Abstract
Copper (Cu)-toxic effects on Citrus grandis growth and Cu uptake, as well as gene expression and physiological parameters in leaves were investigated. Using RNA-Seq, 715 upregulated and 573 downregulated genes were identified in leaves of C. grandis seedlings exposed to Cu-toxicity (LCGSEC). Cu-toxicity altered the expression of 52 genes related to cell wall metabolism, thus impairing cell wall metabolism and lowering leaf growth. Cu-toxicity downregulated the expression of photosynthetic electron transport-related genes, thus reducing CO2 assimilation. Some genes involved in thermal energy dissipation, photorespiration, reactive oxygen species scavenging and cell redox homeostasis and some antioxidants (reduced glutathione, phytochelatins, metallothioneins, l-tryptophan and total phenolics) were upregulated in LCGSEC, but they could not protect LCGSEC from oxidative damage. Several adaptive responses might occur in LCGSEC. LCGSEC displayed both enhanced capacities to maintain homeostasis of Cu via reducing Cu uptake by leaves and preventing release of vacuolar Cu into the cytoplasm, and to improve internal detoxification of Cu by accumulating Cu chelators (lignin, reduced glutathione, phytochelatins, metallothioneins, l-tryptophan and total phenolics). The capacities to maintain both energy homeostasis and Ca homeostasis might be upregulated in LCGSEC. Cu-toxicity increased abscisates (auxins) level, thus stimulating stomatal closure and lowering water loss (enhancing water use efficiency and photosynthesis).
Highlights
Copper (Cu) is required for the proper growth and development of plants, but it is extremely toxic in excess [1,2]
We identified five downregulated and one upregulated pectinesterase genes involved in pectin de-esterification, one downregulated omega-hydroxypalmitate O-feruloyl transferase involved in the cell wall pectin biosynthetic process, and two downregulated and two upregulated Wall-associated receptor kinases (WAKs) in LCGSEC, implying that pectin biosynthesis and the degree of pectin methylesterification were downregulated and upregulated in these leaves, respectively, decreasing and increasing Cu concentration in the cell wall and cytoplasm, respectively, and lowering Cu-tolerance
Thirty-two upregulated and 20 downregulated genes related to cell wall metabolism were identified in LCGSEC, implying that Cu-toxicity might impair cell wall metabolism, reducing leaf growth and lowering Cu-tolerance
Summary
Copper (Cu) is required for the proper growth and development of plants, but it is extremely toxic in excess [1,2]. In order to control fruit and leaf fungal diseases, long-term and heavy application of Cu-based fungicides has caused Cu accumulation in soil of Citrus orchards. The content of available Cu in the soil increases with Citrus planting years. In the old Citrus orchards, excessive accumulation of Cu in soil is a common problem limiting. Most studies have focused on phenomena occurring at the roots, because Cu is mainly accumulated in roots exposed to Cu toxicity, and the reduction of root growth has been shown to be usually earlier than that of shoot growth [1,7,8,9]. Growing evidence has shown that Cutoxicity influences biosynthesis of photosynthetic pigments, photosynthetic electron transport chain (PETC), CO2 assimilation [2,10], production and detoxification of reactive oxygen species (ROS) [11,12], phenol metabolism [13], hormone biosynthesis [14], nitrogen (N) and carbohydrate metabolism [2,15], and cell wall formation [16]
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