SlCESTA Is a Brassinosteroid-Regulated bHLH Transcription Factor of Tomato That Promotes Chilling Tolerance and Fruit Growth When Over-Expressed.

Brassinosteroids (BRs) are required for various aspects of plant growth and development, but also participate in stress responses. The hormones convey their activity through transcriptional regulation and posttranslational modification of transcription factors and one class are basic helix-loop-helix (bHLH) proteins of the BR Enhanced Expression (BEE) subfamily, which in Arabidopsis thaliana include BEE1-3 and CESTA (CES). CES and the BEEs promote the expression of different BR-responsive genes, including genes encoding gibberellin (GA) biosynthetic and catabolizing enzymes, as well as cold-responsive genes. Interestingly, in terms of an application, CES could promote both fruit growth and cold stress tolerance when over-expressed in A. thaliana and here it was investigated, if this function is conserved in the fruit crop Solanum lycopersicum (cultivated tomato). Based on amino acid sequence similarity and the presence of regulatory motifs, a CES orthologue of S. lycopersicum, SlCES, was identified and the effects of its over-expression were analysed in tomato. This showed that SlCES, like AtCES, was re-localized to nuclear bodies in response to BR signaling activation and that it effected GA homeostasis, with related phenotypes, when over-expressed. In addition, over-expression lines showed an increased chilling tolerance and had altered fruit characteristics. The possibilities and potential limitations of a gain of SlCES function as a breeding strategy for tomato are discussed.

DA-6 promotes germination and seedling establishment from aged soybean seeds by mediating fatty acid metabolism and glycometabolism.

Soybean seeds contain higher concentrations of oil (triacylglycerol) and fatty acids than do cereal crop seeds, and the oxidation of these biomolecules during seed storage significantly shortens seed longevity and decreases germination ability. Here, we report that diethyl aminoethyl hexanoate (DA-6), a plant growth regulator, increases germination and seedling establishment from aged soybean seeds by increasing fatty acid metabolism and glycometabolism. Phenotypic analysis showed that DA-6 treatment markedly promoted germination and seedling establishment from naturally and artificially aged soybean seeds. Further analysis revealed that DA-6 increased the concentrations of soluble sugars during imbibition of aged soybean seeds. Consistently, the concentrations of several different fatty acids in DA-6-treated aged seeds were higher than those in untreated aged seeds. Subsequently, quantitative PCR analysis indicated that DA-6 induced the transcription of several key genes involved in the hydrolysis of triacylglycerol to sugars in aged soybean seeds. Furthermore, the activity of invertase in aged seeds, which catalyzes the hydrolysis of sucrose to form fructose and glucose, increased following DA-6 treatment. Taken together, DA-6 promotes germination and seedling establishment from aged soybean seeds by enhancing the hydrolysis of triacylglycerol and the conversion of fatty acids to sugars.

ABI4 regulates the floral transition independently of ABI5 and ABI3.

Emerging evidence suggests that the stress hormone abscisic acid (ABA) is also involved in the floral transition control. The transcription factors ABA INSENSITIVE4 (ABI4) and ABI5 negatively regulate flowering by directly promoting FLOWERING LOCUS C expression, and ABI3 also negatively regulates the floral transition. However, the genetic relationships between ABI4 and both ABI5 and ABI3 remain elusive. Here, we generated transgenic plants overexpressing ABI4 in the abi5 (OE-ABI4::abi5) and abi3 backgrounds (OE-ABI4::abi3). The flowering phenotypic analysis demonstrated that OE-ABI4::abi5 and OE-ABI4::abi3 plants exhibited delayed flowering. These findings suggest that ABI4 independently regulates floral transition but not through ABI5 and ABI3 cascades.

Exogenous auxin represses soybean seed germination through decreasing the gibberellin/abscisic acid (GA/ABA) ratio.

Auxin is an important phytohormone which mediates diverse development processes in plants. Published research has demonstrated that auxin induces seed dormancy. However, the precise mechanisms underlying the effect of auxin on seed germination need further investigation, especially the relationship between auxins and both abscisic acid (ABA) and gibberellins (GAs), the latter two phytohormones being the key regulators of seed germination. Here we report that exogenous auxin treatment represses soybean seed germination by enhancing ABA biosynthesis, while impairing GA biogenesis, and finally decreasing GA1/ABA and GA4/ABA ratios. Microscope observation showed that auxin treatment delayed rupture of the soybean seed coat and radicle protrusion. qPCR assay revealed that transcription of the genes involved in ABA biosynthetic pathway was up-regulated by application of auxin, while expression of genes involved in GA biosynthetic pathway was down-regulated. Accordingly, further phytohormone quantification shows that auxin significantly increased ABA content, whereas the active GA1 and GA4 levels were decreased, resulting insignificant decreases in the ratiosGA1/ABA and GA4/ABA.Consistent with this, ABA biosynthesis inhibitor fluridone reversed the delayed-germination phenotype associated with auxin treatment, while paclobutrazol, a GA biosynthesis inhibitor, inhibited soybean seed germination. Altogether, exogenous auxin represses soybean seed germination by mediating ABA and GA biosynthesis.

Salt Stress Represses Soybean Seed Germination by Negatively Regulating GA Biosynthesis While Positively Mediating ABA Biosynthesis.

