Dual effect of phytochrome A on hypocotyl growth under continuous red light

Abstract

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.

Sequential and coordinated action of phytochromes A and B during Arabidopsis stem growth revealed by kinetic analysis

“…The hallmark long-hypocotyl phenotype of phyB, when grown for several days under R C , a situation in which phyA contributes much less to hypocotyl inhibition (24), has fostered the view that R C acts primarily through phyB to inhibit growth (10,25). However, the analysis presented here demonstrates that phyA initiated this response to R C (ignoring for the moment the initial transient inhibition), and prevailed as the primary photoreceptor for the first 3 hr of irradiation.…”

The function of phytochrome A

“…The second phase of the response to R is a LFR, because it is R/FR reversible and shows reciprocity, in contrast to HIR. The VLFR is absent in the phyA and phyA phyB mutants but normal in the phyB mutant (Mazzella et al 1997; Fig. 1).…”

“…At least in Arabidopsis, hypocotyl growth inhibition by FR is a HIR because it shows reciprocity failure (Mazzella et al 1997). Overexpression of oat phyA in transgenic tobacco (McCormac et al 1991), Arabidopsis (Whitelam et al 1992) and rice (Casal, Clough & Vierstra 1996), rice phyA in transgenic tobacco (Emmler et al 1995) and potato phyA in transgenic potato (Heyer et al 1995) enhances the response to continuous FR.…”

Evolutionary Studies Illuminate the Structural-Functional Model of Plant Phytochromes

“…Light-sensitive germination under canopies is also controlled by phytochromes acting in the FR-HIR (Botto et al, 1996). Moreover, under low R:FR ratios, several of the morphological and biochemical changes associated with reduced LFR may antagonize those associated with FR-HIR (McCormac et al, 1992;Reed et al, 1994;Mazzella et al, 1997;Cerdá n et al, 1999;Devlin et al, 2003;Salter et al, 2003). For example, reduction in LFR results in an increase in elongation growth and a decrease in leaf development and chlorophyll production (shade avoidance responses), whereas FR-HIRs result in a decrease in elongation growth and an increase in leaf development and chlorophyll production.…”

“…R-mediated development in the LFR mode is found across embryophytes and in Charales; elements of shade avoidance also are widespread, occurring as deeply in the tree as Charales (Mathews, 2006). The remarkable innovation in the PHYN/A lineage is more suggestive of adaptive change, possibly in response to the evolution of forests and the value of possessing a mechanism to counteract phyB activities under canopies and leaf litter (McCormac et al, 1992;Reed et al, 1994;Yanovsky et al, 1995;Mazzella et al, 1997;Cerdá n et al, 1999;Devlin et al, 2003;Mathews et al, 2003;von Wettberg and Schmitt, 2005).…”