Bioactive gibberellins (GAs) are diterpene plant hormones that are biosynthesized through complex pathways and control diverse aspects of growth and development. Biochemical, genetic, and genomic approaches have led to the identification of the majority of the genes that encode GA biosynthesis and deactivation enzymes. Recent studies have highlighted the occurrence of previously unrecognized deactivation mechanisms. It is now clear that both GA biosynthesis and deactivation pathways are tightly regulated by developmental, hormonal, and environmental signals, consistent with the role of GAs as key growth regulators. In some cases, the molecular mechanisms for fine-tuning the hormone levels are beginning to be uncovered. In this review, I summarize our current understanding of the GA biosynthesis and deactivation pathways in plants and fungi, and discuss how GA concentrations in plant tissues are regulated during development and in response to environmental stimuli.

Gibberellin Metabolism and its Regulation

What determines the number and size of the seeds produced by a plant? How often should it reproduce them? How often should a plant produce them? Why and how are seeds dispersed, and what are the implications for the diversity and composition of vegetation? These are just some of the questions tackled in this wide-ranging review of the role of seeds in the ecology of plants. The authors bring together information on the ecological aspects of seed biology, starting with a consideration of reproductive strategies in seed plants and progressing through the life cycle, covering seed maturation, dispersal, storage in the soil, dormancy, germination, seedling establishment, and regeneration in the field. The text encompasses a wide range of concepts of general relevance to plant ecology, reflecting the central role that the study of seed ecology has played in elucidating many fundamental aspects of plant community function.

The ecology of seeds

Seed Dormancy and Germination

Florigen, the mobile signal that moves from an induced leaf to the shoot apex and causes flowering, has eluded identification since it was first proposed 70 years ago. Understanding the nature of the mobile flowering signal would provide a key insight into the molecular mechanism of floral induction. Recent studies suggest that the Arabidopsis FLOWERING LOCUS T (FT) gene is a candidate for encoding florigen. We show that the protein encoded by Hd3a, a rice ortholog of FT, moves from the leaf to the shoot apical meristem and induces flowering in rice. These results suggest that the Hd3a protein may be the rice florigen.

https://science.sciencemag.org/content/sci/316/5827/1033.full.pdf

Hd3a protein is a mobile flowering signal in rice

The power of molecular genetics has dramatically advanced our understanding of all aspects of gibberellin (GA) signaling. Many genes encoding GA response pathway components have been identified using Arabidopsis and cereal mutants, and more elaborate genetic screens are producing additional mutants

/pdf/gibberellin-signaling-biosynthesis-catabolism-and-response-4ze6agdblf.pdf

Gibberellin signaling: biosynthesis, catabolism, and response pathways.

We have examined the seed germination in Arabidopsis thaliana of wild type (wt), and phytochrome A (PhyA)- and B (PhyB)-mutants in terms of incubation time and environmental light effects. Seed germination of the wt and PhyA-null mutant (phyA) was photoreversibly regulated by red and far-red lights of 10-1,000 micromol m-2 when incubated in darkness for 1-14 hr, but no germination occurred in PhyB-null mutant (phyB). When wt seeds and the phyB mutant seeds were incubated in darkness for 48 hr, they synthesized PhyA during dark incubation and germinated upon exposure to red light of 1-100 nmol m-2 and far-red light of 0.5-10 micromol m-2, whereas the phyA mutant showed no such response. The results indicate that the seed germination is regulated by PhyA and PhyB but not by other phytochromes, and the effects of PhyA and PhyB are separable in this assay. We determined action spectra separately for PhyA- and PhyB-specific induction of seed germination at Okazaki large spectrograph. Action spectra for the PhyA response show that monochromatic 300-780 nm lights of very low fluence induced the germination, and this induction was not photoreversible in the range examined. Action spectra for the PhyB response show that germination was photoreversibly regulated by alternate irradiations with light of 0.01-1 mmol m-2 at wavelengths of 540-690 nm and 695-780 nm. The present work clearly demonstrated that PhyA photoirreversibly triggers the germination upon irradiations with ultraviolet, visible and far-red light of very low fluence, while PhyB controls the photoreversible effects of low fluence.

https://www.pnas.org/content/pnas/93/15/8129.full.pdf

Action spectra for phytochrome A- and B-specific photoinduction of seed germination in Arabidopsis thaliana

We have isolated phytochrome B (phyB) and phyC mutants from rice (Oryza sativa) and have produced all combinations of double mutants. Seedlings of phyB and phyB phyC mutants exhibited a partial loss of sensitivity to continuous red light (Rc) but still showed significant deetiolation responses. The responses to Rc were completely canceled in phyA phyB double mutants. These results indicate that phyA and phyB act in a highly redundant manner to control deetiolation under Rc. Under continuous far-red light (FRc), phyA mutants showed partially impaired deetiolation, and phyA phyC double mutants showed no significant residual phytochrome responses, indicating that not only phyA but also phyC is involved in the photoperception of FRc in rice. Interestingly, the phyB phyC double mutant displayed clear R/FR reversibility in the pulse irradiation experiments, indicating that both phyA and phyB can mediate the low-fluence response for gene expression. Rice is a short-day plant, and we found that mutation in either phyB or phyC caused moderate early flowering under the long-day photoperiod, while monogenic phyA mutation had little effect on the flowering time. The phyA mutation, however, in combination with phyB or phyC mutation caused dramatic early flowering.

