The recently discovered group of
plant hormones, the strigolactones, have been implicated in regulating photomorphogenesis. We examined this extensively in our
strigolactone synthesis and response mutants and could find no evidence to support a major role for
strigolactone signaling in classic seedling photomorphogenesis (e.g. elongation and leaf expansion) in pea (Pisum sativum), consistent with two recent independent reports in Arabidopsis. However, we did find a novel effect of strigolactones on adventitious rooting in darkness.
Strigolactone-deficient mutants, Psccd8 and Psccd7, produced significantly fewer adventitious roots than comparable wild-type seedlings when grown in the dark, but not when grown in the light. This observation in dark-grown plants did not appear to be due to indirect effects of other factors (e.g. humidity) as the constitutively de-etiolated mutant, lip1, also displayed reduced rooting in the dark. This role for strigolactones did not involve the MAX2 F-Box
strigolactone response pathway as Psmax2 f-box mutants did not show a reduction in adventitious rooting in the dark compared with wild-type plants. The
auxin-deficient mutant bushy also reduced adventitious rooting in the dark, as did
decapitation of wild-type plants. Rooting was restored by the application of
indole-3-acetic acid (IAA) to decapitated plants, suggesting a role for
auxin in the rooting response. However,
auxin measurements showed no accumulation of IAA in the epicotyls of wild-type plants compared with the
strigolactone synthesis mutant Psccd8, suggesting that changes in the gross
auxin level in the epicotyl are not mediating this response to
strigolactone deficiency.