Cereals are the most planted crops and common person’s food all over the world. Maize provides vitamins, starch, fiber, protein, and sugar; 100 g of maize grains contains 361 calories. Maize at its vegetative stage is also used as animal feed/fodder and provides the chief source of energy for livestock and poultry feeding (Arain, 2013). Due to the rapid increase in the world’s population, the food demand has increased and hence cereals production needs to be increased with every passing day. However, the annual yield and productivity of cereals are declining in the developing world mainly in Asia. The use of balanced inputs (fertilizers, pesticides, herbicides) in the recent era is the key to gaining maximum production of cereals (Pingali and Heisey, 2001). The reason behind the lower production of maize in Pakistan includes water scarcity, non-availability at critical stages and the rising cost of chemical (fertilizers and pesticides), and the availability of certified hybrid seed in the market. Moreover, climate change is causing a 15-20% decline in the production of hybrid varieties to their actual potential (Ayub et al., 2021). Furthermore, climate change is posing serious threats to crop production in developing countries like Pakistan and shifting them from food surplus to food dearth countries (Abbas, 2022).

Imbalanced utilization of macronutrients, diminished use of organic fertilizers and natural manure, decreased incorporation of plant residues, and exhaustive maize yield in the previous decade have caused micronutrient inadequacies in the soils of Pakistan (Bashir et al., 2021). The greater parts of the soil in maize growing territories of Pakistan have turned out to be inadequate in Zn and Fe. Microorganisms in combination with chemical and organic fertilizers are increasing the uptake of micronutrients, which results in a higher concentration of micronutrients particularly Fe and Zn in plants (Steven, 1991; Mishra et al., 2004; Shivay et al., 2010). The nitrogen requirements of maize are increasing to get higher yields of maize. Balanced nitrogen application along with other macro/micronutrients is important for acquiring efficient grain yield and quality (Zebrath et al., 2009). Nitrogen deficiency is the most significant yield-limiting factor for grain crops (Shah et al., 2003). The use of phosphorus alongside Rhizobium inoculation improves root and plant development and ultimately yield of crops (Gentili and Huss-Danell, 2003; Fatima et al., 2007). Qualities of oat, for example, grain yield, chlorophyll, and protein contents were improved when PGPR were associated with roots (Tawfiq and Ahmad, 2014). Bio-fertilizers are helpful in the incorporation of microbes in the soil that colonize plant roots and expedite plant development by various mechanisms (Glick, 1995). The utilization of PGPR is consistently expanding in agribusiness because it enhances fertilizer use efficiency, decreases fertilizer requirements for crops, and in addition utilization of synthetic pesticides and different agrochemicals (Rana et al., 2012). The PGPR can enhance the uptake and micronutrient biofortification (N, P, Fe, Zn, and Cu) in cereal grains, through nutrient solubilization, siderophores, and exopolysaccharides production (Jalal et al., 2021; Shahane and Shivay, 2022). Biofortification through rhizobacteria has gained popularity to improve Zn and other micronutrient contents in grain crops. For instance, Gopalakrishnan et al. (2016) revealed the species from the genera Pseudomonas, Brevibacterium, Bacillus, Enterobacter, and Acinetobacter as potential candidates for biofortification and biocontrol in plants. These microorganisms also can incorporate micronutrients inside eatable plant tissues through solubilization of their indigenous insoluble sources present in the soil (Khalid et al., 2015; Gopalakrishnan et al., 2016). The Consultative Group on International Agricultural Research (CGIAR) has reported the hereditarian potential for increment in bioavailable Fe and Zn contents in grain crops for example; rice, wheat, and maize after harvest (Cakmak et al., 2010; de Santiago et al., 2011; Galindo et al., 2021).

