Abstract
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.
Highlights
The legume species soybean (Glycine max L.) is one of the most important oil crops in the world, and is an important source of plant oil and protein for human consumption[1]
In the model plant Arabidopsis, for instance, the Abscisic acid (ABA) biosynthesis mutants nced[3], nced[5], nced[6] and aba[211,12], and the ABA signaling mutants abi[313], abi[414], and abi[515] exhibited a faster-germinating phenotype compared to the wild-type, whereas mutants overexpressing either ABA2 or ABI4 exhibited a deep level of dormancy, resulting in the delayed-germination phenotype[14,16]
Earlier studies revealed that exogenous auxin application significantly suppressed Arabidopsis seed germination under salt stress conditions[29], indicating that auxin plays an important role in seed germination in response to environmental stimuli
Summary
The legume species soybean (Glycine max L.) is one of the most important oil crops in the world, and is an important source of plant oil and protein for human consumption[1]. The oxidation of oil and protein causes seeds to deteriorate during storage, resulting in decreased soybean seed germination and seedling emergence in the field[2,3]. The ratios including GA1/ABA and GA4/ABA is the key determinant of seed germination[4,6] Both ABA and GA signaling/level play key roles in the regulation of seed germination. Another important phytohormone, auxin, is involved in almost all stages of plant development, including root growth, apical dominance, fruit growth and response to environmental signals[19,20,21,22,23,24]. Auxin was demonstrated to be a regulator of several phytohormone signaling pathways, including ABA and GA, and, as a result, regulated tomato fruit formation[27]
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