3.1.3  Root exudates as interplant signals
Plant release large amounts of root exudates into the soil where they act as carbon and nitrogen sources or cues for rhizosphere organisms. The profile of root exudates can be affected by biotic and abiotic stimuli. Soil dwelling organisms and roots of neighboring plants use some of these root exudates as a source of information (Bais, 2006, Biedrzycki et al. , 2010, Carvalhais et al. , 2015, Khashi u Rahman, Zhou & Wu, 2019, Sharifi & Ryu, 2017). Plants employ root exudates to discriminate between kin and non-kin neighbor plants. Rice use allantoin as a cue to recognize kin cultivars and respond to it by shifting root biomass allocation and increasing grain yield (Yanget al. , 2018). An allantoin mutant of Arabidopsis was reported to be susceptible to Pseudomonas syringae and Pectobacterium carotovorum . Exogenous application of allantoin also induces JA-responsive genes, such as MYC2 (Table 1) (Takagi, Ishiga, Watanabe, Konishi, Egusa, Akiyoshi, Matsuura, Mori, Hirayama, Kaminaka, Shimada & Sakamoto, 2016). Root exudates such as (–)-loliolide, jasmonic acid and salicylic acid act as interplant signals and plant defense inducers (Cheol Song, Sim, Kim & Ryu, 2016, Kong et al. , 2018b, Liet al. , 2020a). (–)-loliolide from barnyardgrass (Echinochloa crus-galli ) root exudates induces the biosynthesis of the rice allelochemicals momilactone B and tricin (Li et al. , 2020a), which can reduce the disease severity of several rice pathogens, such as Piricularia oryzae , Rhizoctonia solani andFusarium oxysporum (Table 1) (Kong, Xu, Zhang & Zhang, 2010, Zhao, Cheng, Guo, Duan & Che, 2018).