The main functions of plant roots are water and nutrient uptake, soil anchorage, and interaction with soil-living biota. Root hairs, single cell tubular extensions of root epidermal cells, facilitate or enhance these functions by drastically enlarging the absorptive surface. Root hair development is constantly adapted to changes in the root’s surroundings, allowing for optimization of root functionality in heterogeneous soil environments. The underlying molecular pathway is the result of a complex interplay between position-dependent signalling and feedback loops. Phytohormone signalling interconnects this root hair signalling cascade with biotic and abiotic changes in the rhizosphere, enabling dynamic hormone-driven changes in root hair growth, density, length, and morphology. This review critically discusses the influence of the major plant hormones on root hair development, and how changes in rhizosphere properties impact on the latter.

Assembly of cell wall polysaccharides into specific patterns is required for plant growth. A complex of RAPID ALKALINIZATION FACTOR 4 (RALF4) and its cell wall–anchored LEUCINE-RICH REPEAT EXTENSIN 8 (LRX8)–interacting protein is crucial for cell wall integrity during pollen tube growth, but its molecular connection with the cell wall is unknown. Here, we show that LRX8-RALF4 complexes adopt a heterotetrametric configuration in vivo, displaying a dendritic distribution. The LRX8-RALF4 complex specifically interacts with demethylesterified pectins in a charge-dependent manner through RALF4’s polycationic surface. The LRX8-RALF4-pectin interaction exerts a condensing effect, patterning the cell wall’s polymers into a reticulated network essential for wall integrity and expansion. Our work uncovers a dual structural and signaling role for RALF4 in pollen tube growth and in the assembly of complex extracellular polymers.

Plant roots fulfill important functions as they serve in water and nutrient uptake, provide anchorage of the plant body in the soil and in some species form the site of symbiotic interactions with soil-living biota. Root hairs, tubular-shaped outgrowths of specific epidermal cells, significantly increase the root's surface area and aid in these processes. In this review we focus on the molecular mechanisms that determine the hair and non-hair cell fate of epidermal cells and that define the site on the epidermal cell where the root hair will be initiated (= planar polarity determination). In the model plant Arabidopsis, trichoblast and atrichoblast cell fate results from intra- and intercellular position-dependent signaling and from complex feedback loops that ultimately regulate GL2 expressing and non-expressing cells. When epidermal cells reach the end of the root expansion zone, root hair promoting transcription factors dictate the establishment of polarity within epidermal cells followed by the selection of the root hair initiation site at the more basal part of the trichoblast. Molecular players in the abovementioned processes as well as the role of phytohormones are discussed, and open areas for future experiments are identified.

Ca²⁺ signaling is central to plant development and acclimation. While Ca²⁺-responsive proteins have been investigated intensely in plants, only a few Ca²⁺-permeable channels have been identified, and our understanding of how intracellular Ca²⁺ fluxes is facilitated remains limited. Arabidopsis thaliana homologs of the mammalian channel-forming mitochondrial calcium uniporter (MCU) protein showed Ca²⁺ transport activity in vitro. Yet, the evolutionary complexity of MCU proteins, as well as reports about alternative systems and unperturbed mitochondrial Ca²⁺ uptake in knockout lines of MCU genes, leave critical questions about the in vivo functions of the MCU protein family in plants unanswered. Here, we demonstrate that MCU proteins mediate mitochondrial Ca²⁺ transport in planta and that this mechanism is the major route for fast Ca²⁺ uptake. Guided by the subcellular localization, expression, and conservation of MCU proteins, we generated an mcu triple knockout line. Using Ca²⁺ imaging in living root tips and the stimulation of Ca²⁺ transients of different amplitudes, we demonstrated that mitochondrial Ca²⁺ uptake became limiting in the triple mutant. The drastic cell physiological phenotype of impaired subcellular Ca²⁺ transport coincided with deregulated jasmonic acid-related signaling and thigmomorphogenesis. Our findings establish MCUs as a major mitochondrial Ca²⁺ entry route in planta and link mitochondrial Ca²⁺ transport with phytohormone signaling.