Genomic and biochemical insights into millet adaptation: A pathway to climate-resilient agriculture

Introduction

Millets are a group of hardy, drought-resistant crops that have gained significant attention for their potential to contribute to sustainable agriculture, particularly in the face of climate change. These resilient crops are well-suited to thrive in environments that are too harsh for many other cereals, including rice and wheat. One of the most notable features of millets is their short growing period, typically ranging from 3 to 4 months, allowing them to escape the peak periods of environmental stress that longer-growing crops, such as rice and wheat, endure, which typically require 5 to 6 months. This ability to avoid prolonged exposure to adverse conditions gives millets a distinct advantage in regions prone to erratic weather patterns or water scarcity [1] . Millets possess a variety of physiological and morphological characteristics that help them withstand extreme environmental conditions. Their relatively short stature, small leaf size, and thickened cell walls enable them to minimize water loss and reduce the surface area exposed to the elements. Moreover, their deep and robust root systems allow them to access water from deeper soil layers, enhancing drought tolerance. These traits, coupled with their efficiency in nitrogen and water usage, make millets highly adaptable to challenging growing environments.

At the biochemical level, millets benefit from the C4 photosynthetic pathway, which is a key factor in their stress tolerance. This unique photosynthetic process allows millets to utilize carbon dioxide more efficiently by reducing photorespiration, a process that competes with photosynthesis and can reduce plant growth, particularly under hot and dry conditions. The C4 pathway not only enhances the efficiency of carbon fixation but also improves the crop's overall growth and heat tolerance. This is particularly advantageous in environments with high temperatures, where C3 plants like rice and wheat may experience reduced productivity due to increased photorespiration [2] . In addition to the C4 photosynthesis pathway, millets have a remarkable ability to grow and thrive with significantly less water than other major cereals such as maize and wheat. Their water use efficiency is superior, allowing them to survive in regions with limited water availability. This makes millets an ideal crop for regions prone to droughts or irregular rainfall, which are becoming more common due to climate change. The ability of millets to maintain efficient growth, biomass distribution, and heat tolerance further underscores their potential as a valuable crop for developing climate-resilient agricultural systems. Structural and functional characterization of VDAC along with heterologous over-expression in yeast which showed tolerance to several abiotic stresses Overall, the combination of their unique physiological traits and efficient biochemical processes, such as C4 photosynthesis, enables millets to survive and even flourish in environments where other crops might fail. These characteristics position millets as a promising candidate for addressing the challenges posed by climate change, offering a sustainable solution for ensuring food security in a rapidly changing world. As climate conditions become more unpredictable, the cultivation of millets holds great promise for creating agricultural systems that are both resilient and resource-efficient, supporting the need for crops that can withstand the rigors of a warming planet. These traits make millets a valuable crop for developing climate-resilient plants [1,2] .

Millets: Model for Experiment to Stress Tolerance

Drought, heat, salinity, and light, such type of stress can be easily handled by millets, making them as an example of great model to examine what are plants responds to these type of stresses. There are so many results about millets adaptability towards stress in different ways. For instance, Pearl millet changes its flowering time based on rainfall [3] . and that little millet grows longer roots under stress [4] . Specific changes in biochemical aspects like higher antioxidants levels and stress-related proteins, have also been recorded in foxtail millet [5] and teff [6] . Additionally, several genes governing stress tolerance have been discovered in millets (Table 1) [7] .

Foxtail Millet: An Absolute Model for Stress Tolerance

The millet whose study has been done widely is Foxtail millet as compared to other millets, with help of so many genetic engineering tools and techniques developed for it particularly [8] . From the whole genome sequencing, it has been recorded that it contains 1,517 genes from which 586 genes are related to water response only and these help the plant survived dry and drought conditions. Several studies also recognised important genes for the C4 photosynthesis process, with foxtail millet having more genes for malate dehydrogenase (7 genes) and pyruvate orthophosphate dikinase (3 genes) than other crops. From phylogenetic analysis it has been reported that the expression of Ft_CA1 gene is highl in the mesophyll and that can be important for analysing the Foxtail millet C4 pathway [9] . However, the understanding is not clear at molecular level for stress tolerance in millets. There is a need of further research to unfold the factors at genetic level for stress resistance, and screening different millet cultivars can help to explore the complete potential. This will lead to develop more resilient crops through breeding and genetic tools and technologies, which is important to enhance crop's response against climate change.

