SciCombinator

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Concept: Abscisic acid

28

Strigolactones (SLs) constitute a new class of plant of hormones which are active as germination stimulants for seeds of parasitic weeds of Striga, Orobanche and Pelipanchi spp, in hyphal branching of arbuscular mycorrhizal (AM) fungi and as inhibitors of shoot branching. In this review the focus is on molecular features of these SLs. The occurrence of SLs in root exudates of host plants is described. The naming protocol for SL according to the IUPAC rules and the “at a glance” method is explained. The total synthesis of some natural SLs is described with details for all eight stereoisomers of strigol. The problems encountered with assigning the correct structure of natural SLs are analyzed for orobanchol, alectrol and solanacol. The structure-activity relationship of SLs as germination stimulants leads to the identification of the bioactiphore of SLs. Together with a tentative mechanism for the mode of action, a model has been derived that can be used to design and prepare active SL analogues. This working model has been used for the preparation of a series of new SL analogues such as Nijmegen-1, and analogues derived from simple ketones, keto enols and saccharine. The serendipitous finding of SL mimics which are derived from the D-ring in SLs (appropriately substituted butenolides) is reported. For SL mimics also a mode of action is proposed. Recent new results support this proposal. The stability of SLs and SL analogues toward hydrolysis is described and some details of the mechanism of hydrolysis are discussed as well. The attempted isolation of the protein receptor for germination and current status concerning the biosynthesis of natural SLs 1 are briefly discussed. Some non-SLs as germinating agents are mentioned.The structure-activity relationship for SLs in hyphal branching of AM fungi and in repression of shoot branching is also analyzed. For each the principle functions a working model for the design of new active SL analogues is described and its applicability and implications are discussed. It is shown, that the three principal functions use a distinct perception system. The importance of stereochemistry for bioactivity has been described for the various functions.

Concepts: Protein, Plant, Fungus, Hormone, Receptor, Seed, Abscisic acid, Strigolactone

28

Strigolactones (SLs) are a novel class of plant hormones and rhizosphere communication signals, although the molecular mechanisms underlying their activities have not yet been fully determined. Nor is their application in agriculture well developed. The importance of plant hormone agonists has been demonstrated in both basic and applied research, and chemicals that mimic strigolactone functions should greatly facilitate strigolactone research. Here, we report our discovery of a new phenoxyfuranone compound, 4-Br debranone (4BD), that shows similar activity to that of the major strigolactone (SL) analogue GR24 in many aspects of a biological assay on plants. 4BD strongly inhibited tiller bud outgrowth in the SL-deficient rice mutant d10 at the same concentration as GR24, with no adverse effects, even during prolonged cultivation. This result was also observed in the Arabidopsis thaliana SL-deficient mutants max1, max3, and max4. However, the application of 4BD to the Arabidopsis SL-insensitive mutant max2 induced no morphological changes in it. The expression of SL biosynthetic genes was also reduced by 4BD treatment, probably via negative feedback regulation. However, in a seed germination assay on Striga hermonthica, a root parasitic plant, 4BD showed far less activity than GR24. These results suggest that 4BD is the first plant-specific strigolactone mimic.

Concepts: Pharmacology, Feedback, Negative feedback, Seed, Abscisic acid, Gibberellin, Strigolactone

27

The demand for increased crop productivity and the predicted challenges related to plant survival under adverse environmental conditions have renewed the interest in research in root biology. Various physiological and genetic studies have provided ample evidence in support of the role of plant growth regulators in root development. The biosynthesis and transport of auxin and its signaling play a crucial role in controlling root growth and development. The univocal role of auxin in root development has established it as a master regulator. Other plant hormones, such as cytokinins, brassinosteroids, ethylene, abscisic acid, gibberellins, jasmonic acid, polyamines and strigolactones interact either synergistically or antagonistically with auxin to trigger cascades of events leading to root morphogenesis and development. In recent years, the availability of biological resources, development of modern tools and experimental approaches have led to the advancement of knowledge in root development. Research in the areas of hormone signal perception, understanding network of events involved in hormone action and the transport of plant hormones has added a new dimension to root biology. The present review highlights some of the important conceptual developments in the interplay of auxin and other plant hormones and associated downstream events affecting root development.

