Suresh Malhotra

The rhizosphere is a dynamic environment in which multiple microbial activities elicit phenotypical, physiological, and molecular crop responses. For a better understanding of the rhizosphere microbiome, researchers are utilizing next-generation sequencing to focus on microbiome regulations with an emphasis on multi-functional microbes. There are two main concepts currently being focused on: identifying microbial antagonists (between beneficial microbes and plant pathogens) from predominant stocks of plant-growth-promoting microbes, preferably with an aim towards bioprospecting soil-plant health; and secondly, developing a more microbially active rhizosphere through a process called rhizosphere hybridization (RH). The present review is focused on some recent studies on the outcome of RH in citrus cultivars, showing renewed functional corridors of the rhizosphere characterized by secondary metabolites providing a load-supporting functional dichotomy through elevated nutrient-supply, activated soil enzyme profiles, and improvements in root- shoot systems and plant defense enzymes. These response trade-offs collectively contributed to higher quality yield coupled with possibly a better shelf life of fruits. The rhizobiome of heritage trees viz., Azadirachta, Ficus, Dendrocalamus, Populus, Sasa, Acer, Alnus, Quercus, and Phyllostachys, could be effectively used in exercising RH. These observations on RH mean the concept could be expanded in other fruit crops, with an emphasis on developing a robust holobiont (climate-smart suppressive soils and engineering rhizosphere microbiomes for microbially engineered plants) as a part of regenerative agriculture.

Fruits crops by the virtue of their nutritional qualities have already emerged as a major alternative, cutting short the menacing load on the consumption of traditional monotonous cereal/tuber crop-based diet. Huge microbial diversity has displayed different magnitude of synergism with fruit crops, which played a catalytic role in unlocking the productivity stagnation through improved efficacy of applied nutrients. Perennial fruit crops are better equipped to be benefitted through nutrient-microbe synergy because of their perennial framework and root configuration. However, use of multiple inoculation through crop specific microbial consortium, especially AM-based consortium in combination with nutrients (organic or inorganic in nature) provided a much better option in fruits with an added element of much better labile pool of microbial (taxonomic, function, and metabolic diversity) and nutrient pool of the rhizosphere for stronger soil carbon sink ultimately. The concept of “rhizosphere hybridization” is, therefore, advocated to harness the value added benefit of nutrient -microbe synergy, besides providing dynamism to microbial consortium suiting to wide range of perennial fruits. Microbial consortium augers well, with fertigation option as well, as a pretreatment of soil before injecting soluble mineral fertilizers into the wetting zone of drippers in order to improve upon the fertilizer use efficiency.