Phytochrome‐mediated control of respiratory activities of mitochondria in cotyledons of dark‐grown cucumber seedlings

Abstract

Mitochondria isolated from cotyledons of dark‐grown cucumber (Cucumber sativus L., cv. Shimotsuki‐Aonaga) seedlings after illumination with continuous far‐red light showed an increased capacity for oxidation of malate or α‐ketoglutarate, as compared with those from cotyledons of non‐illuminated seedlings. This increase is supposed to be caused by phytochrome action (high irradiance response). Exogenous NAD+ had no effect on the rate of the oxidation of α‐ketoglutarate or malate by mitochondria isolated from far‐red light‐treated cotyledons, but it enhanced the oxidation rate of mitochondria from control cotyledons to the level of mitochondria from light‐treated ones. The NAD (NAD++ NADH) content was higher in mitochondria isolated from continuously far‐red light‐treated cotyledons than in mitochondria from controls. The NAD content was also increased by the treatment with a red light pulse and this response was reversed by a subsequent far‐red light pulse. It is proposed that phytochrome controls respiratory activities of cucumber mitochondria by changing the size of the NAD pool in the mitochondria.