and the role of cAMP as a second messenger in plants. Adenylate cyclase activity in TIR1/AFB, the canonical auxin receptor, has an essential role in auxin-mediated root growth inhibition in land plants.

Directionality in the intercellular transport of the plant hormone auxin is determined by polar plasma membrane localization of PIN-FORMED (PIN) auxin transport proteins. However, apart from PIN phosphorylation at conserved motifs, no further determinants explicitly controlling polar PIN sorting decisions have been identified. Here we present Arabidopsis WAVY GROWTH 3 (WAV3) and closely related RING-finger E3 ubiquitin ligases, whose loss-of-function mutants show a striking apical-to-basal polarity switch in PIN2 localization in root meristem cells. WAV3 E3 ligases function as essential determinants for PIN polarity, acting independently from PINOID/WAG-dependent PIN phosphorylation. They antagonize ectopic deposition of de novo synthesized PIN proteins already immediately following completion of cell division, presumably via preventing PIN sorting into basal, ARF GEF-mediated trafficking. Our findings reveal an involvement of E3 ligases in the selective targeting of apically localized PINs in higher plants.

The analysis of dynamic cellular processes such as plant cytokinesis stands and falls with live-cell time-lapse confocal imaging. Conventional approaches to time-lapse imaging of cell division in Arabidopsis root tips are tedious and have low throughput. Here, we describe a protocol for long-term time-lapse simultaneous imaging of multiple root tips on a vertical-stage confocal microscope with automated root tracking. We also provide modifications of the basic protocol to implement this imaging method in the analysis of genetic, pharmacological or laser ablation wounding-mediated experimental manipulations. Our method dramatically improves the efficiency of cell division time-lapse imaging by increasing the throughput, while reducing the person-hour requirements of such experiments.Key wordsCytokinesis Arabidopsis Root meristemConfocal microscopyTime-lapse imagingLaser ablationAutomation

Auxin is a key regulator of plant growth and development. Local auxin biosynthesis and intercellular transport generates regional gradients in the root that are instructive for processes such as specification of developmental zones that maintain root growth and tropic responses. Here we present a toolbox to study auxin-mediated root development that features: (i) the ability to control auxin synthesis with high spatio-temporal resolution and (ii) single-cell nucleus tracking and morphokinetic analysis infrastructure. Integration of these two features enables cutting-edge analysis of root development at single-cell resolution based on morphokinetic parameters under normal growth conditions and during cell-type-specific induction of auxin biosynthesis. We show directional auxin flow in the root and refine the contributions of key players in this process. In addition, we determine the quantitative kinetics of Arabidopsis root meristem skewing, which depends on local auxin gradients but does not require PIN2 and AUX1 auxin transporter activities. Beyond the mechanistic insights into root development, the tools developed here will enable biologists to study kinetics and morphology of various critical processes at the single cell-level in whole organisms. Auxin gradients regulate plant root growth and development. Here the authors manipulate auxin synthesis in specific root cell types and use single-cell nucleus tracking and morphokinetics to map directional auxin flow in the root and quantify the kinetics of meristem skewing.