A novel Ca2+-binding protein that can rapidly transduce auxin responses during root growth

Signaling cross talks between auxin, a regulator of plant development, and Ca2+, a universal second messenger, have been proposed to modulate developmental plasticity in plants. However, the underlying molecular mechanisms are largely unknown. Here, we report that in Arabidopsis roots, auxin elicits specific Ca2+ signaling patterns that spatially coincide with the expression pattern of auxin-regulated genes. We have identified the single EF-hand Ca2+-binding protein Ca2+-dependent modulator of ICR1 (CMI1) as an interactor of the Rho of plants (ROP) effector interactor of constitutively active ROP (ICR1). CMI1 expression is directly up- regulated by auxin, whereas the loss of function of CMI1 associates with the repression of auxin-induced Ca2+ increases in the lateral root cap and vasculature, indicating that CMI1 represses early auxin responses. In agreement, cmi1 mutants display an increased auxin response including shorter primary roots, longer root hairs, longer hypocotyls, and altered lateral root formation. Binding to ICR1 affects subcellular localization of CMI1 and its function. The interaction between CMI1 and ICR1 is Ca2+-dependent and involves a conserved hydrophobic pocket in CMI1 and calmodulin binding-like domain in ICR1. Remarkably, CMI1 is monomeric in solution and in vitro changes its secondary structure at cellular resting Ca2+ concentrations ranging between 10−9 and 10−8 M. Hence, CMI1 is a Ca2+-dependent transducer of auxin-regulated gene expression, which can function in a cell-specific fashion at steady-state as well as at elevated cellular Ca2+ levels to regulate auxin responses

Chloroplast-specific in vivo Ca2+ imaging using Yellow Cameleon fluorescent protein sensors reveals organelle-autonomous Ca2+ signatures in the stroma

In eukaryotes, subcellular compartments such as mitochondria, the endoplasmic reticulum, lysosomes and vacuoles have the capacity for Ca2+ transport across their membranes to modulate the activity of compartmentalized enzymes or to convey specific cellular signaling events. In plants, it has been suggested that chloroplasts also display Ca2+ regulation. So far monitoring of stromal Ca2+ dynamics in vivo has exclusively relied on using the luminescent Ca2+ probe aequorin. This technique is, however, limited in resolution and can only provide a readout averaged over chloroplast populations from different cells and tissues. Here we present a toolkit of Arabidopsis Ca2+ sensor lines expressing plastid-targeted FRET-based Yellow Cameleon (YC) sensors. We demonstrate that the probes reliably report in vivo Ca2+ dynamics in the stroma of root plastids in response to extracellular ATP and of leaf mesophyll and guard cell chloroplasts during light-to-low intensity blue light illumination transition. Applying YC sensing of stromal Ca2+ dynamics to single chloroplasts we confirm findings of gradual, sustained stromal Ca2+ increases at the tissue level after light-to-low intensity blue light illumination transitions, but monitor transient Ca2+ spiking as a distinct and previously unknown component of stromal Ca2+ signatures. Spiking was dependent on the availability of cytosolic Ca2+ but not synchronized between the chloroplasts of a cell. In contrast, the gradual sustained Ca2+ increase, occurred independent of cytosolic Ca2+ suggesting intra-organellar Ca2+ release. We demonstrate the capacity of the YC sensor toolkit to identify novel, fundamental facets of chloroplast Ca2+ dynamics and to refine the understanding of plastidial Ca2+ regulation

Constitutive cyclic GMP accumulation in Arabidopsis thaliana compromises systemic acquired resistance induced by an avirulent pathogen by modulating local signals

The infection of Arabidopsis thaliana plants with avirulent pathogens causes the accumulation of cGMP with a biphasic profile downstream of nitric oxide signalling. However, plant enzymes that modulate cGMP levels have yet to be identified, so we generated transgenic A. thaliana plants expressing the rat soluble guanylate cyclase (GC) to increase genetically the level of cGMP and to study the function of cGMP in plant defence responses. Once confirmed that cGMP levels were higher in the GC transgenic lines than in wild-type controls, the GC transgenic plants were then challenged with bacterial pathogens and their defence responses were characterized. Although local resistance was similar in the GC transgenic and wild-type lines, differences in the redox state suggested potential cross-talk between cGMP and the glutathione redox system. Furthermore, large-scale transcriptomic and proteomic analysis highlighted the significant modulation of both gene expression and protein abundance at the infection site, inhibiting the establishment of systemic acquired resistance. Our data indicate that cGMP plays a key role in local responses controlling the induction of systemic acquired resistance in plants challenged with avirulent pathogens