Salicylic Acid-Mediated Stress-Induced Flowering - Salicylic Acid: Plant Growth and Development - page 168

in a dose-dependent manner, and this effect was completely eliminated with SA at
10 -4 M (Wada et al. 2010b ). These results suggest that SA is involved in stress-
induced flowering, but phenylpropanoids and dihydrokaempferol-7-O-b-D-glu-
coside are not. AOA also inhibits 1-aminocyclopropane-1-carboxylic acid (ACC)
synthase. ACC synthase catalyzes the conversion of S-adenosylmethionine (SAM)
to ACC, which is subsequently converted to ethylene. However, the involvement
of ethylene in the stress-induced flowering is excluded because flowering was
completely inhibited in the presence of ACC (Hatayama and Takeno 2003 ). This is
consistent with the observation that ethylene inhibits the photoperiodic flowering
of P. nil (Suge 1972 ). SAM is also a precursor of polyamines; the excess SAM
may have been metabolized to polyamines. Polyamines, especially putrescine,
have been reported to induce flowering of P. nil under poor nutritional conditions
(Wada et al. 1994 ). Recently, AOA and L-2-aminooxy-3-phenylpropionic acid
(AOPP), which are known inhibitors of PAL (Appert et al. 2003 ), were reported to
inhibit the biosynthesis of IAA (Soeno et al. 2010 ). However, IAA is known to
inhibit flowering of P. nil (Takeno 1996 ).
Stress increases PAL activity and induces SA biosynthesis (Rasmussen et al.
1991 ; Wen et al. 2005 ). Stress also induces reactive oxygen species (Leon et al.
1995 ; Okuda et al. 1991 ), and reactive oxygen species promote the conversion of
benzoic acid to SA (Gidrol et al. 1996 ; León et al. 1995 ; Mauch-Mani and Slu-
sarenko 1996 ; Neuenschwander et al. 1995 ; Summermatter et al. 1995 ). The
treatment of P. nil with SA, its precursor benzoic acid and some benzoic acid
derivatives prior to low-temperature treatment enhances the flower-inducing effect
of low temperature (Shinozaki 1985 ; Shinozaki et al. 1982 , 1985 ). In addition to
these effects, the flower-inhibiting effects of PAL inhibitors, which might have
decreased the endogenous SA level in P. nil, provided new evidence to suggest
that SA acts as an endogenous regulator of stress-induced flowering (Wada et al.
2010b ). The flowering response of cultured plumules excised from SD treated P.
nil seedlings was enhanced with benzoic acid (Ishioka et al. 1990 ). Amagasa et al.
( 1992 ) reported that AOA inhibited the photoperiodic flowering of P. nil. These
observations suggest that SA is also involved in photoperiodic flowering.
5.3 PAL Gene Expression, PAL Activity and SA Content
Based on these results, we hypothesize that stress-induced flowering is regulated
through SA, which is synthesized in the pathway mediated by PAL. However,
there is no evidence to indicate that endogenous SA levels increase when P. nil
plants are induced to flower through the application of stress factors. Accordingly,
the gene expression and enzyme activity of PAL were determined, and the increase
in the endogenous SA content in P. nil during flowering under poor-nutrition stress
was calculated (Wada 2012 ). P. nil plants were induced to flower in 1/100-strength
nutrient solution under LD conditions for 10-20 days. The cotyledons were har-
vested at the end of the stress treatment to analyze PAL expression. The PAL