Abstract
In Arabidopsis, NAC (NAM, ATAF and CUC) transcription factors have been found to promote lateral root number through the auxin signaling pathway. In the present study, the role of water stress–inducible soybean GmNAC003 and GmNAC004 genes in the enhancement of lateral root development under water deficit conditions was investigated. Both genes were highly expressed in roots, leaves and flowers of soybean and were strongly induced by water stress and moderately induced by a treatment with abscisic acid (ABA). They showed a slight response to treatment with 2,4-dichlorophenoxyacetic acid (2,4-D). The transgenic Arabidopsis plants overexpressing GmNAC004 showed an increase in lateral root number and length under non-stress conditions and maintained higher lateral root number and length under mild water stress conditions compared to the wild-type (WT), while the transgenic plants overexpressing GmNAC003 did not show any response. However, LR development of GmNAC004 transgenic Arabidopsis plants was not enhanced in the water-stressed compared to the well-watered treatment. In the treatment with ABA, LR density of the GmNAC004 transgenic Arabidopsis was less suppressed than that of the WT, suggesting that GmNAC004 counteracts ABA-induced inhibition of lateral root development. In the treatment with 2,4-D, lateral root density was enhanced in both GmNAC004 transgenic Arabidopsis and WT plants but the promotion was higher in the transgenic plants. Conversely, in the treatment with naphthylphthalamic acid (NPA), lateral root density was inhibited and there was no difference in the phenotype of the GmNAC004 transgenic Arabidopsis and WT plants, indicating that auxin is required for the action of GmNAC004. Transcript analysis for a number of known auxin and ABA related genes showed that GmNAC004's role may suppress ABA signaling but promote auxin signaling to increase lateral root development in the Arabidopsis heterologous system.
Highlights
Soybean root architecture is defined by tap root length and lateral root (LR) length, density and distribution
GmNAC003 and GmNAC004 are induced by water stress To investigate the response of GmNAC003 and GmNAC004 to water deficit in various soybean organs, a greenhouse experiment was conducted in which water was withheld to allow progressive water stress development to mimic slow soil-drying under drought conditions in the field
Significant advances in understanding of molecular networks regulating LR development in the past few years enabled us to further investigate the molecular regulation of root architecture under water deficit
Summary
Soybean root architecture is defined by tap root length and lateral root (LR) length, density and distribution. Regulation of root growth and development in response to environmental stresses to optimize water and nutrient extraction has been observed in various species including soybean [2,3,4], barley [5], Arabidopsis [6] and common bean [7,8]. Under mild water deficit conditions, root growth rate is relatively less inhibited or even stimulated, while shoot growth is significantly inhibited [2,4,11]. Primary root lengths of these lines remained unchanged while shoot growth was reduced. These data suggest that soybean plants can optimize resources to prioritize LR development to adapt to water stress
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