In seedlings of the Scots pine (Pinus sylvestr& L.), alanine aminotransferase (A1AT EC 2.6.1.2.) is present in the shoot and in the primary root but most activity is found in the cotyledons. During the experimental period (from 6 to 12 d after sowing), A1AT activity increased steadily. Anion exchange chromatography and native polyacrylamide gel electrophoresis were used to show that A1AT activity in extracts from cotyledons is associated with two isoforms of the enzyme. One isoform (A1AT 1) dominated in the cotyledons of lightgrown seedlings, but was absent from primary roots. Its accumulation was strongly increased by light, and both phytochrome and cryptochrome were shown to be involved in this effect. Results of experiments using dichromatic irradiation indicate that cryptochrome acts indirectly by establishing responsiveness towards phytochrome. When plastids were damaged by photooxidation, the accumulation of A1AT 1 decreased; however, A1AT 1 which had accumulated before the onset of photooxidative treatment seemed to remain undamaged. Therefore, and because of the absence of A1AT 1 from primary roots, it is suggested that this isoform is localized in leaf peroxisomes. The isoform A1AT 2 is the only one found in primary roots, and the predominant one in the cotyledons of dark-grown seedlings. It is unaffected by light. Upon photodestruction of plastids, a pronounced increase of its activity was found. This is taken as evidence that A1AT 2 is a cytosolic enzyme. Total A1AT activity in cotyledons was unaffected by feeding nitrate Abbreviations and symbols: A1AT = alanine aminotransferase (EC 2.6.1.2.); B= blue light; c=continuous; D=darkness; Fd-GOGAT = ferredoxin-dependent glutamate synthase (EC 1.4.7.1.); FR = far-red light; HPR = hydroxypyruvate reductase (EC 1.1.1.81 .); FPLC = fast protein liquid chromatography; PAGE = polyacrylamide gel electrophoresis; R = red light; RG9 = long-wavelength far-red light defined by the properties of the Schott glass filter RG9 (~RO9<0.01); ~;~=Pfr/Ptot=far-redabsorbing form of phytochrome/total phytochrome, wavelengthdependent photoequilibrium of the phytochrome system * To whom correspondence should be addressed; FAX: 49 (761) 203 2712 to the seedlings; supplying exogenous ammonium led to a considerably slower accumulation of AIAT compared with water controls. In contrast, AIAT accumulation in the primary roots was augmented by up to 45 % if nitrogenous ions were supplied, ammonium being more effective than nitrate.