Involvement of SA in Stress Induced Flowering

When plants are stressed, they generate stress substances that regulate gene expression to adapt to the stress conditions. The stress substances include reactive oxygen species, nitric acid, jas-monic acid, SA, ethylene, and abscisic acid (Xiong et al. 2002; Moreau et al. 2010; Liu and Zhang 2004; Hey et al. 2010; Jaspers and

Fig. 17.3 Effects of L-2-aminooxy-3-phenylpropionic acid (AOPP) on flowering in Perilla frutescens grown under nonstress light conditions and low-intensity light stress conditions. Red-leaved P frutescens was grown under long-day conditions with a normal light intensity of 120 mmol m-2 s-1 (left) or a low-intensity light of 30 mmol m-2 s-1 (right), and were treated with AOPP for

4 weeks. The treated plants were moved to normal light conditions and grown for an additional 3 (120 mmol m-2 s-1) or 5 (30 mmol m-2 s-1) weeks to score the flowering response; % flowering (open column) and total number of flowers and inflorescences/plant (closed column). n.f., no flowering occurred. Data adapted from Wada et al. (2010b)

Fig. 17.3 Effects of L-2-aminooxy-3-phenylpropionic acid (AOPP) on flowering in Perilla frutescens grown under nonstress light conditions and low-intensity light stress conditions. Red-leaved P frutescens was grown under long-day conditions with a normal light intensity of 120 mmol m-2 s-1 (left) or a low-intensity light of 30 mmol m-2 s-1 (right), and were treated with AOPP for

4 weeks. The treated plants were moved to normal light conditions and grown for an additional 3 (120 mmol m-2 s-1) or 5 (30 mmol m-2 s-1) weeks to score the flowering response; % flowering (open column) and total number of flowers and inflorescences/plant (closed column). n.f., no flowering occurred. Data adapted from Wada et al. (2010b)

Kangasjarvi 2010). Among these stress substances, SA and ethylene have been reported to induce flowering. Ethylene induces flowering in the Bromeliaceae, including pineapple. However, this is an exceptional case, and ethylene generally inhibits flowering in many plant species. The most likely stress substance involved in stress-induced flowering may be SA.

UV-C light stress promotes flowering in wildtype A. thaliana, but not in SA-deficient nahG transgenic plants (Martinez et al. 2004). UV-C irradiation increased the expression of the SA-responsive PR1 gene in Col, but not in nahG plants. The transcript of the SA induction deficient 2/isochorismate synthase 1 (SID2/ICS1) gene encoding the SA biosynthetic enzyme increased under UV-C irradiation in Col, but not in nahG plants. These results suggest the involvement of SA in the UV-C stress-induced flowering of A. thaliana. Exogenous application of SA at 100 mM accelerated flowering of Col, but the nahG plants were not responsive to the SA treatment (Fig. 17.4). SA also regulates flowering time in nonstressed plants. SA-deficient nahG is late flowering (Martinez et al. 2004). The siz1 mutant that has elevated SA level is early flowering under short days, and this phenotype is suppressed by expression of nahG (Jin et al. 2008).

Fig. 17.4 Effect of the exogenous application of salicylic acid (SA) on flowering time in Arabidopsis thaliana. Wild-type Col and SA-deficient nahG transgenic plants were treated daily with 100 mM SA solution (closed column) or not (open column). Total leaf number (rosette plus cauline leaves) was scored when the plants bolted. Data adapted from Martinez et al. (2004)

Fig. 17.4 Effect of the exogenous application of salicylic acid (SA) on flowering time in Arabidopsis thaliana. Wild-type Col and SA-deficient nahG transgenic plants were treated daily with 100 mM SA solution (closed column) or not (open column). Total leaf number (rosette plus cauline leaves) was scored when the plants bolted. Data adapted from Martinez et al. (2004)

When L. paucicostata 6746 was induced to flower by poor-nutrition stress, a larger amount of SA was detected in the flowered plants than in the control plants (Shimakawa 2011). This result suggests the involvement of SA in the stress-induced flowering of L. paucicostata. It is well-known that exogenously applied SA induces flowering in L. paucicostata, L. gibba, and the other-Lemanceous plants (Cleland and Ajami 1974; Cleland and Tanaka 1979; Cleland et al. 1982). However, SA is not considered to be an endogenous flower-regulating factor in Lemna because the endogenous benzoic acid (SA precursor) level is not altered by photoperiodic conditions (Fujioka et al. 1983) ; SA may be the endogenous flower-regulating factor in stress-induced flowering, but not in the photoperiodic flowering of Lemna.

The treatment of P. nil with SA and benzoic acid, a precursor of SA, or some benzoic acid derivatives prior to low-temperature treatment enhances the flower-inducing effect of low temperature (Shinozaki 1985; Shinozaki et al. 1982, Shinozaki and Takimoto 1983). In addition to these effects of exogenous application, the flower-inhibiting effects of PAL inhibitors, which may have decreased the endogenous SA level in P. nil and P frutescens, provided new evidence to suggest that SA acts as an endogenous regulator of stress-induced flowering (Wada et al. 2010a, b). The flowering response of cultured plumules excised from short-day-treated P. nil seedlings was enhanced by benzoic acid (Ishioka et al. 1990). Amagasa et al. (1992) reported that AOA inhibited the photoperi odic flowering of P. nil. These observations suggest that SA is also involved in photoperiodic flowering. However, SA did not induce flowering at any concentrations in P. nil and P. frutescens under nonstress conditions (Wada et al. 2010a, b). SA did not enhance the flowering response under the weak stress conditions. SA may be necessary, but is not sufficient for the induction of flowering. Stress conditions may induce not only SA biosynthesis but also other essential factors to induce flowering.

9 The Genes Involved in Stress-Induced Flowering

Expression of the CO, FT, and SOC1 genes that promote flowering was analyzed in A. thaliana under UV-C stress conditions (Martinez et al. 2004) . UV-C induced expression of FT, moderately induced expression of CO, and did not induce SOC1 expression in wild type (Fig. 17.5). Exogenous SA treatment reduced expression levels of the flower-inhibiting gene FLC. Thus, flowering promoted by UV-C requires the enhanced expression of FT and the reduced expression of FLC. SA application induced expression of the sunflower FT homolog, HAFT, in sunflower (Dezar et al. 2010). The flowering of A. thaliana

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