Salicylic Acid-Mediated Stress-Induced Flowering - Salicylic Acid: Plant Growth and Development - page 172

in P. frutescens. To verify which hypothesis is correct, we examined whether the
PAL inhibitor could induce flowering under normal-intensity light conditions in P.
frutescens plants and whether the PAL inhibitor could suppress low-intensity light-
induced flowering. AOPP, a PAL inhibitor, did not induce flowering when applied
under non-inductive normal-intensity light conditions and inhibited flowering
when applied under inductive low-intensity light conditions (Wada et al. 2010a ).
Treatment with another PAL inhibitor, AOA, showed the same results. Flowering
induced through low-intensity light was inhibited with AOPP, and the inhibition
was partially rescued with SA. These results suggest that stress increased PAL
activity in P. frutescens and P. nil, suggesting that the same mechanism is involved
in the flowering induced through low-intensity light in P. frutescens and poor-
nutrition stress in P. nil.
The preliminary analysis using RT-PCR suggested that the expression of the
PAL gene was suppressed under low-intensity light conditions in red-leaved P.
frutescens plants (Wada 2007 ). Thus, we must consider that AOPP and AOA did
not suppress flowering through an inhibition of PAL activity, but rather through
some unknown mechanism. Although AOPP is a specific inhibitor of PAL
(Kessmann et al. 1990 ; Mavandad et al. 1990 ; Ni et al. 1996 ; Orr et al. 1993 ), it
was recently reported that AOPP and AOA also inhibited the synthesis of IAA
(Ishii et al. 2010 ). We therefore consider the possibility that AOPP and AOA
suppresses flowering through the inhibition of IAA synthesis. Further investigation
is required to determine the mechanism that regulates the low light-induced
flowering.
7 Salicylic Acid-Mediated Stress-Induced Flowering
in Lemna paucicostata
7.1 Stress-Induced Flowering in L. paucicostata
Three strains of the SD plant L. paucicostata, 151, 441 and 6746 were induced to
flower in tap water without addition of nutrients under non-inductive LD condi-
tions (Shimakawa et al. 2012 ). The control plants cultured in a nutrient medium
showed almost no flowering response under LD conditions. The flowering
response under the poor-nutrition conditions was the strongest in strain 6746. In
comparison with the flowering response under SD conditions, the poor-nutrition
conditions produced a weaker flowering response in each strain examined. How-
ever, virtually no flowering occurred under nutrient conditions. The number of
fronds per flask cultured in tap water was much smaller than that cultured in the
nutrient medium for all of the tested strains. The vegetative multiplication of
the fronds was restricted when cultured in tap water, which indicates that the
vegetative growth of the plants was suppressed in tap water, implying that the
plants were stressed (Hatayama and Takeno 2003 ). Therefore, poor-nutrition stress