KAI2- and MAX2-mediated responses to karrikins and strigolactones are largely independent of HY5 in Arabidopsis seedlings
Waters, MT and Smith, SM, KAI2- and MAX2-mediated responses to karrikins and strigolactones are largely independent of HY5 in Arabidopsis seedlings, Molecular Plant, 6, (1) pp. 63-75. ISSN 1674-2052 (2013) [Refereed Article]
Karrikins are butenolide compounds released from burning vegetation that stimulate seed germination and
enhance seedling photomorphogenesis. Strigolactones are structurally similar plant hormones that regulate shoot and
root development, and promote the germination of parasitic weed seeds. In Arabidopsis, the F-box protein MAX2 is
required for responses to karrikins and strigolactones, and the α/β hydrolase KAI2 is necessary for responses to karrikins.
Both MAX2 and KAI2 are essential for normal light-dependent seedling development. The bZIP transcription factor HY5
acts downstream of multiple photoreceptors and promotes photomorphogenesis, but its relationship with MAX2 and
KAI2 in terms of seedling development and responses to karrikins and strigolactones is poorly defined. Here, we demonstrate
that HY5 action is genetically separable from that of MAX2 and KAI2. While hy5 mutants have weak hypocotyl
elongation responses to karrikins and the artificial strigolactone GR24, they have normal transcriptional responses,
suggesting that HY5 is not involved in perception or action of karrikins or strigolactones. Furthermore, we show that
overexpression of KAI2 is sufficient to enhance responses to both karrikins and GR24 in wild-type seedlings, and that
KAI2 overexpression partially suppresses the hy5 long hypocotyl phenotype. These results suggest that KAI2 and MAX2
define a regulatory pathway that largely operates independently of HY5 to mediate seedling responses to abiotic signals
such as smoke and light.