Rice is still the most significant food crop on the planet, which is grown in a wide range of ecological conditions (Gaballah et al., 2021). Rice is one of the most economically significant staple food crops, especially in Asia, where most rice is produced and consumed (Aravind et al., 2015). Paddy rice is important to the country’s economy since it ensures food security, produces rural jobs and generates export earnings (Le, 2016; Xu et al., 2021). In the whole history of agriculture, the last fifty years have seen the highest levels of productivity. Because of the “Green Revolution,” which was made possible by advancements in agricultural science and technology, an estimated 1 billion people have been spared the misery of hunger and even starvation (Xu, 2010). However, weeds are the most serious biotic stresses (Rahaman et al., 2021), which typically outweigh the damages caused by any other kind of agricultural pest, including rodents, insects, diseases, and so on (Abouziena & Haggag, 2016). Indeed, the application of chemicals in controlling these bio-constraints leads to develop herbicide resistance and environmental pollution. Integration of crop allelopathy with existing agronomic practices can provide a more sustainable and novel approach to agricultural production methods (Li et al., 2020).

Allelopathic rice varieties produce and discharge allelochemicals that limit weed development and establishment, and might be included in a component of a combined weed-managing approach (Serra Serra et al., 2021). Allelopathy is an ecological phenomenon that occurs when plants, microbes, allelochemicals, and the environment interact (Abbas et al., 2021). Allelopathy occurs when a plant emits allelochemicals into the agroecosystem that interfere with neighbouring plants’ physiology, germination of seeds, typical growth and development, and longevity (Das et al., 2021). Allelochemicals, the rhizosphere microbiome, and their bio interaction are the heart of this natural weed-control strategy. Allelochemicals have enormous potential to manufacture biocontrol products. Thanks to the search for innovative active ingredients and the development of new manufacturing techniques brought about by the advent of biotechnology (Chaïb et al., 2021). Biological management in agriculture through the use of naturally occurring allelopathic substances has thus become essential and useful. Allelochemicals can be employed as useful techniques for increasing agricultural yields through sustainable weed management (Chou, 1999).

Understanding the link between allelochemicals and certain bacteria might assist to speed up the application of allelopathic qualities in farming. The effects of allelopathy depend on both species and concentration. However, the edaphic environment’s accumulation of allelochemicals affects soil chemistry (Lal et al., 2021). Agricultural crop seed germination, stand establishment, growth, yield, and physiology are all affected by the allelochemicals emitted by weeds (Zohaib et al., 2016). However, the allelochemicals in rice are a helpful tool to defend the growth and development of different weed species in the crop. Momilactones in rice, benzoxanoids in the rye, tabanone in cogongrass, alkaloids and flavonoids in fescue, anthratectone and naphthotectone in the teak (Tectona grandis), sorgoleone in sorghum, caffeic acid and calorigenic acids in sunflower, and abscisic acid beta-d-glucopyranosy have all been found as allelochemicals implicated in weed control (Ferguson et al., 2013). Eventually, plant allelopathy is often a win-win technique for managing weeds in the fields because it is an ecologically sound and resource-saving practice (Li et al., 2020). The microbial decomposition of rice husks produces hazardous chemicals such as p-coumaric acid, p-hydroxybenzoic acid, syringic acid, vanillic acid, ferulic acid, and o-hydroxy phenylacetic acid (Amb and Ahluwalia, 2016). Eventually, allelopathic rice cultivars may create and release allelochemicals that prevent paddy weeds from growing and establishing, making them an effective part of integrated weed control (Serra Serra et al., 2021). This review article exclusively focuses on different aspects of allelopathy especially in rice to help farmers for sustainable rice farming.

A thorough literature review revealed that rice has appreciable potential as an allelopathic crop, however, there is no comprehensive review published earlier in the literature covering multiple allelopathic aspects of this important food crop. In order to bridge the gap of information this review is aimed to provide a deep insight into the allelochemicals in various traits of rice, techniques of investigating allelopathic attributes, modes of action and pathways of allelochemicals production. Moreover, different factors influencing the production of rice allelochemicals along with the significance of rice allelopathy in sustainable agriculture are covered. Understanding these biological phenomena may thus aid in the development of new and novel weed-control tactics while allowing farmers to manage weeds in an environmentally friendly manner.

