Abstract
BackgroundFoxtail millet [Setaria italica (L.) P. Beauv.] is an excellent crop known for its superior level of drought tolerance across the world. Especially, less water is needed during its germination period than the other cereal crops. However, the knowledge of the mechanisms underlying the abiotic stress effects on seed germination of foxtail millet is largely unknown.ResultsThe water uptake pattern of foxtail millet seeds was ploted during germination period, according to which the germination time course of millet was separated into three phases. We sequenced the transcriptome of foxtail millet seeds, which were treated by PEG during different germination phases after sowing. The transcriptional studies revealed that more DEGs were identified during the further increase in water uptake period (phase III) than during the rapid initial uptake period (phase I) and the plateau period (phase II) under PEG stress. The pathway analysis of DEGs showed that the highly enriched categories were related to phenylpropanoid biosynthesis, plant hormone signal transduction and phenylalanine metabolism during phase III. The 20 phenylpropanoids-related genes of germinating foxtail millet were found to be down-regulated during the further increase in water uptake period under PEG stress. Further expression analysis identified 4 genes of phenylalanine ammonia-lyase, 4-coumarate-CoA ligase 3, cinnamoyl-CoA reductase 1, cationic peroxidase SPC4 in phenylpropanoids-related pathway, which played important roles in foxtail millet in response to PEG stress during different germination periods. The studies of metabolites in phenylpropanoid biosynthesis pathway revealed that higher amount of cinnamic acid was accumulated in germinating seeds under PEG stress, while the contents of p-coumaric acid, caffeic acid, ferulic acid and sinapic acid were decreased. And the effects of five phenolic compounds on germination and growth of foxtail millet showed that 1 mM concentration of cinnamic acid inhibited shoot and root growth, especially root development. Ferulic acid, caffeic acid, sinapic acid and p-coumaric acid could increase the root length and root/sprout in lower concentration.ConclusionsThese findings suggest that key genes and metabolites of foxtail millet related with phenylpropanoids pathway may play prominent roles in the regulation of resistance to drought during germination. Foxtail millet can probably avoid drought by regulating the levels of endogenous allelochemicals.
Highlights
The water uptake pattern of foxtail millet seeds was ploted during germination period, according to which the germination time course of millet was separated into three phases
We sequenced the transcriptome of foxtail millet seeds, which were treated by polyethylene glycol (PEG) during different germination phases after sowing
The transcriptional studies revealed that more differentially expressed genes (DEGs) were identified during the further increase in water uptake period than during the rapid initial uptake period and the plateau period under PEG stress
Summary
The knowledge of the mechanisms underlying the abiotic stress effects on seed germination of foxtail millet is largely unknown. Drought is one of the most frequent and severe abiotic stress factors, which adversely affects plant growth and crop productivity in many arid and semiarid regions [1,2,3]. All the cellular and metabolic events occur in the nondormant seeds (before the completion of germination) and the imbibed dormant seeds, but the metabolic activities of the latter are subtly different from those of the former [5]. The imbibed mature seed is sensitive to different environmental factors during germination process. The knowledge about the physiological and molecular mechanisms underlying the environmental effects on germination was largely lacking [8, 9]
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