Optimal plant growth is hampered by deficiency of the essential macronutrient phosphate in most soils. Plant roots can, however, increase their root hair density to efficiently forage the soil for this immobile nutrient. By generating and exploiting a high-resolution single-cell gene expression atlas of Arabidopsis roots, we show an enrichment of TARGET OF MONOPTEROS 5 / LONESOME HIGHWAY (TMO5/LHW) target gene responses in root hair cells. The TMO5/LHW heterodimer triggers biosynthesis of mobile cytokinin in vascular cells and increases root hair density during low phosphate conditions by modifying both the length and cell fate of epidermal cells. Moreover, root hair responses in phosphate deprived conditions are TMO5 and cytokinin dependent. In conclusion, cytokinin signaling links root hair responses in the epidermis to perception of phosphate depletion in vascular cells.

Physical damage to cells leads to the release of immunomodulatory peptides to elicit a wound defense response in the surrounding tissue. In Arabidopsis thaliana, the plant elicitor peptide 1 (Pep1) is processed from its protein precursor, PRECURSOR OF PEP1 (PROPEP1). We demonstrate that upon damage, both at the tissue and single-cell levels, the cysteine protease METACASPASE4 (MC4) is instantly and spatiotemporally activated by binding high levels of Ca2+ and is necessary and sufficient for Pep1 maturation. Cytosol-localized PROPEP1 and MC4 react only after loss of plasma membrane integrity and prolonged extracellular Ca2+ entry. Our results reveal that a robust mechanism consisting of conserved molecular components links the intracellular and Ca2+-dependent activation of a specific cysteine protease with the maturation of damage-induced wound defense signals.

Arabidopsis thaliana UV RESISTANCE LOCUS 8 (UVR8) is a UV-B photoreceptor that initiates photomorphogenic responses underlying acclimation and UV-B tolerance in plants. UVR8 is a homodimer in its ground state, and UV-B exposure results in its instantaneous monomerization followed by interaction with CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1), a major factor in UV-B signaling. UV-B photoreception by UVR8 is based on intrinsic tryptophan aromatic amino acid residues, with tryptophan-285 as the main chromophore. We generated transgenic plants expressing UVR8 with a single amino acid change of tryptophan-285 to alanine. UVR8 W285A appears monomeric and shows UV-B-independent interaction with COP1. Phenotypically, the plants expressing UVR8 W285A exhibit constitutive photomorphogenesis associated with constitutive activation of target genes, elevated levels of anthocyanins, and enhanced, acclimation-independent UV-B tolerance. Moreover, we have identified COP1, REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 and 2 (RUP1 and RUP2), and the SUPPRESSOR OF PHYA-105 (SPA) family as proteins copurifying with UVR8 W285A. Whereas COP1, RUP1, and RUP2 are known to directly interact with UVR8, we show that SPA1 interacts with UVR8 indirectly through COP1. We conclude that UVR8 W285A is a constitutively active UVR8 photoreceptor variant in Arabidopsis, as is consistent with the crucial importance of monomer formation and COP1 binding for UVR8 activity.signal transduction | abiotic stress | ultraviolet-B P lants react to UV-B radiation (UV-B; 280-315 nm) with a photomorphogenic response that generates acclimation to this environmental stress factor (1-3). The associated specific signaling pathway is characterized molecularly by the involvement of the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor (4, 5). Loss of UVR8 in Arabidopsis results in the loss of a broad range of molecular and physiological UV-B responses, including reduced UV-B acclimation and tolerance (6-11). Perception of UV-B photons by UVR8 homodimers results in UVR8 monomerization (4). The crystal structure of UVR8 shows that the homodimer is maintained by salt-bridge interactions between charged amino acids at the dimeric interaction surface (12, 13). Destabilization of the salt bridges upon absorption of UV-B photons by tryptophan-285, and to a lesser extent tryptophan-233, underlies UVR8 monomerization and signal initiation (12, 13). The UVR8 photoreceptor can revert to the ground state in vivo by redimerization (14,15). This process is facilitated by RE-PRESSOR OF UV-B PHOTOMORPHOGENESIS 1 and 2 (RUP1 and RUP2), consequently disrupting the key interaction of UVR8 with CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) (14, 16). The reversibility of UVR8 between inactive homodimer and active monomer conformations results in continuous sensitivity to the ambient UV-B environment (14,15). It can be assumed that UVR8 cycles between the dimeric and monomeric forms in vivo, and thus the resulting UVR8 dimer/monomer photoequilibrium is a measure of the ambient UV-B levels experienced by the ...