Host developmental stage is associated with shifts in the exosphere microbiome of urban-farmed Asian green leafy vegetables

Green leafy vegetables (GLV’s) comprise a significant part of urban South-East Asian (SEA) diets and are intensively farmed in SEA cities, including Singapore. Urban farming practices and urban-adapted vegetable cultivars likely select for specific above- and below-ground microbial communities – microbiomes – that assemble in close proximity to the plant tissues – the exosphere. A healthy exosphere microbiome is important for plant growth and safe human consumption. Using 16S rDNA gene amplicon sequencing compositional analyses, we show here that the exosphere microbiome of two commonly-consumed GLV’s – Choy Sum ( Brassica oleracea Alboglabra Group ) and Gai Lan ( Brassica chinensis var. parachinensis ) – dominated by Gammaproteobacteria, Alphaproteobacteria, Bacteroidia and Actinobacteria . Shifts in exosphere microbiome composition were strongly associated with plant developmental stage. Finally, microbial taxa consistently detected in the exosphere comprise a small subset, which are predicted to harbour plant-beneficial traits. Significance Among plant crops, GLVs form an integral part of the Asian diet, especially so in Southeast Asia. Some of these GLVs have short life-cycles (∽30-45 days), which makes them suitable for urban farms in terms of cost advantage as short cycle crops are preferred in urban farms. From a food-security perspective, GLVs forms an important target food group and efforts are being made to increase its productivity to meet the increasing food demands. Current farming practices often place lot of importance on chemical fertilizers and nutrient inputs to improve the fertility of non-arable urban lands to increase the crop productivity. Furthermore, farms in urban settings are also associated with anthropogenic inputs and eutrophic conditions. These together, contribute to negative environmental externalities questioning the sustainability and eco-sustenance of urban farming. Microbial based management systems can not only resolve these challenging issues, but can also enhance plant growth, nutrient use efficiency and disease tolerance. However, their use as microbial adjuncts to agricultural practices is currently limited in urban environments, which could possibly be due to the restricted knowledge-base on these urban phytobiomes.