Study On MicroRNAs For The Sucrose Signaling And Copper Homeostasis In Arabiodopsis

Sucrose induces the expression of miR398and the production of miR398is controlled by SQUAMOSA promoter binding protein-like7(SPL7) transcription factor under copper deficiency in Arabidopsis thaliana. Little else, however, is known about the sucrose-regulated copper homeostasis in Arabidopsis. Here, we employed the microarray technology to screen for sucrose-responsive microRNAs (miRNAs), and identified several candidates using northern blotting. In addition to miR398, miR408, miR319, and miR160are also responsive to sucrose. Furthermore, the induction of miR398and miR408by sucrose depends on the presence of SPL7. MiR398and miR408induction by high sucrose was eliminated by high copper. Meanwhile, high sucrose treatment strongly suppressed copper accumulation. Knock-down of SPL7dramatically reduced copper accumulation in plants under low sucrose, but this effect was compromised in the presence of high levels of sucrose. Taken together, these results suggest a connection between sucrose signaling and copper accumulation in cells, and that sucrose-regulated copper accumulation is both SPL7-dependent and SPL7-independent in Arabidopsis.Sucrose (C12H22O11), the major sugar transportation molecule in plants, not only serves as an essential carbon and energy resource for plant growth, but also plays a central role in plant sensing and signaling (disaccharide sensing/signaling. In plants, the transport of sucrose from sources to sinks is largely driven by sucrose-cleaving enzymes. Two types of sucrose-cleaving enzymes are widespread in specific plant cellular compartments:invertases and sucrose synthases. Sucrose is frequently converted into glucose/UDP-glucose and fructose by these two types of enzymes. Fast conversion of sucrose into glucose may facilitate the rapid switching of sucrose to glucose signaling, making it difficult to distinguish the effects of sucrose from glucose. A more general term, sugar signaling, is preferably used to describe the sucrose or glucose signaling in plants.Genes regulated by sugar are involved in many aspects of plant life cycle, such as seed germination, vegetative and reproductive growth, plant senescence, and in response to abiotic and biotic stresses. Endogenous sucrose produced mainly by photosynthesis, can be transported from sources (mature leaves) to sinks (growing and expanding roots, stems, meristems, flowers, and immature seeds). The endogenous sucrose auto-regulate its production and/or storage during photosynthesis. For example, low levels of sucrose stimulate photosynthesis and the transport of photosynthetic products, while high levels of sucrose repress sucrose transport and lead to sucrose storage. The coordination of photosynthesis and sucrose storage is well controlled through both metabolic regulation and specific sugar-signaling mechanisms. In addition to the endogenous sucrose produced by photosynthesis, plants can also grow by feeding on exogenous sucrose. In response to exogenous sucrose, plants selectively activate or suppress specific genes to use this exogenous carbon resource efficiently.Cellular copper is an indispensable micronutrient element and is also essential for photosynthesis. Copper acts as a cofactor of scavenger of reactive oxygen species (ROS), and participates in ethylene signaling. The most abundant copper containing protein plastocyanin (PC) is required for photosynthetic electron transport within the thylakoid lumen. Another major copper containing protein, Cu/Zn superoxide dismutase (CSD), is essential for detoxifying ROS and scavenging superoxide in plants. Arabidopsis thaliana contains three CSD isozymes cytoplasmic CSD1, chloroplast stromal CSD2, and the peroxisomal CSD3. To effectively utilize copper, plants allocate and redistribute their limited copper in response to copper deficiency. The molecular mechanisms underlying the response to copper deficiency have been investigated both in the unicellular green alga Chlamydomonas reinhardtii and the vascular plant Arabidopsis. In Chlamydomonas, the transcription factor copper response regulator1(Crr1) is a SQUAMOSA promoter binding protein, which regulates the switch of the photosynthesis machinery in response to copper deficiency. In Arabidopsis, an SBP homolog, SPL7, regulates the expression of multiple genes upon copper deficiency.Sucrose and copper homeostasis are closely related to plant growth and development. During the production of sucrose through photosynthesis, highly toxic ROS (Reactive Oxygen Species) are produced in plant cells, including superoxide radicals. Plants employ CSD1and CSD2to detoxify and scavenge superoxide radicals. On the other hand, the expression of CSD1and CSD2is tightly regulated by miR398, through specific cleavage of CSD1and CSD2transcripts. In Arabidopsis, the function of CSD1and CSD2can be replaced by Fe-SOD, which is not controlled by miRNAs. Abiotic and biotic stresses, such as, intense light, salinity, UV-B and infection by bacterial and fungal pathogens, specifically down-regulated miR398and activated CSD1and CSD2in Arabidopsis. In contrast, copper starvation and/or high sucrose strongly induced the expression of miR398and suppressed the expression of CSD1and CSD2. In both cases, the expression of CSD1and CSD2is strictly controlled by miR398through a post-transcriptional gene silencing (PTGS) mechanism. Down-regulation of CSD1and CSD2releases copper ions and thus modulates cellular copper homeostasis. The copper ions are preferably transported and incorporated into plastocyanin (PC) proteins as a form of copper storage as well as an essential component of the photosynthetic apparatus.Although the induction of miR398and the down-regulation of CSD1and CSD2by high sucrose and the impacts of copper deficiency on the expression of genes involved in copper homeostasis have been extensively studied, the molecular mechanism underlying the intersection of sucrose signaling and copper homeostasis has not been fully revealed. In this study, we present evidence that sucrose is related strongly to copper homeostasis. High levels of sucrose suppress the accumulation of copper in Arabidopsis, while high copper can fully eliminate miR398and miR408induction by high sucrose. SPL7is the key transcription factor that activates the expression of miR398and miR408in response to high sucrose in Arabidopsis. However, the regulation of copper accumulation by sucrose is not fully controlled by SPL7.