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The diversification of flowering time in response to natural environments is critical for the spread of crops to diverse geographic regions. In contrast with recent advances in understanding the molecular basis of photoperiodic flowering in rice (Oryza sativa), little is known about how flowering-time diversification is structured within rice subspecies. By analyzing genome sequencing data and a set of 429 chromosome segment substitution lines (CSSLs) originating from 10 diverse rice accessions with wide distributions, we revealed diverse effects of allelic variations for common flowering-time quantitative trait loci in the recipient's background. Although functional variations associated with a few loci corresponded to standing variations among subspecies, the identified functional nucleotide polymorphisms occurred recently after rice subgroup differentiation, indicating that the functional diversity of flowering-time gene sequences was not particularly associated with phylogenetic relationship between rice subspecies. Intensive analysis of the Hd1 genomic region identified the signature of an early introgression of the Hd1 with key mutation(s) in aus and temperate japonica accessions. Our data suggested that, after such key introgressions, new mutations were selected and accelerated the flowering-time diversity within subspecies during the expansion of rice cultivation area. This finding may imply that new genome-wide changes for flowering-time adaptation are one of the critical determinants for establishing genomic architecture of local rice subgroups. In-depth analyses of various rice genomes coupling with the genetically confirmed phenotypic changes in a large set of CSSLs enabled us to demonstrate how rice genome dynamics has coordinated with the adaptation of cultivated rice during the expansion of cultivation area., (© 2018 The Authors The Plant Journal © 2018 John Wiley & Sons Ltd.)

The short-day plant, Lemna paucicostata (synonym Lemna aequinoctialis), was induced to flower when cultured in tap water without any additional nutrition under non-inductive long-day conditions. Flowering occurred in all three of the tested strains, and strain 6746 was the most sensitive to the starvation stress conditions. For each strain, the stress-induced flowering response was weaker than that induced by short-day treatment, and the stress-induced flowering of strain 6746 was completely inhibited by aminooxyacetic acid and l-2-aminooxy-3-phenylpropionic acid, which are inhibitors of phenylalanine ammonia-lyase. Significantly higher amounts of endogenous salicylic acid (SA) were detected in the fronds that flowered under the poor-nutrition conditions than in the vegetative fronds cultured under nutrition conditions, and exogenously applied SA promoted the flowering response. The results indicate that endogenous SA plays a role in the regulation of stress-induced flowering., (Copyright © 2012 Elsevier GmbH. All rights reserved.)

The stress-sensitive short-day plant Pharbitis nil var. Kidachi flowers under a 16-h light and 8-h dark regime and non-stress conditions when grown for long periods of time. Such flowering was found to occur from the third week, and the floral buds were formed from the eighth node of the main stem. When young plants were grafted onto aged plants, the scions were induced to flower early. This flower induction by grafting was more effective when older plants were used as rootstocks. Grafting experiments using a single leaf as a donor revealed that younger leaves are more responsive to flower induction, suggesting that this age-mediated flowering response is not induced by aging or senescence of individual leaves. Rather, the plant may obtain the ability to flower as the whole plant ages. Flowering does not occur under continuous light conditions. A night break given in the 8-h dark period inhibits flowering. These results suggest that 8-h dark conditions, which are normally considered to be long-day conditions, actually correspond to short-day conditions for this plant. The 8-h dark conditions caused early flowering more efficiently in older plants. The critical dark length determined by a single treatment was 12 h in 0-week-old plants and was reduced to 6 h in 2- and 4-week-old plants. These results suggest that the critical dark length becomes shorter when plants get older. The expression of PnFT1 and PnFT2, orthologs of the flowering gene flowering locus T, was analyzed by reverse transcription-polymerase chain reaction revealing that the expression of PnFT at the end of dark period is correlated with flowering.

Many plant species can be induced to flower by responding to stress factors. The short-day plants Pharbitis nil and Perilla frutescens var. crispa flower under long days in response to the stress of poor nutrition or low-intensity light. Grafting experiments using two varieties of P. nil revealed that a transmissible flowering stimulus is involved in stress-induced flowering. The P. nil and P. frutescens plants that were induced to flower by stress reached anthesis, fruited and produced seeds. These seeds germinated, and the progeny of the stressed plants developed normally. Phenylalanine ammonia-lyase inhibitors inhibited this stress-induced flowering, and the inhibition was overcome by salicylic acid (SA), suggesting that there is an involvement of SA in stress-induced flowering. PnFT2, a P. nil ortholog of the flowering gene FLOWERING LOCUS T (FT) of Arabidopsis thaliana, was expressed when the P. nil plants were induced to flower under poor-nutrition stress conditions, but expression of PnFT1, another ortholog of FT, was not induced, suggesting that PnFT2 is involved in stress-induced flowering.