The major allelochemicals in rice are presented in Table 1 . Allelochemicals in rice have been divided into two categories; phenolic acids belong to one category, whereas terpenoids and flavonoids belong to the other (Li et al., 2020). Allelochemicals from rice exudates include phenolics, fatty acids, benzoxazinoids, and terpenoids. The compounds present in momilactone that inhibits weed growth are a diterpenoid flavone (5,7,4′-trihydroxy-3′,5′- dimethoxyflavone (Tricin)); a cyclohexenone flavone (5,7,4′-trihydroxy-3′,5′- dimethoxy flavone); and a cyclohexanone (3- isopropyl-5-acetoxycyclohex-2-enone) (Iqbal et al., 2019). Chung et al. (2002) noted that the husk extracts of the rice variety, Janganbyeo contained nine compounds, including salicylic acid, which had the highest inhibiting impact on the total seedling length and dry weight of barnyard grass.

The pathway of the release of allelochemicals in nature is shown in Figure 1 . Volatile chemicals are released from live plant parts, water-soluble toxins are leached from above-ground portions in response to rain, and others are exuded from below-ground parts (Nega and Gudeta, 2019).

Figure 1 illustrates the procedure of releasing allelochemicals from the donor plant, then retention, transport or transformation in the soil, and finally how these go to the receiver plant. Therefore, water-soluble allelochemicals from the leaves and plant body surfaces are washed by rain, dew, mist etc. and come to the soil. Some detached leaves on the soil surface are decomposed by the action of microbes and then release allelochemicals which also go to the soil. The living roots and below-ground parts of the donor plants release allelochemicals through exudation. Some dead roots are also decomposed in the soil releasing the allelochemicals. All these four ways are the pathways of releasing allelochemicals from the allelopathic plants. After accumulating in the soil, these may retain in soils, or transform into other compounds before being reached to receptor plant, or it goes directly to the receptor plants.

During the biosynthesis of allelochemicals in rice, it may follow any of the three routes, i) the shikimic acid, ii) the acetic acid pathway and iii) the mevalonic acid (M.V.A.) pathways (Zhang et al., 2019). The shikimic acid pathway yields phenolic compounds and flavonoids; the acetic acid pathway generates fatty acids and flavonoids; the mevalonic acid (M.V.A.) pathway produces terpenoids and steroids; the shikimic acid and acetic acid pathways combinedly make flavonoids (Buchanan et al., 2015). The biosynthesis of major allelochemicals is presented in Figure 2 .

The factors which influence the development of allelochemicals in plants can be categorized as a) genetic factors e.g., species of crop, variety of crop, the growth stage of the crop, exponential growing character etc., which are regulated by different genes, and b) environmental factors e.g. Temperature, UV radiation, drought stress, pH level, damages caused by insects, disease pressure, nutrient deficiency etc.

Advanced agricultural activities such as the application of excessive fertilizers, herbicides, fungicides and nematicides etc., have been damaging the soil’s physical and chemical qualities and polluting the soil and water. According to Peterson et al. (Peterson et al., 2018), 51 weed species became common in rice fields as a consequence of extensive pesticide use, which led to the development of herbicide resistance. For the sake of all living things, including humans, natural resources must be conserved and used wisely to preserve the long-term viability of the planet (Chou, 1999).

Allelochemicals found naturally in plants are a promising source of ecological pesticides because they help plants tolerate, suffer, or adapt to insect pest stress. Nowadays, nanotechnology has been contributing to the development of bio-herbicides using allelochemicals. Using allelochemicals to create nanoparticles, nanoemulsions, and nanocapsules and using them as nano-herbicides opens up new possibilities for weed management. The controlled release of nano-herbicides at their smallest effective concentration guarantees the least harmful impact on target field crops and that their bioavailability is maintained (Das et al., 2021). Weed management in farming systems may benefit from the development of allelochemicals-based herbicides. An allelochemicals-based herbicide made of benzothiazine derivatives was patented by Zhao et al. (2019).

By producing secondary metabolites as signalling molecules, allelopathy begins communication between plants and microbes in both agricultural and natural habitats. This ultimately affects the growth and development of nearby plants. Allelochemicals produced by these crops control weeds while simultaneously boosting microbial activity underground, demonstrating their allelopathic potential (Jabran et al., 2015). When allelochemicals or allelopathic plants were present, crop development and soil microorganisms had considerable connections. In other words, allelopathic plant species, important mediators, and soil organisms have all been found as significant indicators of how allelopathic interactions between plants will take place (Zeng, 2014).