Genomic Resources in Millets

The knowlwdge of genetic and genomic resources are essential for crop improvement. While genomic resources provide great insights to identify useful genes, alleles, and Quantitative Trait Locis for enhancing crop enhancement but for the breeding purpose genetic resources have major role. There are abundant genetic resources present in millets but genomic resources are limited [10,11] . These genomic resources includes molecular markers and genetic maps are more abundant in major cereals. Integrated marker database and different molecular markers having high throughput have been developed from the genome sequence of foxtail millet [12][13][14][15][16][17][18] . So for the study about large-scale genotyping, resources like genetic diversity analysis and nutritional trait mapping QTLs are the key. These tools have been used in 200 foxtail millet and identified microsatellite markers linked to agronomic traits specifically yield traits, which contributes up to 25% of phenotypic variation. These markers will help in uplifting agronomic as well as nutritional characters in millets [19] Genome-assisted breeding has received a velocity in the direction improvement of crops combining next-generation sequencing with traditional and molecular breeding tools techniques. The gene sequence of Foxtail millet makes a perfect match for Genome-assisted breeding, from which other millets are also getting benifits. Thousands of SSR markers also recorded from the transcript to some analysis of finger millet particularly analysing low and high calcium genotypes [20] . Similarly, SSR markers have been developed for tef [21] . Genome sequencing of pearl millet and finger millet is also ongoing, further expanding genomic resources for crop improvement.

Biochemical changes in finger millet [Eleusine coracana (L.) Gaertn.] under drought stress Biochemical changes Occur in Millets due to stress

An increase in proline content was observed under drought stress compared to the controls in all genotypes of millets. This increase in proline accumulation was more pronounced in drought-sensitive varieties than in drought-tolerant varieties [22] . While proline accumulation has been widely studied in relation to drought stress, its exact role in drought tolerance remains unclear. Some studies have suggested that proline accumulation is a universal symptom of leaf dehydration, indicating stress susceptibility, while others have found that drought-tolerant varieties tend to accumulate more proline than sensitive ones. Despite the variations, the present study suggested that proline may not be a reliable indicator of drought tolerance in the selected genotypes. Additionally, drought stress resulted in an increase in water-soluble carbohydrates (WSCs), particularly sucrose, in the leaves of all genotypes [23] . Sucrose accumulation was significantly higher in sensitive varieties, and the levels varied according to the growth stage and genotype. Other soluble sugars, such as glucose and fructose, were also elevated, likely due to a decrease in respiration rates and the downregulation of glycolysis during drought stress. These sugars, along with raffinose and stachyose, are linked to drought stress tolerance mechanisms, potentially playing a role in mitigating oxidative damage [24] . Drought-induced oxidative stress was reflected by the increased accumulation of malondialdehyde (MDA) in the leaves, a marker of lipid peroxidation. The extent of MDA accumulation was higher in sensitive varieties compared to the drought-tolerant varieties, indicating greater membrane damage in the sensitive varieties under stress. The activities of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), and guaiacol peroxidase (GPX), were elevated under drought conditions, indicating an active defense mechanism against oxidative stress [25] . However, genotype-specific variations in the enzyme activities were observed, with SOD activity being higher in sensitive varieties at certain stages, while CAT activity was higher in tolerant varieties. Different biochemical changes has been shown in Fig 3, 4 & 5 [26] .

Conclusion

Millets have great potential to improve food security and nutrition, especially with rising agricultural costs, climate change, and a growing global population. They are nutritious, provide extra health benefits, need fewer resources to grow, and are naturally resistant to many stresses. These qualities make millets an ideal choice as we face climate challenges. To make the most of millets, we need to study the genetic differences in their varieties and work on improving important traits like yield and nutrition. New technologies like next-generation sequencing (NGS) and genome-wide association studies (GWAS) make it faster and easier to find genes linked to these traits, helping to develop better crops. The growing focus on millets also benefits other crops, like cereals and bioenergy grasses, since they share a common genetic background and have similar genomes.

References

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