The short-day plants Pharbitis nil (synonym Ipomoea nil), var. Violet and Tendan were grown in a diluted nutrient solution or tap water for 20 days under long-day conditions. Violet plants were induced to flower and vegetative growth was inhibited, whereas Tendan plants were not induced to flower, although vegetative growth was inhibited under these conditions. The Violet plants induced to flower by poor-nutrition stress produced fertile seeds and their progeny developed normally. Defoliated Violet scions grafted onto the rootstocks of Violet or Tendan were induced to flower under poor-nutrition stress conditions, but Tendan scions grafted onto the Violet rootstocks were not induced to flower. These results indicate that a transmissible flowering stimulus is involved in the induction of flowering by poor-nutrition stress. The poor-nutrition stress-induced flowering was inhibited by aminooxyacetic acid, a phenylalanine ammonia-lyase inhibitor, and this inhibition was almost completely reversed by salicylic acid (SA). However, exogenously applied SA did not induce flowering under non-stress conditions, suggesting that SA may be necessary but not sufficient to induce flowering. PnFT2, a P. nil ortholog of the flowering gene FLOWERING LOCUS T (FT) of Arabidopsis thaliana, was expressed when the Violet plants were induced to flower by growing in tap water, but expression of PnFT1, another ortholog of FT, was not induced, suggesting the specific involvement of PnFT2 in stress-induced flowering., (Copyright 2009 Elsevier GmbH. All rights reserved.)

An obligatory short-day plant, Perilla frutescens var. crispa was induced to flower under long-day conditions when grown under low-intensity light (30 micromol m(-2) s(-1)). Plant size was smaller under lower light intensity, indicating that the low-intensity light acted as a stress factor. The phenomenon is categorized as stress-induced flowering. Low-intensity light treatment for 4 weeks induced 100% flowering. The plants responded to low-intensity light immediately after the cotyledons expanded, and the flowering response decreased with increasing plant age. The induced plants produced fertile seeds, and the progeny developed normally. The plants that flowered under low-intensity light had greener leaves. This greening was because of the decrease in anthocyanin content, and there was a negative correlation between the anthocyanin content and percent flowering. Treatment with L-2-aminooxy-3-phenylpropionic acid, an inhibitor of phenylalanine ammonia-lyase (PAL), did not induce flowering under non-inductive light conditions and inhibited flowering under inductive low-intensity light conditions. The metabolic pathway regulated by PAL may be involved in the flowering induced by low-intensity light.

The flower-inducing effect of 5-azacytidine, a DNA demethylating reagent, was examined in several plant species with a stable or unstable photoperiodically induced flowering state under non-inductive photoperiodic conditions. The long day plant Silene armeria, whose flowering state is stable and the short day plant Pharbitis nil, whose flowering state is unstable were induced to flower by 5-azacytidine under a non-inductive condition. Thus, the replacement of photoinduction by 5-azacytidine treatment is not specific to Perilla frutescens. On the other hand, 5-azacytidine did not induce flowering in Xanthium strumarium whose flowering state is stable and Lemna paucicostata whose flowering state is unstable. Thus, epigenetics caused by DNA demethylation may be involved in the regulation of photoperiodic flowering irrespective of the stability of the photoperiodically induced flowering state.

The red-leaved form of Perilla frutescens var. crispa was induced to flower by low-intensity light stress. The leaves of this form are normally red, but turned green under low-intensity light due to anthocyanin depletion in the epidermis. Flowering did not occur when plants were grown under light passed through a red-colored cellophane paper, which has an absorption spectrum similar to that of anthocyanins. High-concentration anthocyanins may play the role of a red-colored optical filter under normal light conditions, and this filter effect may be lost under low-intensity light, causing a change in the wavelength characteristics of the light with which the mesophyll cells are irradiated. This change may induce a photobiological effect leading to flowering. The gene expression and enzyme activity of phenylalanine ammonia-lyase (PAL), the key enzyme for anthocyanin biosynthesis, decreased under low-intensity light. L-2-aminooxy-3-phenylpropionic acid (AOPP), which is widely used as a PAL inhibitor, inhibited low-intensity light stress-induced flowering and increased PAL activity and anthocyanin content. The inhibition of flowering by AOPP in P. frutescens may be through different mechanisms than PAL inhibition. [ABSTRACT FROM AUTHOR]