Distinct and Cooperative Functions of Phytochromes A, B, and C in the Control of Deetiolation and Flowering in Rice

We examined whether spectrally active phytochrome A (PhyA) and phytochrome B (PhyB) play specific roles in the induction of seed germination in Arabidopsis thaliana (L.) Heynh., using PhyA- and PhyB-null mutants, fre1-1 (A. Nagatani, J.W. Reed, J. Chory [1993] Plant Physiol 102: 269-277) and hy3-Bo64 (J. Reed, P.Nagpal, D.S. Poole, M. Furuya, J. Chory [1993] Plant Cell 5: 147-157). When dormant seeds of each genotype imbibed in the dark on aqueous agar plates, the hy3 (phyB) mutant did not germinate, whereas the fre1 (phyA) mutant germinated at a rate of 50 to 60%, and the wild type (WT) germinated at a rate of 60 to 70%. By contrast, seeds of all genotypes germinated to nearly 100% when plated in continuous irradiation with white or red light. When plated in continuous far-red light, however, frequencies of seed germination of the WT and the fre1 and hy3 mutants averaged 14, nearly 0, and 47%, respectively, suggesting that PhyB in the red-absorbing form prevents PhyA-dependent germination under continuous far-red light. When irradiated briefly with red or far-red light after imbibition for 1 h, a typical photoreversible effect on seed germination was observed in the fre1 mutant and the WT but not in the hy3 mutant. In contrast, when allowed to imbibe in the dark for 24 to 48 h and exposed to red light, the seed germination frequencies of the hy3 mutant were more than 40%. Immunoblot analyses of the mutant seeds showed that PhyB apoprotein accumulated in dormant seeds of the WT and the fre1 mutant as much as in the seeds that had imbibed. In contrast, PhyA apoprotein, although detected in etiolated seedlings grown in the dark for 5 d, was not detectable in the dormant seeds of the WT and the hy3 mutant. The above physiological and immunochemical evidence indicates that PhyB in the far-red-absorbing form was stored in the Arabidopsis seeds and resulted in germination in the dark. Hence, PhyA does not play any role in dark germination but induces germination under continuous irradiation with far-red light. Finally, we examined seeds from a signal transduction mutant, det1, and a det1/hy3 double mutant. The det1 seeds exhibited photoreversible responses of germination on aqueous agar plates, and the det1/hy3 double mutant seeds did not. Hence, DET1 is likely to act in a distinct pathway from PhyB in the photoregulation of seed germination.

https://www.jstor.org/stable/info/4275630

The Induction of Seed Germination in Arabidopsis thaliana Is Regulated Principally by Phytochrome B and Secondarily by Phytochrome A.

To elucidate phytochrome A (phyA) function in rice, we screened a large population of retrotransposon (Tos17) insertional mutants by polymerase chain reaction and isolated three independent phyA mutant lines. Sequencing of the Tos17 insertion sites confirmed that the Tos17s interrupted exons of PHYA genes in these mutant lines. Moreover, the phyA polypeptides were not immunochemically detectable in these phyA mutants. The seedlings of phyA mutants grown in continuous far-red light showed essentially the same phenotype as dark-grown seedlings, indicating the insensitivity of phyA mutants to far-red light. The etiolated seedlings of phyA mutants also were insensitive to a pulse of far-red light or very low fluence red light. In contrast, phyA mutants were morphologically indistinguishable from wild type under continuous red light. Therefore, rice phyA controls photomorphogenesis in two distinct modes of photoperception—far-red light–dependent high irradiance response and very low fluence response—and such function seems to be unique and restricted to the deetiolation process. Interestingly, continuous far-red light induced the expression of CAB and RBCS genes in rice phyA seedlings, suggesting the existence of a photoreceptor(s) other than phyA that can perceive continuous far-red light in the etiolated seedlings.

Isolation and Characterization of Rice Phytochrome A Mutants

Elementary processes of photoperception by phytochrome A (PhyA) for the high-irradiance response (HIR) of hypocotyl elongation in Arabidopsis were examined using a newly designed irradiator with LED. The effect of continuous irradiation with far-red (FR) light could be replaced by intermittent irradiation with FR light pulses if given at intervals of 3 min or less for 24 h. In this response, the Bunsen-Roscoe law of reciprocity held in each FR light pulse. Therefore, we determined the action spectrum for the response by intermittent irradiation using phyB and phyAphyB double mutants. The resultant action spectrum correlated well with the absorption spectrum of PhyA in far-red-absorbing phytochrome (Pfr). Intermittent irradiation with 550 to 667 nm of light alone had no significant effect on the response. In contrast, intermittent irradiation with red light immediately after each FR light pulse completely reversed the effect of FR light in each cycle. The results indicate that neither red-absorbing phytochrome synthesized in darkness nor photoconverted Pfr are physiologically active, and that a short-lived signal is induced during photoconversion from Pfr to red-absorbing phytochrome. The mode of photoperception by PhyA for HIR is essentially different from that by PhyA for very-low-fluence responses and phytochrome B for low-fluence responses.

Tomoko Shinomura

Papers

Action spectra for phytochrome A- and B-specific photoinduction of seed germination in Arabidopsis thaliana

Distinct and Cooperative Functions of Phytochromes A, B, and C in the Control of Deetiolation and Flowering in Rice

The Induction of Seed Germination in Arabidopsis thaliana Is Regulated Principally by Phytochrome B and Secondarily by Phytochrome A.

Isolation and Characterization of Rice Phytochrome A Mutants

Elementary processes of photoperception by phytochrome A for high-irradiance response of hypocotyl elongation in Arabidopsis.