As the sun tracks daily through the sky from east to west, different parts of the canopy are exposed to high light (HL). The extent of and mechanisms by which a systemic acquired acclimation (SAA) response might preacclimate shaded leaves that will be subsequently exposed to full sunlight is largely undefined. We investigated the role of an Arabidopsis thaliana zinc finger transcription factor, ZAT10, in SAA. ZAT10 overexpression resulted in enhanced tolerance to photoinhibitory light and exogenous H 2 O 2 , increased expression of antioxidative genes whose products are targeted to multiple subcellular compartments. Partial HL exposure of a leaf or leaves rapidly induced ZAT10 mRNA in distal, shaded photosynthetic tissues, including the floral stem, cauline leaves, and rosette, but not in roots. Fully 86% of fivefold HL-upregulated and 71% of HL-downregulated genes were induced and repressed, respectively, in distal, shaded leaves. Between 15 and 23% of genes whose expression changed in the HL and/or distal tissues were coexpressed in the ZAT10 overexpression plants, implicating ZAT10 in modulating the expression of SAA-regulated genes. The SAA response was detectable in plants with mutations in abscisic acid, methyl jasmonate, or salicylic acid synthesis or perception, and systemic H 2 O 2 diffusion was not detected. Hence, SAA is distinct from pathogen-stimulated systemic acquired resistance and apparently involves a novel signal or combination of signals that preacclimate photosynthetic tissues to HL.

Organelle-nuclear retrograde signaling regulates gene expression, but its roles in specialized cells and integration with hormonal signaling remain enigmatic. Here we show that the SAL1-PAP (3′-phosphoadenosine 5′- phosphate) retrograde pathway interacts with abscisic acid (ABA) signaling to regulate stomatal closure and seed germination in Arabidopsis. Genetically or exogenously manipulating PAP bypasses the canonical signaling components ABA Insensitive 1 (ABI1) and Open Stomata 1 (OST1); priming an alternative pathway that restores ABA-responsive gene expression, ROS bursts, ion channel function, stomatal closure and drought tolerance in ost1-2. PAP also inhibits wild type and abi1-1 seed germination by enhancing ABA sensitivity. PAP-XRN signaling interacts with ABA, ROS and Ca2+; up-regulating multiple ABA signaling components, including lowly-expressed Calcium Dependent Protein Kinases (CDPKs) capable of activating the anion channel SLAC1. Thus, PAP exhibits many secondary messenger attributes and exemplifies how retrograde signals can have broader roles in hormone signaling, allowing chloroplasts to fine-tune physiological responses.DOI: http://dx.doi.org/10.7554/eLife.23361.001

Greater availability of leaf dark respiration (Rdark) data could facilitate breeding efforts to raise crop yield and improve global carbon cycle modelling. However, the availability of Rdark data is limited because it is cumbersome, time consuming, or destructive to measure. We report a non‐destructive and high‐throughput method of estimating Rdark from leaf hyperspectral reflectance data that was derived from leaf Rdark measured by a destructive high‐throughput oxygen consumption technique. We generated a large dataset of leaf Rdark for wheat (1380 samples) from 90 genotypes, multiple growth stages, and growth conditions to generate models for Rdark. Leaf Rdark (per unit leaf area, fresh mass, dry mass or nitrogen, N) varied 7‐ to 15‐fold among individual plants, whereas traits known to scale with Rdark, leaf N, and leaf mass per area (LMA) only varied twofold to fivefold. Our models predicted leaf Rdark, N, and LMA with r2 values of 0.50–0.63, 0.91, and 0.75, respectively, and relative bias of 17–18% for Rdark and 7–12% for N and LMA. Our results suggest that hyperspectral model prediction of wheat leaf Rdark is largely independent of leaf N and LMA. Potential drivers of hyperspectral signatures of Rdark are discussed.