Spatiotemporal Analysis of Copper Homeostasis in Populus trichocarpa Reveals an Integrated Molecular Remodeling for a Preferential Allocation of Copper to Plastocyanin in the Chloroplasts of Developing Leaves      

Abstract

Plastocyanin, which requires copper (Cu) as a cofactor, is an electron carrier in the thylakoid lumen and essential for photoautotrophic growth of plants. The Cu microRNAs, which are expressed during Cu deprivation, down-regulate several transcripts that encode for Cu proteins. Since plastocyanin is not targeted by the Cu microRNAs, a cofactor economy model has been proposed in which plants prioritize Cu for use in photosynthetic electron transport. However, defects in photosynthesis are classic symptoms of Cu deprivation, and priorities in Cu cofactor delivery have not been determined experimentally. Using hydroponically grown Populus trichocarpa (clone Nisqually-1), we have established a physiological and molecular baseline for the response to Cu deficiency. An integrated analysis showed that Cu depletion strongly reduces the activity of several Cu proteins including plastocyanin, and consequently, photosynthesis and growth are decreased. Whereas plastocyanin mRNA levels were only mildly affected by Cu depletion, this treatment strongly affected the expression of other Cu proteins via Cu microRNA-mediated transcript down-regulation. Polyphenol oxidase was newly identified as Cu regulated and targeted by a novel Cu microRNA, miR1444. Importantly, a spatiotemporal analysis after Cu resupply to previously depleted plants revealed that this micronutrient is preferentially allocated to developing photosynthetic tissues. Plastocyanin and photosynthetic electron transport efficiency were the first to recover after Cu addition, whereas recovery of the other Cu-dependent activities was delayed. Our findings lend new support to the hypothesis that the Cu microRNAs serve to mediate a prioritization of Cu cofactor use. These studies also highlight poplar as an alternative sequenced model for spatiotemporal analyses of nutritional homeostasis.

Plastocyanin Controls the Stabilization of the Thylakoid Cu-transporting P-type ATPase PAA2/HMA8 in Response to Low Copper in Arabidopsis

“…In this context, we recently reported that, when Cu is resupplied to Cu-starved Populus trichocarpa, Cu is preferentially allocated to PC and the photosynthetic activity is quickly recovered (36). By contrast CCS and CSD2 recovery in the stroma was delayed (36). The stabilization of PAA2 is likely an additional regulatory mechanism, acting collectively with the SPL7-mediated responses, in order to facilitate the flow of Cu ions into the thylakoid lumen.…”

“…In the chloroplast, CCS and CSD2 are down-regulated so that the Cu pool in this compartment might now be used in "economy mode" for only the most essential functions. In this context, we recently reported that, when Cu is resupplied to Cu-starved Populus trichocarpa, Cu is preferentially allocated to PC and the photosynthetic activity is quickly recovered (36). By contrast CCS and CSD2 recovery in the stroma was delayed (36).…”

Influence of Soil Chemistry and Plant Physiology in the Phytoremediation of Cu, Mn, and Zn

“…However, 5 -20 μg g −1 DW seems to be optimal, as toxicity symptoms appear above and deficiency symptoms below this critical range (Hansch and Mendel, 2009;White and Brown, 2010). Cu deficiency/toxicity affects plant physiological processes and ultimately plant production (Ravet et al, 2011;Sanchez-Pardo et al, 2012).…”

Arabidopsis Pollen Fertility Requires the Transcription Factors CITF1 and SPL7 That Regulate Copper Delivery to Anthers and Jasmonic Acid Synthesis

“…An uptake-based Cu homeostatic mechanism involves the upregulated expression of Cu acquisition and transport genes under Cu deficiency (Burkhead et al, 2009). The Cu economy or "metal switch" mechanism, originally discovered in the green alga Chlamydomonas reinhardtii, includes the downregulation of the expression of potentially redundant Cu proteins for the metabolic reutilization of cellular Cu reserves (Burkhead et al, 2009;Blaby-Haas and Merchant, 2017;Kropat et al, 2015;Ravet et al, 2011;Shahbaz et al, 2015). In Arabidopsis thaliana, both processes are controlled by a conserved transcription factor, SPL7 (SQUAMOSA PROMOTER BINDING PROTEIN LIKE7), a homolog of the algal Cu sensor, CRR1 (COPPER RESPONSE REGULATOR1) (Yamasaki et al, 2009;Bernal et al, 2012;GarciaMolina et al, 2014;Sommer et al, 2010;Kropat et al, 2005).…”

“…However, the same chemical property can result in toxicity when free Cu ions accumulate in cells in excess because they promote oxidative stress (Burkhead et al, 2009). To maintain Cu homeostasis, plants regulate cellular Cu uptake and economize on Cu during deficiency (Ravet et al, 2011;Shahbaz et al, 2015;Burkhead et al, 2009). An uptake-based Cu homeostatic mechanism involves the upregulated expression of Cu acquisition and transport genes under Cu deficiency (Burkhead et al, 2009).…”