Engineering rhizobacterial communities via functional copolymer hydrogels towards grain production under moisture stress

Ongoing impacts of climatic change on ecosystems remain a global challenge including the effects of drought and climatic warming where they appear as drying seasons in many grain-producing areas of the world. Additionally, these abiotic effects directly influence soil microbial community size and population negatively. Previously we have shown that polymeric hydrogels, able to provide specific interactions with soil microbial communities, enhance the dynamics and selectivity of ingress and colonisation of plant beneficial bacteria at model soil microcosm scales. Here we show that a copolymer hydrogel incorporating lignocellulose (PAA-L) provides enhanced microbial ingress, and continuous wetting pathways when in close contact with developing wheat root systems. Under moisture stress, such osmotic hydrogels maintain moisture in preference to surrounding bulk soil suggesting these wetting films enhance viability of rhizobacterial communities.Sequencing studies, carried out in semi-arid wheat production soils undergoing a season of low average rainfall clearly showed microbial population increases and taxa selectivity in the rhizosphere and PAA-L hydrogel regions of the root zone compared to the bulk soil. Suggesting that PAA-L amendments consistently select their own distinct microbiome and represent nodes (hotspots) of microbial abundance within soils, without affecting the surrounding rhizosphere and bulk soil microbial habitats. Molecular data from wheat field incubations showed the most upregulated prokaryotes associated with PAA-L hydrogels were the Oxalobacteraceae family initially alongside an increased microbial biomass compared to both the bulk and rhizosphere soils. In the longer term other proteobacterial taxa (Burkholderiaceae, Xanthomonadaceae and Sphingobacteriaceae also linked to plant-growth promotion and disease suppression became dominant, suggesting functional redundancy between different rhizobacterial taxa while ensuring a continuous potential plant-growth promotion. In-field test plots during wheat seasons having significant water stress, particularly during grain filling, showed an increase in yield of about 20% with PAA-L addition compared to the no hydrogel control. Addition at a rate of 10-20 kg ha-1 at sowing, representing about one hydrogel granule per seed, increased the wheat yields by about 15% of their average water limited potential wheat yield, compared to no hydrogel amendment. Our results demonstrate that small quantities of functionalised hydrogels specifically situated in close proximity to the developing root zone effectively create extensions to the naturally occurring rhizosphere, which is able to sustain significant beneficial activity during periods of moisture stress. These data clearly suggest that these materials may open new opportunities to maintain grain yields during periods of drying climate.