The utilization of PGPR for biofortification not just encourages us to manage the issue of malnutrition among the population, in addition, improves cereals yield, soil fertility, and biodiversity (Bouis et al., 2003; White and Broadley, 2005; Ditta et al., 2022; Jalal et al., 2022). The soil microbial community is a solid pole to assess soil richness and directs the accessibility of supplements to harvest plants. Any change in the microbial community in this manner can influence the supplement take-up by harvest plants (Nuttall et al., 2017). The presence of Zn in the soil is not an issue in soils but its availability to plants remains an issue throughout the growing season of crops and hence lowers their fortification and fertility status of the soil (Sharma et al., 2013; Majeed et al., 2022). However, the Zn deficiency in Pakistani soils is mainly due to the calcareous and basic nature of our soils. The applied Zn solubilizing PGPR populations help in this regard to solubilize Zn in plant-available form and its uptake by plants (Rana et al., 2012). These microbes help plants to take up more Zn from the soil through Zn solubilization through organic acids secretions (Hussain et al., 2015) and expanding the surface area of roots through the production of auxins in the rhizosphere. Maize is the staple diet of most of the population of the world including Pakistan. Keeping in view the discussion above, it can be hypothesized that the sole and combined application of PGPR may have the potential to improve maize yield and quality. The objective of the present investigation is to biofortify maize by pre-isolated and characterized Zn solubilizing PGPR strains without any harmful effects on the environment from chemical fertilizer application and to cope with nutritional insecurity.

To evaluate the effectiveness of a novel bioinoculant for the biofortification of maize, a pot experiment and two field experiments were conducted at a wirehouse and research area of the Department of Soil Science, the Islamia University of Bahawalpur, respectively. The pre-identified and compatible PGPR strains ZM27 (KX788859), ZM31 (KX788860), ZM63 (KX788861), and S10 (KX788862) and their possible combination were tested in pot and field trials to determine the role of these bacterial strains in zinc and iron uptake and biofortification in maize grin to fulfill the nutritional requirement of zinc of the burgeoning population through natural sources.

The current investigation revealed the role of Zn solubilizing PGPR and their sole, co, and consortium application on maize growth, yield, and micronutrients (Fe and Zn) biofortification. The results of the present study showed that all methods of biofertilizer application (sole, co, and consortium) increase the growth, yield, and nutrient status of the maize but the most promising results were obtained from the consortium (ZM31+ZM63+S10) application. The microbial inoculation of maize with PGPR strains i.e. Bacillus subtilis. ZM63, Bacillus aryabhattai. ZM31, Bacillus aryabhattai. S10 and Paenibacillus polymyxa. ZM27 has increased plant development, just as improving mineral nutrients in shoots and grains of maize. A positive relationship between plant biomass and nutrient content (N, P, Fe) was also recorded.

The role of microbial inoculants in enhancing maize growth and biofortification has already been described the microbial community does so through the degradation of organic matter and releasing entrapped nutrients which add up to the soil fertility status, and in turn, is responsible for improved plant growth (McDonagh et al., 1993; Phoomthaisong et al., 2003; Zou et al., 2019). The microorganisms solubilize the mineral nutrients by releasing organic acids into the rhizosphere and reducing the microsite pH suitable for micronutrients availability (Fe and Zn) and bio-fortify maize (Hussain et al., 2019; Merinero et al., 2022). Moreover, the increase in plant height, and shoot fresh and dry biomass of maize with inoculation of PGPR in the present study were at par with Khalid et al. (2004) and Pereira et al. (2020) and they conclude that the improvement was due to the growth hormones related substance produced by PGPR and better nutrient status of maize. The improvement in plant height, and shoot and root dry biomasses might be due to the enhanced nutrient uptake and partitioning especially higher nitrogen and phosphorus are responsible for growth and development at the early stage of the maize (Aquino et al., 2021). Pearson’s correlation of the growth parameters and nutrient contents of maize in pot trial and field is presented in supplementary data ( Tables S2-S4 ) and also revealed that there was a positive correlation between the mineral nutrients uptake and maize growth parameters, which justifies the increased growth of maize attributed to the more mineral uptake.

