Light inhibition of fumarase in Arabidopsis leaves is phytochrome A–dependent and mediated by calcium

• Expression of the fumarase gene FUM1 in Arabidopsis is inhibited by light.
• Phytochrome A is involved in light inhibition of fumarase expression.
• Phytochrome signal is transmitted via the change of Ca2+ concentration in nucleus.

Inhibition of fumarase activity in the light has been studied in Arabidopsis in relation to the involvement of phytochrome. Using knockout phytochrome mutants, we observed that the main regulator of FUM1 gene transcription, encoding the mitochondrial form of fumarase, is phytochrome A. The active form of phytochrome A suppressed FUM1 expression, while the expression of the FUM2 gene encoding the cytosolic form of fumarase was unaffected both in darkness and in light. The nuclear concentration of Ca2+ was modulated by red and far-red light. We suggest that the signal transduction mechanism operates via Ca2+ activation of expression of the gene encoding the transcription factor PIF3, which binds to promoters of phytochrome-regulated genes and inhibits FUM1 expression.