Soybean is an important and staple oilseed crop worldwide. Salinity stress has adverse effects on soybean development periods, especially on seed germination and post-germinative growth. Improving seed germination and emergence will have positive effects under salt stress conditions on agricultural production. Here we report that NaCl delays soybean seed germination by negatively regulating gibberellin (GA) while positively mediating abscisic acid (ABA) biogenesis, which leads to a decrease in the GA/ABA ratio. This study suggests that fluridone (FLUN), an ABA biogenesis inhibitor, might be a potential plant growth regulator that can promote soybean seed germination under saline stress. Different soybean cultivars, which possessed distinct genetic backgrounds, showed a similar repressed phenotype during seed germination under exogenous NaCl application. Biochemical analysis revealed that NaCl treatment led to high MDA (malondialdehyde) level during germination and the post-germinative growth stages. Furthermore, catalase, superoxide dismutase, and peroxidase activities also changed after NaCl treatment. Subsequent quantitative Real-Time Polymerase Chain Reaction analysis showed that the transcription levels of ABA and GA biogenesis and signaling genes were altered after NaCl treatment. In line with this, phytohormone measurement also revealed that NaCl considerably down-regulated active GA1, GA3, and GA4 levels, whereas the ABA content was up-regulated; and therefore ratios, such as GA1/ABA, GA3/ABA, and GA4/ABA, are decreased. Consistent with the hormonal quantification, FLUN partially rescued the delayed-germination phenotype caused by NaCl-treatment. Altogether, these results demonstrate that NaCl stress inhibits soybean seed germination by decreasing the GA/ABA ratio, and that FLUN might be a potential plant growth regulator that could promote soybean seed germination under salinity stress.

The roles of auxin in seed dormancy and germination.

Seed dormancy and germination are attractive topics in the fields of plant molecular biology as they are key stages during plant growth and development. Seed dormancy is intricately regulated by complex networks of phytohormones and numerous key genes, combined with diverse environmental cues. The transition from dormancy to germination is a very important biological process, and extensive studies have demonstrated that phytohormones abscisic acid (ABA) and gibberellin acid (GA) are major determinants. Consequently, the precise balance between ABA and GA can ensure that the seeds remain dormant under stress conditions and germinate at optimal times. Here we review the role of auxin in seed dormancy and germination. Auxin is one of the classic phytohormones effective during tropism growth and tissue differentiation. Recent studies, however, show that auxin possesses positive effects on seed dormancy, which suggests that auxin is the second phytohormone that induces seed dormancy, besides ABA. We will focus on the synthetic effects in detail between auxin and ABA pathways on seed dormancy and propose future research directions.

Karrikins delay soybean seed germination by mediating abscisic acid and gibberellin biogenesis under shaded conditions.

Karrikins (KAR) are a class of signal compounds, discovered in wildfire smoke, which affect seed germination. Currently, numerous studies have focused on the model plant Arabidopsis in the KAR research field, rather than on crops. Thus the regulatory mechanisms underlying KAR regulation of crop seed germination are largely unknown. Here, we report that KAR delayed soybean seed germination through enhancing abscisic acid (ABA) biosynthesis, while impairing gibberellin (GA) biogenesis. Interestingly, KAR only retarded soybean seed germination under shaded conditions, rather than under dark and white light conditions, which differs from in Arabidopsis. Phytohormone quantification showed that KAR enhanced ABA biogenesis while impairing GA biosynthesis during the seed imbibition process, and subsequently, the ratio of active GA4 to ABA was significantly reduced. Further qRT-PCR analysis showed that the transcription pattern of genes involved in ABA and GA metabolic pathways are consistent with the hormonal measurements. Finally, fluridone, an ABA biogenesis inhibitor, remarkably rescued the delayed-germination phenotype of KAR-treatment; and paclobutrazol, a GA biosynthesis inhibitor, inhibited soybean seed germination. Taken together, these evidences suggest that KAR inhibit soybean seed germination by mediating the ratio between GA and ABA biogenesis.

[Current understanding of signaling transduction pathway and biological functions of Karrikins].

Karrikins are a class of signaling molecules discovered in wildfire smoke, which can significantly promote seed germination in some species (such as Arabidopsis and Avena fatua). The structures of Karrikins were first elucidated in 2004. At present, six different types of Karrikins have been documented, and their biological activities vary significantly. So far, studies for Karrikins have become a hot spot in the plant molecular biology field. Recent advances demonstrate that Karrikins regulate plant photomorphogenesis and leaf differentiation effectively, in addition to the effect on seed germination. Furthermore, Karrikins share highly similar molecular structures and signaling transduction pathways with strigolactone. In this review, we summarize the history of discovery, signaling transduction pathways, physiological functions and ecological significance of Karrikins, and further discuss the future research directions.

Karrikins: Regulators Involved in Phytohormone Signaling Networks during Seed Germination and Seedling Development.

Seed germination and early seedling establishment are critical stages during a plant's life cycle. These stages are precisely regulated by multiple internal factors, including phytohormones and environmental cues such as light. As a family of small molecules discovered in wildfire smoke, karrikins (KARs) play a key role in various biological processes, including seed dormancy release, germination regulation, and seedling establishment. KARs show a high similarity with strigolactone (SL) in both chemical structure and signaling transduction pathways. Current evidence shows that KARs may regulate seed germination by mediating the biosynthesis and/or signaling transduction of abscisic acid (ABA), gibberellin (GA) and auxin [indoleacetic acid (IAA)]. Interestingly, KARs regulate seed germination differently in different species. Furthermore, the promotion effect on seedling establishment implies that KARs have a great potential application in alleviating shade avoidance response, which attracts more and more attention in plant molecular biology. In these processes, KARs may have complicated interactions with phytohormones, especially with IAA. In this updated review, we summarize the current understanding of the relationship between KARs and SL in the chemical structure, signaling pathway and the regulation of plant growth and development. Further, the crosstalk between KARs and phytohormones in regulating seed germination and seedling development and that between KARs and IAA during shade responses are discussed. Finally, future challenges and research directions for the KAR research field are suggested.