Structural modifications and δ‐aminolevulinate dehydratase synthesis/transport in cotyledons of far‐red irradiated radish

Abstract

δ‐Aminolevulinate dehydratase (ALAD) is synthesized in the cytoplasm and subsequently transported into the plastids in cotyledons of tar‐red irradiated radish (Raphanus sativus L. ev. Longue Rave Saumonée). The intra‐cellular localization of ALAD activity was determined under far‐red irradiation and darkness. After 60 h far‐red, ALAD was essentially eytoplasmic; 96 h far‐red corresponded to a peak in total ALAD activity, as well as the onset of massive ALAD transfer into the plastids, and after 120 h a maximum enzyme activity was located in the plastids. Under dark conditions, essentially no transport of ALAD was noted.Structural modifications of plastid‐eytoplasmic relationships were analysed at the three developmental periods under dark and far‐red conditions. 60 h far‐red induced a change in the “elasticity” of plastid envelopes leading to envelope proliferations and amoeboid shaped plastids. After 96 h tar‐red, membrane proliferations formed a highly evaginated plastid periphery. It is suggested that these membranes form a transport network between the two compartments. Also at 96 h, numerous vesicles, which appear to originate from the endoplasmic reticulum (ER), surround the plastid profile. On the basis of the structural observations, it is proposed that ALAD would be synthesized within the ER and transported toward the plastids via ER extensions, which take the form of smooth vesicles. Our hypothesis is that the transition vesicles and proliferations of the outer envelope membrane undergo a fusion process emptying the content of the vesicle into the intermembrane space of the envelope.