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▪ Abstract  The plant hormone abscisic acid (ABA) plays a major role in seed maturation and germination, as well as in adaptation to abiotic environmental stresses. ABA promotes stomatal closure by rapidly altering ion fluxes in guard cells. Other ABA actions involve modifications of gene expression, and the analysis of ABA-responsive promoters has revealed a diversity of potential cis-acting regulatory elements. The nature of the ABA receptor(s) remains unknown. In contrast, combined biophysical, genetic, and molecular approaches have led to considerable progress in the characterization of more downstream signaling elements. In particular, substantial evidence points to the importance of reversible protein phosphorylation and modifications of cytosolic calcium levels and pH as intermediates in ABA signal transduction. Exciting advances are being made in reassembling individual components into minimal ABA signaling cascades at the single-cell level.

This paper examines four key mechanisms of the seed size/number trade-off (SSNT) models to assess their relevance to a general understanding of plant community structure. Mechanism 1 is that large seeds have a greater probability of winning in competition against smaller seeds. I provide interspecific experimental evidence that there is a competitive hierarchy among seedlings based on seed size. Mechanism 2 is that a trade-off exists between the number and size of seeds produced for a given reproductive allocation. Negative correlations between seed size and number were found consistently across a range of species from a range of habitats, from published literature. Mechanism 3, that seedling-seedling competition is an important influence on species composition, was found to exist potentially in a range of environments, including annual-dominated, post-fire and gap-dynamic communities. However, there is little quantitative evidence available and this is likely to be a restrictive mechanism. Mechanism 4, that small seeds are superior colonists due to their greater number, was tested in a field experiment in a calcareous grassland community. No supporting evidence was found, suggesting that the SSNT is not an important determinant of structure in this community. Thus two of the four mechanisms can be considered to hold true generally, while the third mechanism may be valid in particular environments. The fourth mechanism did not apply in the community tested, but could be tested in a wider range of communities.

The mechanisms imposing a gibberellin (GA) requirement to promote the germination of dormant and non-dormant Arabidopsis seeds were analyzed using the GA-deficient mutant ga1, several seed coat pigmentation and structure mutants, and the abscisic acid (ABA)-deficient mutant aba1. Testa mutants, which exhibit reduced seed dormancy, were not resistant to GA biosynthesis inhibitors such as tetcyclacis and paclobutrazol, contrarily to what was found before for other non-dormant mutants in Arabidopsis. However, testa mutants were more sensitive to exogenous GAs than the wild-types in the presence of the inhibitors or when transferred to a GA-deficient background. The germination capacity of the ga1-1 mutant could be integrally restored, without the help of exogenous GAs, by removing the envelopes or by transferring the mutation to a tt background (tt4 and ttg1). The double mutants still required light and chilling for dormancy breaking, which may indicate that both agents can have an effect independently of GA biosynthesis. The ABA biosynthesis inhibitor norflurazon was partially efficient in releasing the dormancy of wild-type and mutant seeds. These results suggest that GAs are required to overcome the germination constraints imposed both by the seed coat and ABA-related embryo dormancy.

Amelioration of salt stress effect on canola seed germination was investigated using 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase-producing plant growth promoting halotolerant bacteria. NaCl at 120 mM concentration reduced canola seed germination by 50 %. The inoculation of ACC deaminase-producing halotolerant Brevibacterium epidermidis RS15 and Bacillus aryabhattai RS341 at 120 mM NaCl significantly increased the seed germination with decreased seed ACC content. Notably, the hydrolytic enzymes activities like amylase, invertase, and protease also increased due to inoculation of RS15 and RS341 compared to uninoculated salt stress imposed germinating canola seeds. Ethylene emission of salt stress exposed eight-day-old canola seedlings was reduced by 35.4 and 41.1 % compared to uninoculated salt stressed control due to respective inoculation of RS341 and RS15. The amelioration of salt stress inhibitory effect on the canola seed germination was attributed to the inoculation of ACC deaminase-producing halotolerant bacteria modulating ethylene emission and inducing hydrolytic enzymes.

Physiological attributes of a set of cyanobacterial strains, isolated from the rhizosphere of wheat (var. HD 2687), identified as belonging to the genera Calothrix (n=3), Westiellopsis (1), Hapalosiphon (2) and Nostoc (2), were axenized and evaluated. The concentrated culture filtrates of three cyanobacterial strains, C. ghosei, H. intricatus and Nostoc sp., were able to enhance germination percentage, radicle and coleoptile length in inhibition experiments with wheat seeds. Indole-3-acetic acid (IAA) production was recorded in light and dark (+0.5 % glucose) incubated cultures. Incubation in the presence of tryptophan significantly enhanced IAA production. Acetylene-reducing activity was higher in light incubated cultures of Nostoc sp. followed by C. ghosei, while in the dark, C. ghosei recorded the highest values. TLC of the filtrates revealed the presence of several amino acids such as histidine and auxin-like compounds. Co-culturing with selected strains recorded significant enhancement in plant chlorophyll levels. Root sections of wheat seedlings co-cultured with C. ghosei revealed the presence of short filaments inside the root hairs and cortical region. Such strains can be promising candidates for developing plant growth promoting associations for wheat crop, besides serving as model systems for understanding the metabolic interactions of cyanobacteria with host plant, such as wheat.

Lectin receptor-like kinases (LecRK) are widespread in higher plants; however, little is known about their physiological roles. In this study, At1g70130 (designated LecRK-b2), an Arabidopsis LecRK gene, has been investigated. LecRK-b2 was predominantly expressed during seed germination, and its expression was ceased following germination. The expression of LecRK-b2 was induced by abscisic acid (ABA), salt, and osmotic stress. LecRK-b2 loss-of-function mutation slightly reduced the ABA sensitivity during seed germination, and this reduced sensitivity was demonstrated not due to lower ABA accumulation level in the seeds. Dual-luciferase transient expression assay confirmed that the transcription factor ABSCISIC ACID INSENSITIVE3 (ABI3) could activate the luciferase under driving of LecRK-b2 promoter. LecRK-b2 transcription level was found to be down-regulated in abi3 during seed germination. Furthermore, LecRK-b2 loss-of-function mutation reduced the salt and osmotic sensitivity during early development stage of Arabidopsis. Taken together, these results suggest that LecRK-b2 functions as a positive regulator of the ABA response during the seed germination and is involved in salt and osmotic stress response in the early development stage.

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

The activity of Rhizobium japonicum against the soilborne pathogens Fusarium solani and Macrophomina phaseolina as causative agents of soybean root rot disease in both culture medium and soil was evaluated. Rhizobial culture filtrate caused an inhibition of the radial growth of Fusarium solani and Macrophomina phaseolina on potato dextrose agar medium amended with the filtrate compared with control. The addition of rhizobial culture suspension to the soil contaminated by the two pathogens, Fusarium solani and Macrophomina phaseolina and their interaction, in pots improved seed germination percentages and reduced the root rot disease index significantly. The sowing of rhizobial coated seeds in soil contaminated by Fusarium solani and Macrophomina phaseolina (separately and in combination) in the field increased seed germination significantly and induced a high reduction in disease severity. These results indicate that rhizobia could be an important element in root rot disease management.