Germination of Arabidopsis seeds is light dependent and under phytochrome control. Previously, phytochromes A and B and at least one additional, unspecified phytochrome were shown to be involved in this process. Here, we used a set of photoreceptor mutants to test whether phytochrome D and/or phytochrome E can control germination of Arabidopsis. The results show that only phytochromes B and E, but not phytochrome D, participate directly in red/far-red light (FR)-reversible germination. Unlike phytochromes B and D, phytochrome E did not inhibit phytochrome A-mediated germination. Surprisingly, phytochrome E was required for germination of Arabidopsis seeds in continuous FR. However, inhibition of hypocotyl elongation by FR, induction of cotyledon unfolding, and induction of agravitropic growth were not affected by loss of phytochrome E. Therefore, phytochrome E is not required per se for phytochrome A-mediated very low fluence responses and the high irradiance response. Immunoblotting revealed that the need of phytochrome E for germination in FR was not caused by altered phytochrome A levels. These results uncover a novel role of phytochrome E in plant development and demonstrate the considerable functional diversification of the closely related phytochromes B, D, and E.In many plants, seed germination is light dependent . Among plant photoreceptors, only phytochromes have been shown to directly mediate induction of germination. Recently, several new insights into the molecular mechanisms of phytochrome action and its physiological role throughout the whole life cycle of plants were achieved (for review, see Whitelam and Devlin, 1997;Casal, 2000; Smith, 2000). In Arabidopsis, phytochrome is a small gene family, consisting of the five members, PHYA to PHYE (Sharrock and Quail, 1989;Clack et al., 1994). PHYB, PHYD, and PHYE are evolutionary related and clearly separated from PHYA and PHYC (Mathews and Sharrock., 1997). Studies using mutants and overexpressor lines demonstrated that individual phytochrome family members have overlapping and distinct functions (Reed et al., 1994; Smith et al., 1997). phyA and phyB are the best characterized phytochromes in Arabidopsis. phyA mediates the high irradiance response in far red light (FR; FR-HIR) and the very low fluence response (VLFR), which can also be induced by FR. In contrast, phyB mediates the red light (R)/FR-reversible low fluence response (LFR;. Less is known about the other phytochromes. An LFR-type action of phyD could be observed in phyB seedlings (Aukerman et al., 1997;Hennig et al., 1999a). The high sequence identity of phyB and phyD (Ͼ80%) suggests very similar roles of these two phytochromes, albeit certain physiological differences have been reported (Hennig et al., 1999a). Finally, for phyE, mainly functions in the shade avoidance syndrome of adult plants have been described (Devlin et al., 1996(Devlin et al., , 1998. Furthermore, phyE has been shown to be capable of signaling to the circadian clock in seedlings (Devlin and Kay, 2000).Light-induced germ...