The hook shape of growing leaves results from an active regulatory process

Extranuclear auxin signaling: a new insight into auxin's versatility

Auxin phytohormone has a role in most aspects of the life of a land plant and is found even in ancient plants such as single-cell green algae. Auxin's ubiquitous but specific effects have been mainly explained by the extraordinary ability of plants to interpret spatiotemporal patterns of auxin concentrations via the regulation of gene transcription. This is thought to be achieved through the combinatorial effects of two families of nuclear coreceptor proteins, that is the TRANSPORT INHIBITOR RESPONSE1 and AUXIN-SIGNALING F-BOX (TIR1/AFB) and AUXIN/INDOLE ACETIC ACID. Recent evidence has suggested transcription-independent roles of TIR1/AFBs localized outside the nucleus and TRANSMEMBRANE KINASE (TMK)-based auxin signaling occurring in the plasma membrane. Furthermore, emerging evidence supports a coordinated action of the intra- and extranuclear auxin signaling pathways to regulate specific auxin responses. Here, we highlight how auxin signaling acts inside and outside the nucleus for the regulation of growth and morphogenesis and propose that the future direction of auxin biology lies in the elucidation of a new collaborative paradigm of intra- and extranuclear auxin signaling.

Multiple mechanisms behind plant bending

To survive, plants constantly adapt their body shape to their environment. This often involves remarkably rapid bending of their organs such as stems, leaves and roots. Since plant cells are enclosed by stiff cell walls, they use various strategies for bending their organs, which differ from bending mechanisms of soft animal tissues and involve larger physical forces. Here we attempt to summarize and link different viewpoints on bending mechanisms: genes and signalling, mathematical modelling and biomechanics. We argue that quantifying cell growth and physical forces could open a new level in our understanding of bending and resolve some of its paradoxes.

Growth asymmetry precedes differential auxin response during apical hook initiation in Arabidopsis

The development of a hook-like structure at the apical part of the soil-emerging organs has fascinated botanists for centuries, but how it is initiated remains unclear. Here, we demonstrate with high-throughput infrared imaging and 2-D clinostat treatment that, when gravity-induced root bending is absent, apical hook formation still takes place. In such scenarios, hook formation begins with a de novo growth asymmetry at the apical part of a straightly elongating hypocotyl. Remarkably, such de novo asymmetric growth, but not the following hook enlargement, precedes the establishment of a detectable auxin response asymmetry, and is largely independent of auxin biosynthesis, transport and signaling. Moreover, we found that functional cortical microtubule array is essential for the following enlargement of hook curvature. When microtubule array was disrupted by oryzalin, the polar localization of PIN proteins and the formation of an auxin maximum became impaired at the to-be-hook region. Taken together, we propose a more comprehensive model for apical hook initiation, in which the microtubule-dependent polar localization of PINs may mediate the instruction of growth asymmetry that is either stochastically taking place, induced by gravitropic response, or both, to generate a significant auxin gradient that drives the full development of the apical hook.

Fluctuations shape plants through proprioception

Plants constantly experience fluctuating internal and external mechanical cues, ranging from nanoscale deformation of wall components, cell growth variability, nutating stems, and fluttering leaves to stem flexion under tree weight and wind drag. Developing plants use such fluctuations to monitor and channel their own shape and growth through a form of proprioception. Fluctuations in mechanical cues may also be actively enhanced, producing oscillating behaviors in tissues. For example, proprioception through leaf nastic movements may promote organ flattening. We propose that fluctuation-enhanced proprioception allows plant organs to sense their own shapes and behave like active materials with adaptable outputs to face variable environments, whether internal or external. Because certain shapes are more amenable to fluctuations, proprioception may also help plant shapes to reach self-organized criticality to support such adaptability.

How kelp in drag lose their ruffles: environmental cues, growth kinematics, and mechanical constraints govern curvature

We reveal how patterns of growth in response to environmental cues can produce curvature in biological structures by setting up mechanical stresses that cause elastic buckling. Nereocystis luetkeana are nearshore kelp with wide ruffled blades that minimize self-shading in slow flow, but narrow flat blades that reduce hydrodynamic drag in rapid flow. Previously we showed that blade ruffling is a plastic trait associated with a transverse gradient in longitudinal growth. Here we consider expansion and displacement of tissue elements due to growth in blades, and find that growth patterns are altered by tensile stress due to hydrodynamic drag, but not by shading or nutrients. When longitudinal stress in a blade is low in slow flow, blade edges grow faster than the midline in young tissue near the blade base. Tissue elements are displaced distally by expansion of younger proximal tissue. Strain energy caused by the transverse gradient in longitudinal growth is released by elastic buckling once the blade grows wide enough, producing ruffles distal to the region where the growth inhomogeneity started. If a blade experiences higher stress in rapid flow, the edges and midline grow at the same rate, so the blade becomes flat as these new tissue elements are displaced distally.

By hook or by crook: how and why do compound leaves stay curved during development?

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Rivière M, Corre Y, Peaucelle A, Derr J, Douady S. 2020. The hook shape of growing leaves results from an active regulatory process. Journal of Experimental Botany 71, 6408–6417.