298The light collecting and electron transfer complexes are dynamically distributed within the thylakoid membrane of Arabidopsis thaliana and assemble into supramolecular structures. Beside the basic structure of PSI and PSII bound to LHCI and LHCII, respectively, we found that these supercomplexes may interact together to form a stable or transient supracomplex (PSI-LHCI-PSII-LHCII). This supracomplex is preferentially formed in an AtWhy1 overexpressing mutant affected in chloroplast biogenesis and may represent an intermediate for energy redistribution and compensation. Spectroscopic data indeed reveals that the distribution of absorbed energy is favoured towards photosystem I in the mutant as evidenced from the absence of a transition from state 2 to state 1. Besides, the absorbed light energy is also efficiently dissipated via non-photochemical quenching as photosystem II is down-regulated, a key process that operates when plants have to deal with extreme stress conditions. Several intermediates of this supracomplex containing only the core complexes of PSI and PSII with or without the PSI antenna (LHCI) have been observed. Furthermore we show that large populations of PSI are bound to LHCI and LHCII, supercomplexes known to be formed under energy state transition. The distribution between these forms varies in abundance, suggesting their oligomerization.