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).