Thursday, July 23, 2015
Interaction between copper deficiency responses and iron homeostasis in Arabidopsis thaliana
Departamento de Bioquímica y Biología Molecular Universitat de Valencia
Copper and iron are essential micronutrients for most living organisms because they participate as cofactors in key biological processes, but are cytotoxic when in excess. In many eukaryotic organisms, including yeast and mammals, copper and iron homeostases are highly interconnected; yet, such interdependence is not well established in higher plants. We have recently found that the high affinity copper transporter COPT2 functions under copper and iron deficiencies in Arabidopsis. The copt2-1 mutant shows increased resistance to simultaneous copper and iron deficiencies, measured as reduced leaf chlorosis and improved maintenance of the photosynthetic apparatus. This phenotype could be mediated by low-phosphate responses that are increased in the copt2-1 mutant. Copper homeostasis under deficiency is regulated by the SQUAMOSA PROMOTER-LIKE7 (SPL7) transcription factor. The daily oscillating expression of two SPL7-dependent copper deficiency markers, COPT2 and IRON SUPEROXIDE DISMUTASE (FSD1), has been followed by quantitative PCR and in promoter:LUCIFERASE transgenic plants. Both genes were mostly regulated diurnally, but COPT2 was also circadianly regulated. Under copper deficiency, although both genes are cyclically expressed, the COPT2 amplitude decreased drastically in continuous darkness. Accordingly, total copper content was slightly reduced in etiolated seedlings under copper deficiency. The expression of SPL7 did not depend on light wavelength, whereas its targets COPT2 and FSD1 were both differently regulated under red and blue lights. On the other hand, the alteration of copper homeostasis in the COPT1OE and spl7 mutants modified the amplitude of a classical clock output, such as the circadian oscillation of cotyledon movements, but the period remained constant in the spl7 mutant. These results suggest a feedback of copper transport on the circadian clock and the integration of rhythmic copper homeostasis into the central oscillator of plants. Taken together, these results are in agreement with the effect that the light on the extension of the period of clock under iron deficiency and point to the involvement of the plastids in the perception of the metal plant nutritional status.
Contact : Stéphane Mari