Structure, variation and assembly processes of bacterial communities in different niches of tomato rhizosphere under periodic drought and nitrogen addition

Abstract

Root-associated bacteria play a vital role in the growth and adaptation of host plants to drought stress. These bacteria can be classified as rhizoplane and rhizosphere bacteria based on their distance from the root surface. Tomato plants are often exposed to periodic droughts and nitrogen (N) addition throughout their life cycle, but the impact of these factors on the plant and root-associated bacteria is not well understood. To gain insight into this relationship, we conducted a study to monitor the effects of periodic droughts and N addition on tomato plants’ rhizoplane and rhizosphere bacteria. Drought and N addition had interactive effects in affecting plant and soil properties, which varied with the timing of drought. There were clear divergences in community traits such as alpha, beta diversity and network topological features between the two types of bacteria. The rhizoplane bacteria showed lower alpha diversity but higher beta diversity and were more sensitive to drought and N addition than the rhizosphere bacteria. Nitrogen addition could downsize the effects of drought on rhizoplane bacterial community compositions. The higher proximity to the root might induce a community to develop more cooperation between different members to cope with plant metabolites, as revealed by the more connected and modularized community network of the rhizoplane bacteria. Drought at the seedling stage had great legacy effects on plant and soil properties. It may enhance selection, cause the dominance of deterministic processes in the assembly of rhizoplane bacteria, and reduce their community network complexity. In conclusion, N addition could interact with drought in affecting tomato plants and their root-associated bacteria, depending on the timing of drought and the finer root niches. The higher sensitivity of rhizoplane bacteria to drought and N addition calls for more research due to their higher proximity and importance to plants in future environmental changes.