Phytochrome E Controls Light-Induced Germination of Arabidopsis

Abstract

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...

Tissue- and isoform-specific phytochrome regulation of light-dependent anthocyanin accumulation in Arabidopsis thaliana

“…52 Thus, fine tuning of distinct aspects of photomorphogenesis by opposing activities of individual phy isoforms is not limited to our observation of the impact of phy family members on anthocyanin accumulation under Rc.…”

“…to negatively impact phyA-mediated seed germination in FR; 51,52 however, phyE promotes phyA-mediated germination under FR. 52 Thus, fine tuning of distinct aspects of photomorphogenesis by opposing activities of individual phy isoforms is not limited to our observation of the impact of phy family members on anthocyanin accumulation under Rc.…”

Evolutionary Studies Illuminate the Structural-Functional Model of Plant Phytochromes

“…With respect to its functions as an FR or dim light sensor, there apparently is little functional redundancy among phyA and the other Arabidopsis phytochromes (Figure 1) save that phyE contributes to germination under continuous FR (Hennig et al, 2002). phyB is the predominant form in light-grown tissue, is stable in light, and mediates LFRs to continuous R and pulsed R Robson et al, 1993;Halliday et al, 1994).…”

“…However, a recent study showing that the phyD null but not phyAphyB lost the ability to adjust the chlorophyll a/b ratio in response to canopy shading (Boonman et al, 2009) suggests that in shade avoidance, phyD but not phyB controls this response. phyE also appears to be a minor player in shade avoidance (Devlin et al, 1998) and has a very limited role in FRinduced responses (Hennig et al, 2002). Together, phyD and phyE may be important for fine-tuning the magnitude of responses to the R:FR ratio in complex light environments (Ballaré , 2009).…”

Molecular characterisation of flowering genes in Pongamia Pinnata

“…promoting association with other protein, displaying light-dependent catalytic activity and DNA binding (Moglich et al, 2010). Besides flowering, photoreceptors are also involved in the regulation of various plants response including shade avoidance, seed germination, phototropism and leaf formation (Shinomura et al, 1996;Schmitt, 1997;Kinoshita et al, 2001;Sakai et al, 2001;Briggs and Christie, 2002;Hennig et al, 2002;Sakamoto and Briggs, 2002). (Smith, 2000).…”