Compared with wild type, the dgd1 mutant of Arabidopsis thaliana exhibited a lower amount of PSI-related Chl-protein complexes and lower abundance of the PSI-associated polypeptides, PsaA, PsaB, PsaC, PsaL and PsaH, with no changes in the levels of Lhca1-4. Functionally, the dgd1 mutant exhibited a significantly lower light-dependent, steady-state oxidation level of P700 (P700(+)) in vivo, a higher intersystem electron pool size, restricted linear electron transport and a higher rate of reduction of P700(+) in the dark, indicating an increased capacity for PSI cyclic electron transfer compared with the wild type. Concomitantly, the dgd1 mutant exhibited a higher sensitivity to and incomplete recovery of photoinhibition of PSI. Furthermore, dgd1 exhibited a lower capacity to undergo state transitions compared with the wild type, which was associated with a higher reduction state of the plastoquinone (PQ) pool. We conclude that digalactosyl-diacylglycerol (DGDG) deficiency results in PSI acceptor-side limitations that alter the flux of electrons through the photosynthetic electron chain and impair the regulation of distribution of excitation energy between the photosystems. These results are discussed in terms of thylakoid membrane domain reorganization in response to DGDG deficiency in A. thaliana.