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

MAZZELLA MA, ALCONADA MAGLIANO MT, CASAL JJ

Plant Cell and environment

Wiley online library

Año: 1997 vol. 20 p. 261 - 267

1365-3040

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.