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Allelopathy involves the release of compounds into the environment that affects the growth and development of other organisms. This phenomenon may lead to the production of compounds less harmful to the environment than traditional herbicides used in weed control. In plants, terpenes have been identified as components of allelochemicals and are synthesized by enzymes named as geranylgeranyl diphosphate synthases (GGPPS). There are about 12 GGPPS genes in Arabidopsis, among which is GGR. This work aims to study the association between the expression levels of GGR and the allelopathic response of sesame seedlings to Arabidopsis leaf extracts. Hence, the GGR gene was inserted into Arabidopsis with the purpose to investigate the allelopathic effects of GGR expression levels on sesame seedlings. GGR expression levels were quantified by RT-PCR in both transgenic and non-transgenic [wild-type (WT)] lines. It has been observed that both wild-type and GGR expressing transgenic lines inhibited the growth of sesame seedlings. However, it is noteworthy that the phytotoxicity of extracts from GGR lines were greater than WT extracts. RT-PCR analysis of GGR expression revealed that WT plants had higher levels of GGR expression than GGR transgenic lines, which suggests that a homologous-dependent gene silencing (HDGS) occurred in GGR lines. GGR is part of an enzyme complex that works as a hub that determines the types of terpenes produced in Arabidopsis chloroplasts. The present data indicates that decreases in GGR expression may have favoured the production of terpenes with stronger allelopathic capacity in Arabidopsis leaves.