A Novel Regulator Of Copper Homeostasis, Ccit1, And Spl7 Connect Copper Homeostasis With Pollen Fertility, Jasmonic Acid Biosynthesis And Cadmium Resistance In Arabidopsis Thaliana

Abstract

The transition metal copper (Cu) is among the most important mineral nutrients and is essential for plant growth, development and fertility. However, Cu is toxic if it is accumulated in cells in excess. To maintain Cu homeostasis, plants have evolved transcriptional regulation of genes involved in Cu uptake, trafficking, tissue partitioning and reallocation among Cu requiring enzymes. SPL7 has been shown to play a central role in this transcriptional regulatory network and is the only transcription factor with a documented role in Cu homeostasis. We have found recently that a member of the bHLH family of TFs, that we designated CCIT1, is transcriptionally regulated by Cu availability, is essential for Cu acquisition and is required for plant growth under low Cu condition. Previous transcriptome analyses have identified CCIT1 among the downstream targets of SPL7. However, the ccit1spl7 double mutant exhibited infertility phenotype due to altered flower morphology, substantiallty reduced pollen production and viability. These results suggest that CCIT1 does not simply act downstream of SPL7, and this interactive regulatory pathway is rather more complex. Y1H and RNA-seq studies identified components of this pathway and the hierarchy of interactions which provide essential molecular evidences of the connection between Cu homeostasis, jasmonic acid biosynthesis and male fertility. On the other hand, cadmium (Cd) is a non-essential, toxic metal that causes plant growth retardation, disrupts micronutrient homeostasis and interferes with photosynthesis and redox balance. The components of the molecular machinery mediating the crosstalk between Cd and essential elements, however, are unknown. We have found recently that the central regulator of Cu homeostasis, SPL7, is important for basal Cd tolerance. Here we show that CCIT1 is induced by Cd toxicity. Loss of CCIT1 function results in hypersensitivity to Cd, and compromised Cu accumulation which is essential for basal Cd tolerance. Transcript abundance comparison between wild-type and ccit1 mutant identified two high-affinity Cu transporters among the targets of CCIT1. Together, the results herein expand the understanding of the interaction between the essential heavy metal Cu and the toxic heavy metal Cd, and might provide novel avenues for biofortification strategies to improve mineral nutrition while avoid toxic metal entry into the food crops.