Our previous investigations Ahmad et al. (2014); Mumtaz et al. (2017); Hussain et al. (2020), and Zahir et al. (2011) also advocated the role of the PGPR in enhancing the growth of different crops by enhancing the uptake of the entrapped nutrients through the release of organic acids. Moreover, various other studies also described the role of biofertilizers in improving the vegetative growth of different crops by enhancing nutrient uptake of crops and their partitioning among the different plant parts (Samavat et al., 2012), and biofortify micronutrients to cope with food insecurity and malnutrition (Ahmad et al., 2008; Farooq et al., 2009; Zafar-ul-Hye et al., 2013). Other growth-promoting traits such as auxin (IAA) production by these microbes is responsible for better root infrastructure development and proliferation in the soil, which is a possible reason for root biomass production in the present experiment (Zeb et al., 2018; Ahmad et al., 2019).

The increase in micronutrients (Fe and Zn) was significant under consortium application as compared to the sole and co-inoculation of the PGPR. The result of the present study is in line with Khan et al. (2019) for Zn biofortification while at par with Mumtaz et al. (2022) for Fe biofortification in maize grains. The higher uptake of the described micronutrients was due to the solubilization of these fixed micronutrients by the application of microorganisms, which are responsible for excreting acidic substances (organic acids) in the maize rhizosphere and lowering the microsite pH of soil. The lowering of pH is responsible for the solubilization of these micronutrients and their uptake in the maize plants and their fortification in maize grain (Sheikh et al., 2022). On the other hand, the microorganisms produce specified compounds like siderophores (Fe-loving compounds) which quench the Fe and the other micronutrients and made them available for plant use and reduce their uptake by pathogens (Singh and Prasanna, 2020). Therefore, the biofortification of Fe and Zn in the present investigation might be due to the production of siderophores by these bacterial strains.

The improvement in vegetative growth is responsible for enhancing crop production and higher outcomes from the same piece of land. The result regarding the grains per cob is slightly higher than Chen et al. (2022) which was due to the better growth of the maize due to optimal nutrition and biofertilizer solubilization of the fixed nutrients in the soil and their translocation to the reproductive organs of maize which is responsible for the increase in size and number of grains of the cobs (Amanullah et al., 2021; Inyang et al., 2021). The 1000-grain weight of the maize in the present experiment was at par with the Magar et al. (2021) which depicted that the grain weight improvement is linked with the optimal nutrition of the maize crop and its partitioning among the grains which results in an increased number of grains per cob and increased grain weight. A similar reason for the higher number of grains and thousand-grain weight has also been reported by many researchers (Naveed et al., 2008; Drury et al., 2017; Anwar et al., 2021; Drulis et al., 2022). The results regarding the yield of the present investigation are in line with the Mumtaz et al. (2022) which were due to the higher nutrient uptake, partitioning, and reproductive growth of maize (Al-Suhaibani et al., 2021; Singh et al., 2021; Ebrahimi Chamani et al., 2022). Pearson’s correlation analysis data between growth, nutrient contents, and yield parameters of maize under Field trials are presented in the supplementary data section as Tables S3 , S4 , which depicted a strong positive correlation between nutrients concentration, growth, and yield improvements in maize by application of these novel strains application under different methods.

It can be concluded that the sole, co, and consortium application of the PGPR significantly increases the growth, development, nutritional status, and yield of maize. The PGPR strains are responsible for solubilizing the essential nutrients in maize nitrogen, phosphorous, iron, zinc, and potassium while we compared them to the non-inoculated set of treatments. The results depicted that the application of the PGPR consortium (ZM27+ZM63+S10) the results was synergistic and caused a significant increase in shoot and root biomasses, nutrient status, and yield of maize when compared to un-inoculated control treatment. So, it is not wrong to say that the best consortium in the present study has the potential to be commercialized as a biofertilizer for biofortification (Fe and Zn) in wheat and sustainable production of maize.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding authors.

Conceptualization, MA and AH. Methodology, MA, ADa, and ML. Software, AH. Validation, AH, ADi, ADa and ML. Formal analysis, MA, ADi, ADa, ML, and ZI. Funding, AE-S. Investigation, MA, AH, ADa, ML, ZI, HA and FN. Resources, AH, ADa, ADi, AE-S, and ML. Data curation, ML and ZI. Writing—original draft preparation, MA. Writing—review and editing, ADa, ADi, AE-S, and ML. Visualization, AH, ZI, ADi, AE-S, and FN. Supervision, MA and AZ. All authors contributed to the article and approved the submitted version.