The Effects of Plant Hormones on Root Growth and GmEXPB2 Expression in Soybean

The root system of a plant performs many important functions including water uptake, nutrient acquisition and anchorage of plants to the ground. Understanding the regulation of root development is therefore of vital importance. Previously, we found that soybean β-expansin gene GmEXPB2 functioned as an intrinsic component of root architectural changes to abiotic stress in soybean. Here we examined the effects of exogenous plant hormones on root growth and GmEXPB2 expression to illustrate the relationship between root growth and plant cell wall extension. Three-day old soybean seedlings were grown in hydroponic solutions containing 0, 0.05, 0.5, 1, or 5 μM of auxin (IAA, IBA, or NAA), gibberellin (GA₃), abscisic acid (ABA) or cytokinin (6-BA) and evaluated after 4 days. The results showed that soybean root morphology was significantly altered by the treatment of different levels of auxins,  GA₃, ABA or 6-BA. Higher concentrations of IAA, IBA, and NAA inhibited the growth of primary root and lateral roots, consequently leading to the decrease of the total root length. Low concentrations of IAA (0.05 μM and 0.5 μM), however, slightly increased the total root length which was associated with increased lateral root growth. Similarly, higher concentrations of GA₃, ABA, and 6-BA inhibited primary and lateral root growth, but increased root diameters with an exception for GA₃, which induced finer root growth. Higher concentrations of 6-BA caused the root tips to curve up. Meanwhile, the expression of GmEXPB2 varied with different types of hormones. The transcription levels of GmEXPB2 increased with higher concentrations of IAA and NAA, but increased first and then decreased by IBA. In contrast, the accumulation of GmEXPB2 transcript was strongly associated with concentrations of 6-BA and ABA, while it was not associated with GA₃. Our results indicate the complexity of plant hormone regulations on root growth, which is closely associated with plant cell wall expansion. Nevertheless, their detailed relationships need to be further elucidated.