Salicylic Acid-Mediated Stress-Induced Flowering - Salicylic Acid: Plant Growth and Development - page 170

induces the flowering of Spirodela polyrrhiza (Krajnˇiˇ and Nemec 1995 ) and
L. minor (Krajnˇiˇ et al. 2006 ); therefore, the influence of JA should be examined
in P. nil.
It is also conceivable that stress enhances the sensitivity to SA. In the induction
of systemic-acquired resistance in potato, injury stress did not change the
endogenous SA level but increased the sensitivity to SA (Yu et al. 1997 ). SA is not
essential for seed germination in A. thaliana, but increased activity under salt
stress conditions was observed (Lee et al. 2010 ). If SA acts only under stress
conditions or SA sensitivity is enhanced by stress in P. nil, it explains the reason
why exogenously applied SA did not induce flowering under non-stress conditions.
The interaction between SA and gene expression should also be considered.
The expression of orthologous gene of the floral pathway integrator gene FT,
PnFT2, was induced in plants that flower under poor-nutrition conditions (Wada
et al. 2010b ). SA application induced the expression of A. thaliana FT and sun-
flower HAFT, which is an ortholog of FT (Dezar et al. 2011 ; Martínez et al. 2004 ),
indicating that FT and SA might regulate flowering. In P. nil, stress could induce
both SA biosynthesis and PnFT2 expression, and SA and the PnFT2 protein might
act together to induce flowering. Moreover, SA might induce the expression of
PnFT2, or the product of PnFT2 could induce the expression of genes involved in
the biosynthesis, response or signaling of SA.
6 Salicylic Acid-Mediated Stress-Induced Flowering
in Perilla frutescens
6.1 Stress-Induced Flowering in P. frutescens
Two forms of P. frutescens var. crispa, an obligatory SD plant, were grown under
LD conditions under different light intensities. All of the red-leaved plants flow-
ered under an intensity of 30 lmol m -2 s -1 , whereas the plants grown under 60 or
120 lmol m -2 s -1 did not (Wada et al. 2010a ). The green-leaved form was also
induced to flower under 30 lmol m -2 s -1 , although the flowering response was
lower than that of the red-leaved form. The stem length was shorter under lower
light intensity than under normal light intensity in both forms. The reduction of
vegetative growth results from stress (Hatayama and Takeno 2003 ), and therefore
the flowering of P. frutescens plants under low-intensity light might be a stress-
induced response. The response to low-intensity light appeared to be a shade
avoidance response. An important component of the shade avoidance syndrome is
an acceleration of flowering, but the most typical phenotype of the shade avoid-
ance response is rapid stem elongation (Adams et al. 2009 ). Because the stem
length of P. frutescens plants was shortened under low-intensity light, the response
of P. frutescens plants to low-intensity light might not be a shade avoidance
response. Photosynthetic activity could be reduced under low-intensity light