al. 2015). Furthermore, there are few biomolecules, such as flavonoid, strigolactones, and terpenoids, which assist plants in specifically attracting the beneficial counter‐partner through root signals (Vranova et al. 2013; Lareen et al. 2016; Massalha et al. 2017). Mutualistic interactions, nodulation and mycorrhizal interactions, signal perception and transduction such as receptor‐like kinases (RLKs) are the signal‐based interaction of plant and microbes (Lagunas et al. 2015).
In the consecutive discussion, root excaudate’s role as a messenger that communicates within rhizosphere is examined. This perspective is well defined in root–root communication involved in an allelopathy phenomenon in the rhizosphere, which contributes in agricultural growth, because rhizospheric allelochemicals are protective in nature for plants. Root microbe communication in the rhizosphere may enhance the plant biomass through an increase in nutrition uptake, secretion of phytohormones, and helps in defense of the plant (Robin et al. 2008).
1.4 Applications of Root Exudation
Root exudate varies across the plant genotype and changes in root exudates of plant can be utilized in the breeding program for the enhancement of nutritional interaction among plants and microbes. For targeted breeding, perspective profiling of root exudates is essential for sustainable agriculture (Kuijken et al. 2015). This phenomenon is well‐known through exudate profiles among 19 natural Arabidopsis accessions, which shows a high variation in glycosylated and sulfated metabolites, plant hormones, salicylic acid catabolites, phenylpropanoids, coumarin scopoletin, and polyamine derivatives (Monchgesang et al. 2016). Similar studies were done and demonstrated about the rhizobacterial community composition influenced by varying exudation profiles (Micallef et al. 2009). Root exudate chemical composition changes in the condition of nutrient deprivation, which can further assist said breeding program. Few studies represent the effect of nutrient deficiency in the exudates, and the effect of phosphate limitation investigated in Arabidopsis results in the high abundance of oligolignols, which is responsible for lignifications and coumarins in low quantity (Ziegler et al. 2016). Particularly Fe‐deficient strawberry root exudates show a high content of dehydroascorbic acid, trans ferulic acid, galactonic acid, sucrose, and thymidine, whereas P‐deficient strawberry root exudates show higher concentration of malic acid, lysine, galactaricacid, butylamine, and simultaneously show a low concentration of ribonic acid, sorbitol 6 phosphate, and proline (Valentinuzzi et al. 2015).
In the environment, plants are exposed to all kinds of friendly and unfriendly microbes with several stresses in the ground. Plant secretion called root exudate has a lot of potential from the perspective of defense. Root exudates possess antimicrobial chemicals, such as phytoanticipins, diterpene rhizathalene A, phytoalexins, phenylpropanoids, t‐cinnamic acids, momilacton A, rosmarinic acids, terpenoids, benzoxazinoids, and the defense signaling molecules, salicylic acid, nitric oxide, and methyl jasmonate, which assist in the defense mechanisms of the plant. In addition, these chemical phenolics and terpenoids act as an antimicrobial apart from defense root exudates, which support the stress condition of plants due to their primary and secondary metabolite present in root exudates (Table 1.2).
Plant root exudation indirectly controls resource competition by altering soil chemistry, soil process, and microbial populations, and thus has an important function in plant development. Root exudation possesses the capacity to alter the soil nutrient availability by changing the soil property in the aspect of its chemistry and biology. Root exudate releases mucilaginous substance from root tip of the plant and is a part of exudation quite essential to maintain the water potential (Susan 2018). Root exudate releases plant carbon compounds (border cells and exudates) and primary metabolites into the rhizospheric soil (Canarini et al. 2019). It is important to understand root‐mediated communication between plants and other organisms, which assists in the enhancement of agricultural production and can be useful in reduction in the demand for chemical fertilizer, such as in legume plants.
1.5 Conclusion and Future Prospects
Today's main concern of agriculture is the quality of productivity in order to feed the world population. For more production and protection of crops, farmers use chemicals that directly or indirectly effects the fertility of the soil as well as the consumers of the crop. To alleviate the chemical inputs in the agriculture requires detailed information about root exudation and its influence on plant development through soil condition modulation in terms of both biotic as well as abiotic components. Production of the crop depends on plant genotype and soil ecosystem. Root exudate is the key factor in the rhizospheric interaction due to nutrient assets. Researchers should emphasize root exudates, through profiling and their impact on microbes and soil in the rhizosphere for more exploration of root exudate. Still there are several unexplored chemicals of root exudates that can create magnificent changes in the soil and plants in terms of fertility, defense, growth, and yield. A few relevant and essential components of root exudates can be used for developing variety through genetic engineering and breeding, which assists in a sustainable agriculture system. Several chemical inducers should be explored, which increases the quantity of root exudates for better performance of the rhizosphere. In conclusional, in revealing root exudation secrets, we can support economical agricultural productivity through achievement of environmental sustainability.
Table 1.2 Developments in root exudate studies and their action mechanism.
| S. No. | Root exudate component(s) | Plant system under study | Mechanism | References |
|---|---|---|---|---|
| 1 | Terpenoid class of compound: strigolactone | — | Recruitment of fungal species and establishment of Arbuscular mycorrhiza | Parniske (2008) |
| 2 | The isoflavones like daidzein, genistein, and coumestrol | Tribe Phaseoleae plants | Induces the nod gene expression in their rhizobial partners | Dakora (2000) |
| 3 | Sugars, sugar acids, amino acids and organic acids | Maize root | Effect of exudate components on the chemosensory systems of Pseudomonas putida KT2440 | Lopez‐Farfan et al. (2019) |
| 4 | A class of indole‐derived plant chemical, benzoxazinoids | Cereal crops | Antifeedant, insecticidal, antimicrobial, and allelopathic activities are related with this exudate component | Wouters et al. (2016) |
| 5 | A phenolic compound luteolin | ‐‐ | Acts as a potent and specific inducer of nodABC gene expression in Rhizobium meliloti. | Caetano‐Anolles et al. (1988) |
| 6 | Catechol and flavonoids catechin, and quercetin | Maize (Zea mays L.) | Silicon‐induced amelioration of aluminum toxicity | Kidd et al. (2001) |
| 7 | Benzoxazinoids, secondary metabolites in grasses | Maize (Zea mays L.) | Effects the interaction between maize and Pseudomonas putida KT2440. | Neal et al. (2012) |