PEA MUTANTS WITH REDUCED SENSITIVITY TO FAR-RED LIGHT DEFINE AN IMPORTANT ROLE FOR PHYTOCHROME-A IN DAY-LENGTH DETECTION

In garden pea (Pisum sativum L.), a long-day plant, long photoperiods promote flowering by reducing the synthesis or transport of a graft-tr ansmissible inhibitor of flowering. Previous physiological studies hav e indicated that this promotive effect is predominantly achieved throu gh a response that requires long exposures to light and for which far- red (FR) light is the most effective. These characteristics implicate the action of phytochrome A (phyA). To investigate this matter further , we screened ethylmethane sulfonate-mutagenized pea seedlings for FR- unresponsive, potentially phyA-deficient mutants. Two allelic, recessi ve mutants were isolated and were designated fun1 for FR unresponsive. The fun1-1 mutant is specifically deficient in the PHYA apoprotein an d has a seedling phenotype indistinguishable from wild type when grown under white light. However, fun1-1 plants grown to maturity under lon g photoperiods show a highly pleiotropic phenotype, with short interno des, thickened stems, delayed flowering and senescence, longer peduncl es, and higher seed yield. This phenotype results in large part from a n inability of fun1-1 to detect day extensions. These results establis h a crucial role for phyA in the control of flowering in pea, and show that phyA mediates responses to both red and FR light. Furthermore, g rafting and epistasis studies with fun1 and dne, a mutant deficient in the floral inhibitor, show that the roles of phyA in seedling deetiol ation and in day-length detection are genetically separable and that t he phyA-mediated promotion of flowering results from a reduction in th e synthesis or transport of the floral inhibitor.