Towards a new approach of controlling endophytic bacteria associated with date palm explants using essential oils, aqueous and methanolic extracts from medicinal and aromatic plants

Abstract

We identified two strains of endophytic bacteria associated with date palm explants by 16S rRNA gene amplification and sequencing, and we explored different approaches to control them. Based on their 16S sequences, the two isolates were identified as Microbacterium testaceum and Serratia marcescens. Antibacterial activity of essential oils, methanolic and aqueous extracts, from seven plant species against these endophytic bacteria was studied using different methods. The essential oils and the aqueous extracts of Artemisia herba-alba, Rosmarinus officinalis and Thymus satureioides inhibited the growth of both isolates through the disc diffusion method. The inhibition zones ranged from 18 to 31.5 mm and from 5 to 7 mm for essential oils and aqueous extracts, respectively. The minimum inhibitory concentration (MIC) and the minimum bacteriocidal concentration (MBC) values ranged from 0.025 to 0.033% and 0.033 to 0.05%, respectively. None of the methanolic extracts had any activity against the bacteria. The incorporation of the extracts into the culture medium showed different results depending on culture phase. During the induction phase, none of the extracts was able to inhibit the bacterial growth without causing phytotoxicity. During shoot bud multiplication, only the essential oils of A. herba-alba at the concentration of 0.1% inhibited the bacterial growth without causing phytotoxicity. Furthermore, the explants showed normal growth with an average number of 13.1 shoot buds per explant. The use of extract-impregnated plugs showed no inhibitory activity against the bacteria, whereas immersing explants in the antibacterial solutions caused browning and death of plant tissues. The endophytic bacteria observed during date palm organogenesis and somatic embryogenesis were identified for the first time ever using 16S sequencing, and a new biological and efficient approach to control it was developed.