Concepts: Root, Plant hormones, Plant hormone, Abscisic acid, Abscission, Ethylene, Auxin, Jasmonic acid

25

Homozygous Golden Rice lines developed in the background of Swarna through marker assisted backcross breeding (MABB) using transgenic GR2-R1 event as a donor for the provitamin A trait have high levels of provitamin A (up to 20 ppm) but are dwarf with pale green leaves and drastically reduced panicle size, grain number and yield as compared to the recurrent parent, Swarna. In this study, we carried out detailed morphological, biochemical and molecular characterization of these lines in a quest to identify the probable reasons for their abnormal phenotype. Nucleotide blast analysis with the primer sequences used to amplify the transgene revealed that the integration of transgene disrupted the native OsAux1 gene, which codes for an auxin transmembrane transporter protein. Real time expression analysis of the transgenes (ZmPsy and CrtI) driven by endosperm-specific promoter revealed the leaky expression of the transgene in the vegetative tissues. We propose that the disruption of OsAux1 disturbed the fine balance of plant growth regulators viz., auxins, gibberellic acid and abscisic acid, leading to the abnormalities in the growth and development of the lines homozygous for the transgene. The study demonstrates the conserved roles of OsAux1 gene in rice and Arabidopsis.

Concepts: DNA, Protein, Gene, Genetics, Molecular biology, Plant hormones, Plant hormone, Abscisic acid

25

Abiotic stresses, such as salinity, heavy metals and drought, are some of the most devastating factors hindering sustainable crop production today. Plants use their own defensive strategies to cope with the adverse effects of these stresses, via the regulation of the expression of essential phytohormones, such as gibberellins (GA), salicylic acid (SA), jasmonates (JA), abscisic acid (ABA) and ethylene (ET). However, the efficacy of the endogenous defensive arsenals of plants often falls short if the stress persists over an extended period. Various strategies are developed to improve stress tolerance in plants. For example, silicon (Si) is widely considered to possess significant potential as a substance which ameliorate the negative effects of abiotic stresses, and improves plant growth and biomass accumulation. This review aims to explain how Si application influences the signaling of the endogenous hormones GA, SA, ABA, JA and ET during salinity, wounding, drought and metal stresses in crop plants. Phytohormonal cross talk plays an important role in the regulation of induced defences against stress. However, detailed molecular and proteomic research into these interactions is needed in order to identify the underlying mechanisms of stress tolerance that is imparted by Si application and uptake.

Concepts: Oxygen, Seed, Plant hormones, Plant hormone, Abscisic acid, Abscission, Salicylic acid, Heavy metal music

23

Sorbitol dehydrogenase (SDH) catalyzes the reversible oxidation of sorbitol, xylitol and ribitol to their corresponding ketoses. In this study, we investigated the expression and role of Arabidopsis SDH in salt and osmotic stress tolerance, and abscisic acid (ABA) response. The expression patterns of SDH were investigated using transgenic Arabidopsis plants expressing beta-glucuronidase (GUS) under the control of promoter with the first intron of SDH. qRT-PCR and histochemical assay of GUS activity were used to study SDH expression regulation by ABA, salt and osmotic stress. The SDH-overexpression lines of Arabidopsis were used to investigate the role of SDH involved in salt and osmotic stress, and ABA response. Arabidopsis SDH was predominantly expressed in source organs such as green cotyledons, fully expanded leaves and sepals, especially in the vascular tissues of theses organs. The SDH expression was inhibited by NaCl and mannitol treatments. Seed germination and post-germination growth of SDH-overexpressing lines exhibited decreased sensitivity toward salt and osmotic stress compared to wild-type plants. The transcript of SDH was induced by ABA. Overexpression of SDH decreased sensitivity to ABA during seed germination and post-germination growth. Expression of AAO3 was increased but ABI5 and MYB2 was decreased in SDH-overexpressing lines after ABA treatment. This study demonstrated that the expression of SDH is regulated by ABA, salt and osmotic stress. SDH functions in plant tolerance to salt and osmotic stress, and ABA response via specific regulating gene expression of ABA synthesis and signaling in Arabidopsis. This article is protected by copyright. All rights reserved.

Concepts: Gene, Gene expression, Transcription, Seed, Abscisic acid, Plant morphology, Germination, Seed dormancy

14

Resistance to pre-harvest sprouting (PHS) is an important objective for the genetic improvement of many cereal crops, including wheat. Resistance, or susceptibility, to PHS is mainly influenced by seed dormancy, a complex trait. Reduced seed dormancy is the most important aspect of seed germination on a spike prior to harvesting, but it is influenced by various environmental factors including light, temperature and abiotic stresses. The basic genetic framework of seed dormancy depends on the antagonistic action of abscisic acid (ABA) and gibberellic acid (GA) to promote dormancy and germination. Recent studies have revealed a role for epigenetic changes, predominantly histone modifications, in controlling seed dormancy. To investigate the role of DNA methylation in seed dormancy, we explored the role of ARGONAUTE4_9 class genes in seed development and dormancy in wheat. Our results indicate that the two wheat AGO4_9 class genes i.e. AGO802 and AGO804 map to chromosomes 3S and 1S are preferentially expressed in the embryos of developing seeds. Differential expressions of AGO802-B in the embryos of PHS resistant and susceptible varieties also relates with DNA polymorphism in various wheat varieties due to an insertion of a SINE-like element into this gene. DNA methylation patterns of the embryonic tissue from six PHS resistant and susceptible varieties demonstrate a correlation with this polymorphism. These results suggest a possible role for AGO802-B in seed dormancy and PHS resistance through the modulation of DNA methylation.

