Salicylic Acid-Mediated Stress-Induced Flowering - Salicylic Acid: Plant Growth and Development - page 167

cotyledons given a single SD treatment, but neither gene was expressed under LD
conditions (Wada et al. 2010a ; Yamada 2011 ). The expression of PnFT2 was
induced in the cotyledons and true leaves of plants grown under the poor-nutrition
conditions for 2 weeks or longer. The level of mRNA expression was closely
correlated with the flowering response. However, PnFT1 was not expressed in the
cotyledons or true leaves regardless of nutritional conditions. These results suggest
that PnFT2, but not PnFT1, is involved in the poor-nutrition stress-induced
flowering of P. nil. PnFT2 is involved in both photoperiodic flowering and stress-
induced flowering, whereas PnFT1 is only involved in photoperiodic flowering.
The two PnFT genes might play different roles in the regulation of flowering
depending on the inductive cue. It is also possible that the essential gene for
flowering is PnFT2 and that PnFT1 expression is induced only under SD treatment
and redundantly enhances the activity of PnFT2.
5.2 Involvement of the Metabolic Pathway Regulated by PAL
in Stress-Induced Flowering
The PAL activity increases when P. nil is induced to flower under poor-nutrition or
low-temperature conditions. Additionally, we observed that the roots of the
stressed plants produced a red color when P. nil was induced to flower in response
to poor-nutrition stress. Accordingly, we determined the anthocyanin content of
roots, stem, cotyledons and true leaves that were harvested from Violet grown in
nutrient solution or tap water for 20 days. The anthocyanin content was increased
in the roots and stem of Violet grown in tap water, although there was no sig-
nificant difference in the anthocyanin content in the cotyledons, and the antho-
cyanin concentration was reduced under stress conditions in the true leaves. PAL
regulates the biosynthesis of anthocyanin. The activity of PAL increases when
plants are stressed (Borsani et al. 2001 ; Dixon and Paiva 1995 ; Larkindale et al.
2005 ; Mateo et al. 2006 ; Scott et al. 2004 ). The PAL inhibitor AOA inhibited low-
temperature or poor-nutrition-induced flowering in Violet; therefore, it is
hypothesized that some compound(s) in the metabolic pathway regulated by PAL
act as flowering stimuli (Hatayama and Takeno 2003 ; Hirai et al. 1995 ). PAL
catalyzes the conversion of phenylalanine to t-cinnamic acid, and several meta-
bolic intermediates are derived from t-cinnamic acid. Dihydrokaempferol-7-O-b-
D-glucoside, which is derived from the pathway from t-cinnamic acid to antho-
cyanin via p-coumaric acid, has been reported to promote the flowering of P. nil
(Nakanishi et al. 1995 ). Therefore, metabolic intermediates were applied simul-
taneously with AOA to the plants grown under low temperature stress conditions.
Among them, t-cinnamic acid and benzoic acid negated the inhibitory effect of
AOA in low-temperature-induced flowering, but p-coumaric acid and caffeic acid
did not (Hatayama and Takeno 2003 ). The AOA-mediated inhibition of flowering
under poor-nutrition conditions was rescued when SA was applied simultaneously