Background: Studies have indicated that graphene oxide (GO) could regulated Brassica napus L. root growth via abscisic acid (ABA) and indole-3-acetic acid (IAA). To study the mechanism and interaction between GO and IAA further, B. napus L (Zhongshuang No. 9) seedlings were treated with GO and IAA accordance with a two factor completely randomized design. Results: GO and IAA cotreatment significantly regulated the root length, number of adventitious roots, and contents of IAA, cytokinin (CTK) and ABA. Treatment with 25 mg/L GO alone or IAA (> 0.5 mg/L) inhibited root development. IAA cotreatment enhanced the inhibitory role of GO, and the inhibition was strengthened with increased in IAA concentration. GO treatments caused oxidative stress in the plants. The ABA and CTK contents decreased; however, the IAA and gibberellin (GA) contents first increased but then decreased with increasing IAA concentration when IAA was combined with GO compared with GO alone. The 9-cis-epoxycarotenoid dioxygenase (NCED) transcript level strongly increased when the plants were treated with GO. However, the NCED transcript level and ABA concentration gradually decreased with increasing IAA concentration under GO and IAA cotreatment. GO treatments decreased the transcript abundance of steroid 5-alpha-reductase (DET2) and isochorismate synthase 1 (ICS), which are associated with brassinolide (BR) and salicylic acid (SA) biosynthesis, but increased the transcript abundance of brassinosteroid insensitive 1-associated receptor kinase 1 (BAK1), cam-binding protein 60-like G (CBP60) and calmodulin binding protein-like protein 1, which are associated with BR and SA biosynthesis. Last, GO treatment increased the transcript abundance of 1-aminocyclopropane-1-carboxylic acid synthase 2 (ACS2), which is associated with the ethylene (ETH) pathway. Conclusions: Treatment with 25 mg/L GO or IAA (> 0.5 mg/L) inhibited root development. However, IAA and GO cotreatment enhanced the inhibitory role of GO, and this inhibition was strengthened with increased IAA concentration. IAA is a key factor in the response of B. napus L to GO and the responses of B. napus to GO and IAA cotreatment involved in multiple pathways, including those involving ABA, IAA, GA, CTK, BR, SA. Specifically, GO and IAA cotreatment affected the GA content in the modulation of B. napus root growth.

To better understand the effects of nanomaterials on plants, tobacco seedlings and graphene oxide were used as research materials for detailed study. The tobacco seedlings were treated with 0 mg/L and 20 mg/L of graphene oxide. Results indicated that 20 mg/L of graphene oxide had a significant effect on the tobacco roots growth. Treatment with 20 mg/L of graphene oxide resulted into shorter seminal root length than control samples after 20 days treatment. Moreover, the numbers of adventitious roots were also dramatically increased in the treated samples when compared to the untreated ones after 35 days treatment, the increase being about two to three times more in the treated samples. The average weight of treated fresh plants’ roots was 64.8 mg after 35 days treatment, which was 1.5 times as great as that of the untreated plants. Graphene oxide treatments also resulted into higher SOD, POD, and CAT activities, and also lower MDA content. The treatment with 20 mg/L of graphene oxide also resulted in the increased transcript levels of IAA3, IAA4, IAA7, ARF2, and ARF8, indicating that the graphene oxide treatment affected the gene transcript levels of the IAA relatives, resulting in enhanced roots’ growth. However, the mechanism by which 20 mg/L of graphene improves part of roots’ growth still needs further study.