Chloroplasts require a fine-tuned control of their internal Ca concentration, which is crucial for many aspects of photosynthesis and for other chloroplast-localized processes. Increasing evidence suggests that calcium regulation within chloroplasts also may influence Ca signaling pathways in the cytosol. To investigate the involvement of thylakoids in Ca homeostasis and in the modulation of chloroplast Ca signals in vivo, we targeted the bioluminescent Ca reporter aequorin as a YFP fusion to the lumen and the stromal surface of thylakoids in Arabidopsis (). Thylakoid localization of aequorin-based probes in stably transformed lines was confirmed by confocal microscopy, immunogold labeling, and biochemical analyses. In resting conditions in the dark, free Ca levels in the thylakoid lumen were maintained at about 0.5 μm, which was a 3- to 5-fold higher concentration than in the stroma. Monitoring of chloroplast Ca dynamics in different intrachloroplast subcompartments (stroma, thylakoid membrane, and thylakoid lumen) revealed the occurrence of stimulus-specific Ca signals, characterized by unique kinetic parameters. Oxidative and salt stresses initiated pronounced free Ca changes in the thylakoid lumen. Localized Ca increases also were observed on the thylakoid membrane surface, mirroring transient Ca changes observed for the bulk stroma, but with specific Ca dynamics. Moreover, evidence was obtained for dark-stimulated intrathylakoid Ca changes, suggesting a new scenario for light-to-dark-induced Ca fluxes inside chloroplasts. Hence, thylakoid-targeted aequorin reporters can provide new insights into chloroplast Ca storage and signal transduction. These probes represent novel tools with which to investigate the role of thylakoids in Ca signaling networks within chloroplasts and plant cells.

To coordinate growth, development and responses to environmental stimuli, plant cells need to communicate the metabolic state between different sub-compartments of the cell. This requires signalling pathways, including protein kinases, secondary messengers such as Ca(2+) ions or reactive oxygen species (ROS) as well as metabolites and plant hormones. The signalling networks involved have been intensively studied over recent decades and have been elaborated more or less in detail. However, it has become evident that these signalling networks are also tightly interconnected and often merge at common targets such as a distinct group of transcription factors, most prominently ABI4, which are amenable to regulation by phosphorylation, potentially also in a Ca(2+)- or ROS-dependent fashion. Moreover, the signalling pathways connect several organelles or subcellular compartments, not only in functional but also in physical terms, linking for example chloroplasts to the nucleus or peroxisomes to chloroplasts thereby enabling physical routes for signalling by metabolite exchange or even protein translocation. Here we briefly discuss these novel findings and try to connect them in order to point out the remaining questions and emerging developments in plant organellar signalling.

Drought is a major cause of losses in crop yield. Under field conditions, plants exposed to drought are usually also experiencing rapid changes in light intensity. Accordingly, plants need to acclimate to both, drought and light stress. Two crucial mechanisms in plant acclimation to changes in light conditions comprise thylakoid protein phosphorylation and dissipation of light energy as heat by non-photochemical quenching (NPQ). Here, we analyzed the acclimation efficacy of two different wheat varieties, by applying fluctuating light for analysis of plants, which had been subjected to a slowly developing drought stress as it usually occurs in the field. This novel approach allowed us to distinguish four drought phases, which are critical for grain yield, and to discover acclimatory responses which are independent of photodamage. In short-term, under fluctuating light, the slowdown of NPQ relaxation adjusts the photosynthetic activity to the reduced metabolic capacity. In long-term, the photosynthetic machinery acquires a drought-specific configuration by changing the PSII-LHCII phosphorylation pattern together with protein stoichiometry. Therefore, the fine-tuning of NPQ relaxation and PSII-LHCII phosphorylation pattern represent promising traits for future crop breeding strategies.

SUMMARY Plants undergo photomorphogenic development in the presence of light. Photomorphogenesis is repressed by the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1), which binds to substrates through their valine–proline (VP) motifs. The UV RESISTANCE LOCUS 8 (UVR8) photoreceptor senses UV‐B and inhibits COP1 through the cooperative binding of its own VP motif and photosensing core to COP1, thereby preventing COP1 binding to substrates, including the basic leucine zipper (bZIP) transcriptional regulator ELONGATED HYPOCOTYL 5 (HY5). As a key promoter of visible light and UV‐B photomorphogenesis, HY5 requires coregulators for its function. The B‐box family transcription factors BBX20–BBX22 were recently described as HY5 rate‐limiting coactivators under red light, but their role in UVR8 signaling was unknown. Here we describe a hypermorphic bbx21‐3D mutant with enhanced photomorphogenesis, carrying a proline‐to‐leucine mutation at position 314 in the VP motif that impairs the interaction with and regulation by COP1. We show that BBX21 and BBX22 are UVR8‐dependently stabilized after UV‐B exposure, which is counteracted by a repressor induced by HY5/BBX activity. bbx20 bbx21 bbx22 mutants under UV‐B are impaired in hypocotyl growth inhibition, photoprotective pigment accumulation and the expression of several HY5‐dependent genes under continuous UV‐B, but the immediate induction of marker genes after exposure to UV‐B remains surprisingly rather unaffected. We conclude that BBX20–BBX22 contribute to HY5 activity in a subset of UV‐B responses, but that additional, presently unknown, coactivators for HY5 are functional in early UVR8 signaling.