Salicylic Acid-Mediated Stress-Induced Flowering - Salicylic Acid: Plant Growth and Development - page 162

1 Introduction
After a period of vegetative growth, plants switch to reproductive growth to produce
the next generation. The switch from vegetative to reproductive growth is called
flowering. Flowering is regulated through both endogenous and environmental
factors. One endogenous factor is the autonomous pathway for the regulation of
flowering, which has been well characterized in Arabidopsis thaliana (Simpson
2004 ). Day-neutral plants switch from vegetative to reproductive growth in response
to endogenous signals after a designated period of time (McDaniel 1996 ). Envi-
ronmental factors that regulate flowering include the duration of the day and night
periods in photoperiodic flowering and low temperature in vernalization (Thomas
and Vince-Prue 1997 ). The flowering stimulus, which is produced in leaves under
adequate photoperiodic conditions, is transferred to the shoot apical meristem, and
induces the transition from the vegetative apical meristem to floral meristem, is
called florigen. The isolation and identification of florigen has been long considered
as an important research subject in plant physiology. Recently, the proteins derived
from the floral pathway integrator gene of A. thaliana, FLOWERING LOCUS T (FT),
and its homologs have been characterized as florigens in several species (Corbesier
et al. 2007 ; Lin et al. 2007 ; Tamaki et al. 2007 ). Epigenetic mechanisms, such as
DNA demethylation, are involved in the regulation of vernalization, i.e., flowering
induced through exposure to low temperatures ranging from 0 to 5 C (Michaels and
Amasino 2000 , 2001 ). Occasionally, flowering that cannot be classified as photo-
periodic flowering or vernalization has also been reported. For some plants, flow-
ering is induced under unsuitable growth conditions. Stress has been implicated in
many cases of non-photoperiodic flowering. This flowering is called stress-induced
flowering. It is becoming apparent that SA is involved in the regulation of the stress-
induced flowering.
2 Stress-Induced Flowering
The short-day (SD) plant, Pharbitis nil (synonym Ipomoea nil), can be induced to
flower under long-day (LD) conditions when grown in tap water (poor-nutrition
conditions), at 12-15 C (low-temperature conditions) or under 15,000-20,000 lux
light (high-intensity light conditions) (Hirai et al. 1993 , 1994 ; Shinozaki 1985 ;
Shinozaki and Takimoto 1982 ; Shinozaki et al. 1982 , 1988a , b , 1994 ; Swe et al.
1985 ). Phenylalanine ammonia-lyase (PAL) activity increases when flowering is
induced under these growth conditions (Hirai et al. 1995 ). Although the factors
that can induce such LD flowering are not related, PAL is involved in flowering
induced under any of these conditions. This observation suggests that these factors
might stimulate flowering through a common signal transduction pathway. Poor
nutrition, low temperature and high-intensity light can be regarded as stress fac-
tors, and PAL activity increases under stress conditions (Dixon and Paiva 1995 ).