75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

405 Nuclear-localized Phytochrome B Can Replace Phytochrome A Functions During
Arabidopsis Seedling Photomorphogenesis
Candace Moore 1 , Tomonao Matsushita 2, 3 , Nathan Miller 1 , Akira Nagatani 2 , Edgar Spalding 1
1
UW-Madison, 2 Kyoto University, 3 UCLA
Phytochromes are red/far-red plant photorecepters that mediate aspects of seedling photomorphogenesis including
hypocotyl growth inhibition and apical hook opening. Phytochrome A has been shown to mediate growth inhibition
during the first three hours of red light, then a coordinated transition to a phytochrome B-dependent mechanism occurs.
Our present research asks how nuclear localization of the PHYB protein affects the timing of the different phases of
hypocotyl growth inhibition and hook opening. Using a novel high-resolution morphometric technique, the responses
to red light of phyAphyB double mutant seedlings expressing either PHYB-GFP or PHYB-GFP containing a nuclear
localization signal were investigated. We found that PHYA mediated the first 12 hours of growth inhibition induced
by 10 μmol m -2 s -1 red light. Targeting PHYB to the nucleus compensated for PHYA action at this fluence rate and at
the higher fluence rate of 50 μmol m -2 s -1 . Hook opening was also found to be PHYA-mediated at 10 μmol m -2 s -1 , and
nuclear-localized PHYB also replaced this PHYA function. At the higher fluence rate of 50 μmol m -2 s -1 , hook opening
was mediated largely by PHYB. Targeting PHYB to the nucleus increased the rate of hook opening in this condition.
These data indicate that photomorphogenic responses at low fluence rates of red light are primarily mediated by PHYA
while those at high fluence rates are mediated by both PHYA and PHYB. Nuclear targeting of PHYB can compensate
for the lack of PHYA and speed up PHYB-dependent processes.
406 Genetic Interactions Between Brassinosteroid-Inactivating Enzymes and
Photomorphogenic Photoreceptors
Katy Tiefenbrun, Leeann Thornton, Michael Neff
Washington University, St. Louis, MO
Activation tagging, a gene-overexpression mutagenesis tool, has been used to identify extragenic suppressors of the longhypocotyl
phenotype conferred by the photoreceptor mis-sense mutation phyB-4. Seven of these sob-D mutants (suppressor of
phyB- dominant) have been identified and cloned in the Neff lab to date, some of which implicate cross talk between various
hormone signaling pathways and photomorphogenic development. BAS1 and SOB7 encode a pair of cytochrome P450
enzymes that inactivate the growth-promoting brassinosteroid hormones. We generated single and double null-mutants of
BAS1 and SOB7 to test the hypothesis that these two genes modulate photomorphogenesis. BAS1 and SOB7 act redundantly
or synergistically with respect to light-mediated hypocotyl elongation inhibition and flowering time, photomorphogenic
processes regulated by the photoreceptors phytochrome A (phyA), phytochrome B (phyB) and cryptochrome 1 (cry1). To test
the hypothesis that P450-mediated brassinosteroid inactivation interacts with one or more of these photoreceptor-signaling
pathways, we generated double-, triple- and quadruple-null mutant combinations between the null alleles bas1-2, sob7-1,
phyA-211, phyB-9 and cry1-103. BAS1 and SOB7 act independently from phyB and cry1 to modulate hypocotyl growth in
response to white, red and blue light. However, in far-red light the hypocotyl growth phenotype conferred by the loss of BAS1
and SOB7 requires phyA, demonstrating a role for this photoreceptor in modulating brassinosteroid inactivation. With respect
to flowering, the bas1-2 sob7-1 double mutant and the phyB-9 single mutant flowers four leaves earlier than the wild type in
long-day growth conditions (8 hrs darkness/16 hrs light). However these phenotypes are not additive since the phyB-9 bas1-2
sob7-1 triple mutant confers the same flowering phenotype as phyB-9 and the bas1-2 sob7-1 double mutant. The phyA-211
mutant flowers nine leaves later than the wild type in long-day growth conditions whereas the phyA-211 bas1-2 sob7-1 triple
mutant flowers the same as the wild type. Surprisingly, the phyA-211 sob7-1 double mutant flowers four leaves later than
phyA-211 even though the loss of SOB7 confers an early flowering phenotype in a bas1-2 null background. In contrast, the
phyA-211 bas1-2 mutant flowers ten leaves earlier than phyA-211. In all cases, the flowering time data are supported by the
number of days to bolting after germination. Together, these data demonstrate a complex interaction between phytochrome
photoreceptors and P450-mediated brassinosteroid inactivation with regard to flowering time.