The allelopathic potential of crop plants contributes to cultivars’ capacity to control weeds. Rice allelopathy has thoroughly been investigated, and numerous rice cultivars have been discovered that limited the development of different weed species when cultivated together (Le Thi et al., 2014). Although it is clear that phytotoxins created during the decomposition of rice stubble in the soil are the primary cause of the drop in the yield of the next rice crop, rice plants also have grown successfully for nearly a century with little loss in production. This suggests that rice plants have evolved adaptive strategies to avoid a severe autotoxic response (Chou, 1999). The development of herbicide resistance is a common occurrence as a result of the application of the same pesticide regularly (El-Naby et al., 2019). Therefore, allelopathic rice cultivars should be cultivated to resist these unwanted plants. In a study, it has been observed that the reduction in grain yield of allelopathic rice accessions was roughly 37% in barnyard-grass infested plots, contrary to 60 to 68% in non-allelopathic varieties (Dilday et al., 2001).

Another method of managing weeds is to extract allelochemicals from rice straw (Oryza sativa L.) and use them as biocides (Ho et al., 2021). In one study it was exhibited that rice straw extracts suppressed radicle growth in lettuce (Lactuca sativa), rice (O. sativa), and barnyard grass (Chung et al., 2001). The allelochemicals rho-Coumaric acid dramatically decreased the germination of lettuce seedlings at a dosage of 1 mM. Rho-coumaric acid, on the other hand, was only effective against barnyard grass at concentrations larger than three mM (Rimando et al., 2001).

In the field, rice cultivars with allelopathic activity produced 2–3 times as much root biomass as cultivars with non-allelopathic activity. Rice cultivars with intermediate maturities had a higher proportion of inhibition. Rice cultivars with allelopathic activity had 2-3 times more root biomass than cultivars without allelopathic activity in the field. In comparison to early maturing (50.2%) and late maturing (56.1%), intermediate maturing rice cultivars showed a higher percentage of inhibition (59.3%). Dehulled rice had a stronger inhibitory impact than hulled rice, rice with coloured hulls was inhibited by 55.9%, while colourless hulls were inhibited by 65.4%. Some traditional varieties such as Siam, Jambok, Wangi, and some newer types such as MR77 and MR84 were shown to have considerable allelopathic activity on lettuce and barnyard grass seedlings (Azmi et al., 2000; Mazid et al., 2018). The first allelopathic rice released for commercial application in China in 2009 is Huagan-3 (Kong et al., 2011) and Rondo is an Americal rice cultivar with substantial weed suppressive ability and high yield potential as well (Gealy and Yan, 2012).

Allelochemicals primarily affect photosynthesis by interfering with photosystem II activity (Wang et al., 2014). Allelochemicals reduce chlorophyll levels, produce free radicals, limit enzyme activity, and alter target plants’ cell membranes and structure, all of which stop weeds from developing (Ghanizadeh et al., 2014). ( Figure 4 ) depicts the mode of action of allelochemicals explaining the mechanisms of allelopathy.

The molecular research on allelopathy started many days ago. In 1996, the International Rice Research Institute (IRRI) started a research program on genetic monitoring of allelopathy and genes implicated in chromosomal allelopathy in rice, as well as an analysis of quantitative properties of allelopathy as a primary tool of genetic research (Courtois and Olofsdotter, 1998). These breeding programs for creating genetic variability in rice varieties might be a viable strategy for improving paddy in weed management (Kong et al., 2011). On the other hand, Jansen and Nap (2001) used DNA markers to map gene expression and to classify allelopathic-linked epistatic quantitative trait loci (QTL).

Researchers have made significant progress in discovering genes that code for momilactones in rice (Xu et al., 2004). Lee et al. (2005) identified QTLs on chromosomes 1, 2, 3, 4, 5, 8, 9, and 12 that regulate the allelopathic effects of rice against E. crus-galli. The most common allelopathic QTLs were found on chromosomes 1 and 5, accounting for 36.5% of overall phenotypic variance. Two methods for producing more allelopathic crops have been proposed modulating gene expression associated with allelochemicals production and inserting genes into non-allelopathic crops for generating allelochemicals (Duke et al., 2001). Allelopathic traits related to various chromosomal regions have also been observed in other crops, such as wheat (Olofsdotter, 1998).

