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The ability of streptomycete actinomycetes to promote growth of tomato through the production of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase was evaluated under gnotobiotic and greenhouse conditions. To achieve this, 64 isolates of Streptomyces spp. obtained from a tomato rhizosphere in the United Arab Emirates were initially selected for their ability to produce ACC deaminase as well as indole-3-acetic acid (IAA) and subsequently for their rhizosphere competence as root colonizers. Of the two selected ACC deaminase-producing isolates showing exceptional rhizosphere competence, S. filipinensis no. 15 produced both ACC deaminase and IAA, whilst S. atrovirens no. 26 did not produce IAA. Under greenhouse conditions, the application of S. filipinensis no. 15 or S. atrovirens no. 26 resulted in the reduction of the endogenous levels of ACC, the immediate precursor of ethylene, in both roots and shoots and increased plant growth. Plant growth promotion was most pronounced in the presence of S. filipinensis no. 15 compared to S. atrovirens no. 26. This relative superiority in performance shows the advantage conferred to S. filipinensis no. 15 due to its ability to produce both IAA and ACC deaminase. In comparison, an ACC deaminase-producing isolate of S. albovinaceus no. 41 which was neither rhizosphere-competent nor capable of producing IAA, failed to promote plant growth compared to S. filipinensis no. 15 or S. atrovirens no. 26 although the growth promotion obtained by S. albovinaceus no. 41 was significant compared to control. The application of S. globosus no. 8, which was not rhizospherecompetent and did not produce detectable levels of ACC deaminase or IAA did not promote plant growth. These results indicate the importance of rhizosphere competence. In conclusion I report the production of ACC deaminase by streptomycete actinomycetes and its ability to enhance plant growth through reduction in the in planta levels of endogenous ACC and the consequent lowering of endogenous ethylene levels in plant tissues.

Aims:  To evaluate the potential of Actinoplanes campanulatus, Micromonospora chalcea and Streptomyces spiralis endophytic in cucumber roots, to promote plant growth and to protect seedlings and mature plants of cucumber from diseases caused by Pythium aphanidermatum, under greenhouse conditions. Methods and Results:  Three endophytic isolates, out of 29, were selected through tests aimed at understanding their mechanisms of action as biocontrol agents and plant growth promoters. When applied individually or in combination, they significantly promoted plant growth and reduced damping‐off and crown and root rot of cucumber. The combination of the three isolates resulted in significantly better suppression of diseases and plant growth promotion, than where the plants were exposed to individual strains. Conclusions:  The three selected actinomycete isolates colonized cucumber roots endophytically for 8 weeks, promoted plant growth and suppressed pathogenic activities of P. aphanidermatum on seedling and mature cucumber plants. Significance and Impact of the Study:  The results clearly show that the endophytic, glucanase‐producing actinomycetes used, especially as a combined treatment, could replace metalaxyl, which is the currently recommended fungicide for Pythium diseases in the United Arab Emirates. These endophytic isolates also have the potential to perform as plant growth promoters, which is a useful attribute for crop production in nutrient impoverished soils.