Ethylene aplenty signals soil compaction: It's tough to drive a spade through compacted soil, and plant roots seem to have the same problem when growing in compacted ground. Pandey et al. found that the problem is not, however, one of physical resistance but rather inhibition of growth through a signaling pathway. The volatile plant hormone ethylene will diffuse through aerated soil, but compacted soil reduces such diffusion, increasing the concentration of ethylene near root tissues. The cellular signaling cascades triggered by too much ethylene stop root growth. Therefore, gaseous diffusion serves as a readout of soil compaction for plant roots growing in search of productive nutrition. Science , this issue p. 276

Cytokinins are mobile multifunctional plant hormones with roles in development and stress resilience. Although their Histidine Kinase receptors are substantially localised to the endoplasmic reticulum, cellular sites of cytokinin perception and importance of spatially heterogeneous cytokinin distribution continue to be debated. Here we show that cytokinin perception by plasma membrane receptors is an effective additional path for cytokinin response. Readout from a Two Component Signalling cytokinin-specific reporter (TCSn::GFP) closely matches intracellular cytokinin content in roots, yet we also find cytokinins in extracellular fluid, potentially enabling action at the cell surface. Cytokinins covalently linked to beads that could not pass the plasma membrane increased expression of both TCSn::GFP and Cytokinin Response Factors. Super-resolution microscopy of GFP-labelled receptors and diminished TCSn::GFP response to immobilised cytokinins in cytokinin receptor mutants, further indicate that receptors can function at the cell surface. We argue that dual intracellular and surface locations may augment flexibility of cytokinin responses.

Significance: Hormones play a crucial role in the coordination of the physiological processes within and between the cells and tissues of plants. However, due to a lack of capable technologies, direct and dynamic interactions with plants’ hormone-signaling systems remains limited. Here, we demonstrate the use of an organic electronic device—the organic electronic ion pump—to deliver the plant hormone auxin to the living root tissues of Arabidopsis thaliana seedlings, inducing differential concentration gradients and modulating plant physiology. Electronically regulated transport of aromatic structures such as auxin in an organic electronic device was achieved by synthesis of a previously unidentified class of dendritic polyelectrolyte. Such bioelectronic technology opens the door for precise, electronically mediated control of a plant’s growth and development.

Summary: Dynamic regulation of the concentration of the natural auxin (IAA) is essential to coordinate most of the physiological and developmental processes and responses to environmental changes. Oxidation of IAA is a major pathway to control auxin concentrations in angiosperms and, along with IAA conjugation, to respond to perturbation of IAA homeostasis. However, these regulatory mechanisms remain poorly investigated in conifers. To reduce this knowledge gap, we investigated the different contributions of the IAA inactivation pathways in conifers. MS‐based quantification of IAA metabolites under steady‐state conditions and after perturbation was investigated to evaluate IAA homeostasis in conifers. Putative Picea abies GH3 genes (PaGH3) were identified based on a comprehensive phylogenetic analysis including angiosperms and basal land plants. Auxin‐inducible PaGH3 genes were identified by expression analysis and their IAA‐conjugating activity was explored. Compared to Arabidopsis, oxidative and conjugative pathways differentially contribute to reduce IAA concentrations in conifers. We demonstrated that the oxidation pathway plays a marginal role in controlling IAA homeostasis in spruce. By contrast, an excess of IAA rapidly activates GH3‐mediated irreversible conjugation pathways. Taken together, these data indicate that a diversification of IAA inactivation mechanisms evolved specifically in conifers.

Electronic control of biological processes with bioelectronic devices holds promise for sophisticated regulation of physiology, for gaining fundamental understanding of biological systems, providing new therapeutic solutions, and digitally mediating adaptations of organisms to external factors. The organic electronic ion pump (OEIP) provides a unique means for electronically‐controlled, flow‐free delivery of ions, and biomolecules at cellular scale. Here, a miniaturized OEIP device based on glass capillary fibers (c‐OEIP) is implanted in a biological organism. The capillary form factor at the sub‐100 µm scale of the device enables it to be implanted in soft tissue, while its hyperbranched polyelectrolyte channel and addressing protocol allows efficient delivery of a large aromatic molecule. In the first example of an implantable bioelectronic device in plants, the c‐OEIP readily penetrates the leaf of an intact tobacco plant with no significant wound response (evaluated up to 24 h) and effectively delivers the hormone abscisic acid (ABA) into the leaf apoplast. OEIP‐mediated delivery of ABA, the phytohormone that regulates plant's tolerance to stress, induces closure of stomata, the microscopic pores in leaf's epidermis that play a vital role in photosynthesis and transpiration. Efficient and localized ABA delivery reveals previously unreported kinetics of ABA‐induced signal propagation.