The rice varieties, MR439 and MR164 among 10 simple-sequence repeats (SSR) markers investigated were the rice genotypes with substantial allelopathic activity (El-Denary et al. (2016). The varieties showed polymorphic DNA patterns, and the primer RM164 exhibited a band with a molecular weight of 296bp in all genotypes and had strong allelopathic activity. In another research, SSR227 was found useful in selecting allelopathic rice using the marker-assisted selection method (El-Naby et al., 2019). It was considered the most superior and prospective genetic material for effective weed control and producing the highest yield (El-Denary et al., 2016). DNA marker-assisted selection has recently been proved as a tool for finding QTLs responsible for allelochemicals production in a range of crops, including rice and rapeseed (Shehzad and Okuno, 2020). In a QTL analysis study, Chung et al. (2020) discovered a QTL on chromosome 8 that limits shoot growth and overall length. All this information emphasizes genetic upgrading of allelopathic features as a necessary pre-requisite for using rice allelopathy for sustainable weed management in rice fields (Chen et al., 2008). Jansen and Nap (2001) noted that there was no considerable association between the root morphology and their allelopathic potential, suggesting that allelopathy in rice was under genetic control irrespective of root morphology. From the results of another study on chromatin immunoprecipitation and HiSeq data, it was confirmed that the gene OsMYB57 transcriptionally controlled a mitogen-activated protein kinase, OsMAPK11 and this OsMAPK11 was interrelated with OsPAL2, which is the gene responsible for rice allelopathy. Therefore, the gene OsMYB57 improved rice allelopathy, which allowed genetic engineering for improving weed control in rice. Furthermore, increasing OsMYB57 expression in rice using the transcription activator VP64 resulted in stronger inhibitory effects against barnyard grass (Fang et al., 2020). Molecular mechanisms of allelopathy, soil dynamics in crop weed competition, genetic mapping of QTLs, and knowledge of the mechanism of action of allelochemicals are all related to the development of allelopathic crop types (Palanivel et al., 2021).

Different methods and techniques have been used by researchers to detect the allelopathic potential in plants. Rice bioassay is a simple and efficient tool for determining the allelopathic potential of rice cultivars during the earliest stages of allelopathic research (Khanh et al., 2007). The stair-step method, hydroponic culture test, relay-seeding technique, agar medium test, cluster analysis using HPLC, and water extract method all are examples of techniques to screen rice for allelopathic potential (Maqbool et al., 2013). A short description of different techniques with their references is given below in Table 2 .

Sustainable agriculture involves optimizing farm resources while relying on the basic minimum of purchased inputs. It also aims to reduce the impact of agricultural activities outside of farm limits. The main goal of sustainable agriculture is to decrease pesticide use and the need to focus on developing alternative weed control strategies that are low-cost, environmentally safe, and long-lasting (Sathishkumar et al., 2020). To attain the goal of sustainable agriculture, plant breeding, soil fertility and tillage, crop protection, and cropping methods must all be thoroughly explored and utilized (Chou, 1999). Substantial alternative technologies are required to protect our planet from being polluted and to support the increased global population in recent decades (Wezel et al., 2014). Even though agricultural yield varies widely worldwide, herbicides have become a common approach for weed control in most places (Peterson et al., 2018). However, the herbicide use should be in a way to reduce the detrimental effects on the crop and the agroecosystem. For example, the benzothiazine derivative is an allelochemicals-based herbicide that is eco-sustainable (Kong et al., 2019).

Weeds infestation in rice fields is a great concern for the people behind rice. Although a good number of chemical herbicides can control the weeds effectively in rice fields; which lead to environmental degradation. Rice allelopathy in this regard is one of the best options for environmentally friendly weed management tactics in rice. The development of allelochemicals-based bio-herbicides and the development of allelopathic rice varieties through breeding techniques might be very useful for weed control in sustainable agricultural systems. At long last, using allelopathic rice variety or introgression of the gene responsible for an allelopathic trait may open a new window for the farmers to get the desired output and help to develop smart agriculture.

Conceptualization, ASJ. Methodology, AJ, MR, MU and LH. Validation, ASJ, MR, MU, and FR. Formal analysis, FR. SM, and HB. Investigation, FR and HB. Resources, FR and AJ. Data curation, FR. Writing—original draft preparation. Editing, RK, ASJ, MR, MU, LH, AC, OY, AH, and BYR. Visualization, FR, MA, and MI. Supervision, AJ, MR, and MU. Project administration, AJ, and FR. All authors have read and agreed to the final version of the manuscript. All authors contributed to the article and approved the submitted version.