8 24 0 8 24 0 8 2A Hours after the irradiation
Fig. 17.5 Effect of UV-C irradiation on the expression of flowering genes in Arabidopsis thaliana wild-type Col (circles) and salicylic acid-deficient nahG transgenic plants (triangles) were irradiated with 200 mJ cm-2 UV-C
light (closed symbols) or not (open symbols), and the expression of each gene at different hours after the irradiation (as indicated in the abscissa) was quantified by RT-PCR. Data adapted from Martinez et al. (2004)
is induced by long-day conditions, vernalization, autonomous cues, and gibberellins, and these factors operate through a common pathway integrated by FT (Boss et al. 2004). It was shown that FT is also involved in stress-induced flowering.
Genome-wide analyses of transcriptomes detected the downregulation of Pathogen and Circadian Controlled 1 (PCC1) in SA-deficient plants of A. thaliana (Segarra et al. 2010). PCC1 was initially characterized as a circadian clock-regulated gene that is rapidly upregulated after pathogen inoculation. The expression of PCC1 was strongly activated by UV-C light irradiation in Col, but not in nahG plants. SA application also activated PCC1 expression. The activation of PCC1 expression required CO . RNAi transgenic plants contained lower levels of FT transcript. The overexpression of PCC1 did not accelerate flowering, but suppression of its expression by RNAi delayed flowering. UV-C light irradiation of plants accelerates flowering through an SA-dependent process in wild-type but not in RNAi transgenic plants with reduced expression of PCC1, suggesting that neither SA nor PCC1 alone is sufficient to accelerate flowering in A. thaliana.
The flowering of A. thaliana is induced by four previously known factors and stress, and these factors function through the activation of FT expression. This suggests that the FThomolog could be involved in stress-induced flowering in other plants. Two orthologs of FT, PnFT1 and PnFT2. have been identified in P. nil. and these genes are expressed under inductive short-day conditions to promote flowering (Hayama et al. 2007) . Therefore, the expression of PnFT genes in response to poor-nutrition stress conditions was examined. P. nil Violet was induced to flower by growth in tap water, the cotyledons and true leaves of these plants were collected, and the expression of PnFT1 and PnFT2 was examined by RT-PCR (Wada et al. 2010a; Yamada 2011). The expression of PnFT1 and PnFT2 was induced in cotyledons by a single short-day treatment, but neither gene was expressed without the short-day treatment. The expression of PnFT2 was induced in the cotyledons and true leaves of plants grown under the poor-nutrition conditions for 2 weeks or longer. The level of mRNA expression was closely correlated with the flowering response. Only weak PnFT2 expression was detected in the true leaves of plants grown under nonstress conditions for 3 weeks. On the other hand, PnFT1 was not expressed in the cotyledons or true leaves regardless of nutritional conditions. These results suggest that PnFT2. but not PnFT1. is involved in the poor-nutrition stress-induced flowering of P. nil. PnFT2 is involved in both photoperiodic flowering and stress-induced flowering, whereas PnFT1 is involved only in photoperiodic flowering. The two PnFT genes might have different roles in the regulation of flowering depending on the inductive cue. It is also possible that the essential gene for flowering is PnFT2 and that PnFT1 expression is induced only by short-day treatment and redundantly enhances the activity of PnFT2 . SA might induce the expression of PnFT2 or the product of PnFT2 might induce the expression of genes involved in the biosynthesis of, response to, or signal transduction of SA.
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