Concepts: DNA, Genetics, Gene expression, Histone, Seed, Abscisic acid, Germination, Seed dormancy

14

Chlorophyll (chl) is essential for light capture and is the starting point that provides the energy for photosynthesis and thus plant growth. Obviously, for this reason, retention of the green chlorophyll pigment is considered a desirable crop trait. However, the presence of chlorophyll in mature seeds can be an undesirable trait that can affect seed maturation, seed oil quality, and meal quality. Occurrence of mature green seeds in oil crops such as canola and soybean due to unfavorable weather conditions during seed maturity is known to cause severe losses in revenue. One recently identified candidate that controls the chlorophyll degradation machinery is the stay-green gene, SGR1 that was mapped to Mendel’s I locus responsible for cotyledon color (yellow versus green) in peas. A defect in SGR1 leads to leaf stay-green phenotypes in Arabidopsis and rice, but the role of SGR1 in seed degreening and the signaling machinery that converges on SGR1 have remained elusive. To decipher the gene regulatory network that controls degreening in Arabidopsis, we have used an embryo stay-green mutant to demonstrate that embryo degreening is achieved by the SGR family and that this whole process is regulated by the phytohormone abscisic acid (ABA) through ABSCISIC ACID INSENSITIVE 3 (ABI3); a B3 domain transcription factor that has a highly conserved and essential role in seed maturation, conferring desiccation tolerance. Misexpression of ABI3 was sufficient to rescue cold-induced green seed phenotype in Arabidopsis. This finding reveals a mechanistic role for ABI3 during seed degreening and thus targeting of this pathway could provide a solution to the green seed problem in various oil-seed crops.

Concepts: Gene, Embryo, Plant, Seed, Abscisic acid, Germination, Seed dormancy, Cotyledon

12

Cuticular layers and seeds are prominent plant adaptations to terrestrial life that appeared early and late during plant evolution, respectively. The cuticle is a waterproof film covering plant aerial organs preventing excessive water loss and protecting against biotic and abiotic stresses. Cutin, consisting of crosslinked fatty acid monomers, is the most abundant and studied cuticular component. Seeds are dry, metabolically inert structures promoting plant dispersal by keeping the plant embryo in an arrested protected state. In Arabidopsis thaliana seeds, the embryo is surrounded by a single cell endosperm layer itself surrounded by a seed coat layer, the testa. Whole genome analyses lead us to identify cutin biosynthesis genes as regulatory targets of the phytohormones gibberellins (GA) and abscisic acid (ABA) signaling pathways that control seed germination. Cutin-containing layers are present in seed coats of numerous species, including Arabidopsis, where they regulate permeability to outer compounds. However, the role of cutin in mature seed physiology and germination remains poorly understood. Here we identify in mature seeds a thick cuticular film covering the entire outer surface of the endosperm. This seed cuticle is defective in cutin-deficient bodyguard1 seeds, which is associated with alterations in endospermic permeability. Furthermore, mutants affected in cutin biosynthesis display low seed dormancy and viability levels, which correlates with higher levels of seed lipid oxidative stress. Upon seed imbibition cutin biosynthesis genes are essential to prevent endosperm cellular expansion and testa rupture in response to low GA synthesis. Taken together, our findings suggest that in the course of land plant evolution cuticular structures were co-opted to achieve key physiological seed properties.

Concepts: Embryo, Fruit, Seed, Abscisic acid, Plant morphology, Germination, Seed dormancy, Dormancy

7

Seed germination is important for grain yield and quality and rapid, near-simultaneous germination helps in cultivation; however, cultivars that germinate too readily can undergo pre-harvest sprouting (PHS), which causes substantial losses in areas that tend to get rain around harvest time. Moreover, our knowledge of mechanisms regulating seed germination in wheat (Triticum aestivum) remains limited. In this study, we analyzed function of a wheat-specific microRNA 9678 (miR9678), which is specifically expressed in the scutellum of developing and germinating seeds. Overexpression of miR9678 delayed germination and improved resistance to PHS in wheat through reducing bioactive gibberellins (GAs) levels; miR9678 silencing enhanced germination rates. We provide evidence that miR9678 targets a long non-coding RNA(WSGAR)and triggers the generation of phased siRNA which play a role in the delay of seed germination. Finally, we found that abscisic acid (ABA) signaling proteins bind the promoter of miR9678 precursor and activate its expression, indicating that miR9678 affects germination by modulating the GA/ABA signaling.

Concepts: Embryo, Seed, Abscisic acid, Plant morphology, Germination, Seedling, Seed dormancy, Cotyledon