Tbe role of pbytocbrome A in tbe control of bypocotyl growtb under continuous red ligbt (Re) was investigated using phyA and phyB mutants of Arabidopsis thaliana, wbicb lack pbytochrome A (pbyA) or pbytocbrome B (pbyB), respeetively, and transgenie seedlings of Nicotiana tabacum overexpressing Avena pbyA, compared to tbe corresponding wild type (WT). In WT seedlings of A. thaliana, hypocotyl growtb inhibition showed a biphasic response to the fluence rate of Re, with a brake at 10 /imol m s . At equal total fluence rate, hourly pulses of red light caused slightly more inhibition than Re. The response to very low fluences of eontinuous or pulsed red light was absent in the phyA and phyA phyB mutants and present in the phyB mutant. The second part of the response was steeper in the phyA mutant than in the WT but was absent in the phyB mutant. In WT tobacco the response to Re was biphasic. Overexpression of Avena phyA enhanced the response only at very low fluence rates of Re (< 10"^ ;UmoI m~^ s"'). In both species, the effect of hourly pulses of far-red light was similar to the maximum inhibition observed in the first phase of the response to Re. Using reciprocity failure (i.e. higher inhibition under continuous than pulsed light) as the operational criterion, a 'true' high-irradianee reaction occurred under continuous far-red light but not under Re or red plus far-red light mixtures. Native and overexpressed phyA are proposed to mediate very low fluence responses under Re. In WT A. thaliana, this effeet is eounteraeted by a negative action of phyA on phyB-mediated low-fluenee responses.

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We used 52 Arabidopsis (Arabidopsis thaliana) accessions and developed a new set of 137 recombinant inbred lines between Landsberg erecta (Ler) and to explore the genetic basis of phytochrome-mediated responses during deetiolation. Unexpectedly, most accessions showed weak or moderate hypocotyl growth and cotyledon unfolding responses to pulses of far-red light (FR). Crosses between Columbia and No-0, two accessions with poor response, segregated seedlings with unfolded cotyledons under pulsed FR, suggesting the occurrence of accession-specific loci in the repression of morphological responses to weak light signals. Confirming the latter expectation, mapping of responses to pulsed FR in the Ler 3 No-0 lines identified novel loci. Despite its weak response to pulsed FR, No-0 showed a response to continuous FR stronger than that observed in Ler. By mapping the differential effect of pulsed versus continuous FR, we identified two high-irradiance response loci that account for the steeper response to continuous FR in No-0. This underscores the potential of the methodology to identify loci involved in the regulation of the shape of signal input-output relationships. Loci specific for a given phytochromemediated response were more frequent than pleiotropic loci. Segregation of these specific loci is predicted to yield different combinations of seedling responsivity to light. Such flexibility in combination of responses is observed among accessions and could aid in the adjustment to different microenvironments.Some fluctuations of the light environment tightly correlate with the occurrence of conditions that impose a challenge to plant survival such as seasonal changes that result in extreme temperatures, organ emergence out of the soil, or competition with neighbor individuals. Subtle light signals, including small changes in photoperiod throughout the year, the transition between darkness and very low fluences of light reaching the top millimeters of the soil, and small reductions in the red light (R) to far-red light (FR) ratio caused by light reflected by neighbors, actually anticipate stressful conditions. Plants are able to perceive these signals, which are translated into regulation of developmental plasticity (Casal et al., 2004). Not surprisingly, fitting developmental decisions to these signals requires an intricate network of molecular players.The use of Arabidopsis (Arabidopsis thaliana) mutants has been the primary approach in the search for players in light signaling. Mutant screens have led to the discovery of the photoreceptors phytochrome A (phyA; Whitelam et al., 1993), phytochrome B (phyB; Koornneef et al., 1980;Reed et al., 1993), cryptochrome 1 (Koornneef et al., 1980;Ahmad and Cashmore, 1993), and cryptochrome 2 (cry2; Koornneef et al., 1980;Guo et al., 1998). However, a complementary approach to mutant analysis has been the exploration of natural allelic variation. The use of recombinant inbred lines (RILs) between the accessions Landsberg erecta (Ler) and Cape Verde Islands (Cvi) helped to iden...