Endogenous Substances Involved in Stress Induced Flowering

CGA and some other phenylpropanoids were found to accumulate in cotyledons during the treatments by poor nutrition, low temperature, or high-intensity light in P. nil (Shinozaki et al. 1988a, b, 1994; Hirai et al. 1993, 1994). Phenylpropanoid synthesis is involved in the stress response (Dixon and Paiva 1995). Stress promotes the metabolism of t-cinnamic acid to SA via ben-zoic acid (Gidrol et al. 1996; Mauch-Mani and Slusarenko 1996). The flowering induced by these conditions is accompanied by an increase in PAL activity (Hirai et al. 1995), and AOA inhibited flowering in P nil (Shinozaki et al. 1988a, 1994; Hatayama and Takeno 2003) . Some compound(s) in the metabolic pathway regulated by PAL might act as flowering stimuli. Phenylpropanoids, such as CGA, were prominent candidates for this in earlier studies (Shinozaki et al. 1988a, b, 1994; Hirai et al. 1993, 1994). However, exoge-nously applied CGA failed to induce flowering (Shinozaki et al. 1988a, 1994; Hatayama and Takeno 2003). No flower-inducing activity was detected in other phenylpropanoids, including 4-0-p-coumaroylquinic acid, 3-0-feruloylquinic acid, dehydrodiconiferylalcohol-13-0- b-D-gluco-side, and (+)-pinoresinol-b-D-glucoside (unpublished data). Therefore, CGA and related phenylpropanoids are not involved in the stress-induced flowering of P. nil. The close positive correlation between CGA content and flowering response was merely coincidence.

In addition to CGA, several compounds, including SA and anthocyanin, are derived from t-cinnamic acid of which conversion from phe-nylalanine is catalyzed by PAL (Dixon and Paiva 1995). Dihydrokaempferol-7-0-D-glucoside derived from the pathway from t -cinnamic acid to anthocyanin via p-coumaric acid has been reported to promote the flowering of P. nil (Nakanishi et al. 1995). Furthermore, AOA inhibits 1-aminocyclopropane-1-carboxylic acid (ACC) synthase. ACC synthase catalyzes the conversion of S-adenosylmethionine to ACC, which is converted to ethylene. Such components or other substances in the metabolic pathways derived from t-cinnamic acid might be involved in stress-induced flowering. Accordingly, the flowering of P. nil was induced by low-temperature or poor-nutrition stress, and AOA treatment was used to inhibit the flowering. Several metabolic intermediates in the pathways were applied together with AOA (Hatayama and Takeno 2003 ; Wada et al. 2010a). Among the intermediates tested, t-cin-namic acid, benzoic acid, and SA were shown to counteract the inhibitory effect of AOA (Fig. 17.2), whereas p-coumaric and caffeic acids did not. These results suggest that SA is involved in the stress-induced flowering of P. nil and that the pathways to CGA and anthocyanin are not involved. Flowering was completely inhibited in the presence of ACC (Hatayama and Takeno 2003). Thus, the ACC route is not involved. This is consistent with the observation that ethylene

Fig. 17.2 Effects of aminooxyacetic acid (AOA) and salicylic acid (SA) on the poor-nutrition stress-induced flowering of Pharbitis nil. The 5-day-old seedlings of P. nil cv. Violet were grown in tap water with or without 3 x 10-5 M AOA and SA at various concentrations under long-day conditions for 20 days, transferred to fresh nutrient solution without AOA and SA, and grown for an additional 2 weeks to score the flowering response; % flowering (open column) and number of floral buds/plant (closed column). Data adapted from Wada et al. (2010a)

Fig. 17.2 Effects of aminooxyacetic acid (AOA) and salicylic acid (SA) on the poor-nutrition stress-induced flowering of Pharbitis nil. The 5-day-old seedlings of P. nil cv. Violet were grown in tap water with or without 3 x 10-5 M AOA and SA at various concentrations under long-day conditions for 20 days, transferred to fresh nutrient solution without AOA and SA, and grown for an additional 2 weeks to score the flowering response; % flowering (open column) and number of floral buds/plant (closed column). Data adapted from Wada et al. (2010a)

derived from ACC inhibits the photoperiodic flowering of P. nil (Suge 1972).

The leaves of red-leaved P. frutescens were deep green when induced to flower under low-intensity light (Wada et al. 2010b). The greening of the leaves was due to a decrease in anthocyanin content. There was a negative correlation between anthocyanin content and percentage flowering. Therefore, the metabolic pathway related to antho-cyanin synthesis may be involved in the regulation of flowering. It is possible that some substances, such as SA, which are synthesized by the common metabolic pathway for anthocyanin synthesis are involved in flowering as mentioned above for P. nil. Low-intensity light may induce the flowering of P. frutescens by influencing the endogenous level of SA through suppression of PAL activity. However, this conflicts with previous reports. Stress generally increases PAL activity and promotes anthocyanin biosynthesis (Christie et al. 1994; Dixon and Paiva 1995; Chalker-Scott 1999). Actually, PAL activity increases in the stress-induced flowering of P. nil as mentioned above. Therefore, it was examined whether the PAL inhibitor could promote or inhibit the low-intensity light stress-induced flowering in P. frutescens (Wada et al. 2010b). The PAL inhibitor AOPP did not induce flowering when applied under noninductive normal-intensity light conditions and inhibited flowering in a dose-dependent manner when applied under inductive low-intensity light stress conditions (Fig. 17.3). The treatment with another PAL inhibitor, AOA, gave the same results. These results suggest that the same mechanism is involved in flowering that is induced by low-intensity light in P. frutescens and the flowering that is induced by several stress factors in P. nil. That PAL inhibitors inhibited stress-induced flowering suggests that the stress increased PAL activity. However, in P. frutescens, the decrease in anthocyanin content under low-intensity light suggests that stress limited the activity of PAL. These contradictory results must be explained in future.

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