Certain group of PGPR including fluorescent pseudomonads and other organisms
is known for biocontrol and protection of a range of crop plants from many
pathogens. Irrespective of antibiotic production, these PGPR elicit ISR in the host
and allow plants to withstand pathogens attacking leaves/roots (Ongena et al. 2004).
The different strains of Pseudomonas spp. (Pseudomonas fluorescens WCS 417r,
Pseudomonas thivervalensis and Pseudomonas fluorescens CHA0) primed
Arabidopsis thaliana, in which the plant reacted more rapidly and sturdily to
pathogen attack, as part of facilitated ISR with JA/ET inducible defensive pathway
(Verhagen et al. 2004). The PGPR, Pseudomonas fluorescens GRP3 promotes ISR
in rice against sheath blight (Pathak et al. 2004). In P. fluorescens GRP3,
rhamnolipids are considered an important determinant of biocontrol, with plant
growth promoting and anti-mycelial activities (along with lysis of zoospore plasma
membrane) against Pythium and Phytophthora caused damping-off in chili and
tomato (Sharma et al. 2007). Molecular characterization of rhamnolipids in strain
GRP3 revealed presence of a number of mono- and di-rhamnolipids that include
rhamnose (Rha)–C8–C10, Rha–C10–C8, Rha–C10–C10, Rha–C10–C12:1, Rha–
C10–C12, Rha–Rha–C8–C10, Rha–Rha C10–C10, Rha–Rha–C10–C10:1, Rha–
Rha–C10–C12, Rha–Rha–C10–C12:1, Rha–Rha–C12–C12:1, and Rha–Rha–
C12–C12. Furthermore, strain GRP3 effectively increased shoot length and
activities of ISR responsive proteins peroxidase and phenylalanine ammonia lyase
(PAL) involved in active lignification. Besides, PGPR produce a complex mixture of
volatiles that can kindle plant growth, stimulate ISR for disease suppression, or
antagonize phytopathogens, nematodes, and insects (Ryu et al. 2004; Vespermann
et al. 2007; Kai et al. 2009; Farag et al. 2013).
AMF, another potential group which is ubiquitous in natural and agricultural
terrestrial ecosystems, is an economically and ecologically important group of
symbiotic fungi that provide varied benefits to plants including enhanced phosphorus nutrition and tolerance towards metal toxicity and drought. Besides, AMF are
considered important BCA of pathogens in the natural agriculture systems for
Aphanomyces, Fusarium, Phytophthora, and Sclerotium (Zambolim and Schenck
1983; Rosendahl 1985; Mark and Cassells 1996; Cordier et al. 1998). The several
mechanisms explained for biocontrol by AMF include stress alleviation, along with
alterations of rhizosphere, root system, nutrient availability status, other biochemical
and anatomical aspects of plants cells, and induction of ISR (Singh and Giri 2017).
In addition, isolates of Trichoderma have the ability to induce plant growth by direct
and indirect mechanisms, enhance photosynthetic activity, and to reduce disease
severity in plants by resilient antagonistic and mycoparasitic effects against
phytopathogens, and inducing systemic resistance by releasing proteins and secondary metabolites (Keswani et al. 2016; Zachow et al. 2016). Moreover, endophytes
produce antimicrobial, insecticidal, antioxidant, anti-tumor, and anti-viral
metabolites. For instance, endophytic fungi can produce alkaloid (perfumoid,
phomoenamide,
joxysporidinone,
alantrypinene,
alantryleunone,
anhydrooxysporidinone, and deoxyoxysporidinone) and other cytotoxic compounds
(nidurufin,
sterigmatocystin,
averantin,
11a-methoxycurvularin
10 Microbe-Mediated Biotic Stress Signaling and Resistance Mechanisms in Plants
313
is known for biocontrol and protection of a range of crop plants from many
pathogens. Irrespective of antibiotic production, these PGPR elicit ISR in the host
and allow plants to withstand pathogens attacking leaves/roots (Ongena et al. 2004).
The different strains of Pseudomonas spp. (Pseudomonas fluorescens WCS 417r,
Pseudomonas thivervalensis and Pseudomonas fluorescens CHA0) primed
Arabidopsis thaliana, in which the plant reacted more rapidly and sturdily to
pathogen attack, as part of facilitated ISR with JA/ET inducible defensive pathway
(Verhagen et al. 2004). The PGPR, Pseudomonas fluorescens GRP3 promotes ISR
in rice against sheath blight (Pathak et al. 2004). In P. fluorescens GRP3,
rhamnolipids are considered an important determinant of biocontrol, with plant
growth promoting and anti-mycelial activities (along with lysis of zoospore plasma
membrane) against Pythium and Phytophthora caused damping-off in chili and
tomato (Sharma et al. 2007). Molecular characterization of rhamnolipids in strain
GRP3 revealed presence of a number of mono- and di-rhamnolipids that include
rhamnose (Rha)–C8–C10, Rha–C10–C8, Rha–C10–C10, Rha–C10–C12:1, Rha–
C10–C12, Rha–Rha–C8–C10, Rha–Rha C10–C10, Rha–Rha–C10–C10:1, Rha–
Rha–C10–C12, Rha–Rha–C10–C12:1, Rha–Rha–C12–C12:1, and Rha–Rha–
C12–C12. Furthermore, strain GRP3 effectively increased shoot length and
activities of ISR responsive proteins peroxidase and phenylalanine ammonia lyase
(PAL) involved in active lignification. Besides, PGPR produce a complex mixture of
volatiles that can kindle plant growth, stimulate ISR for disease suppression, or
antagonize phytopathogens, nematodes, and insects (Ryu et al. 2004; Vespermann
et al. 2007; Kai et al. 2009; Farag et al. 2013).
AMF, another potential group which is ubiquitous in natural and agricultural
terrestrial ecosystems, is an economically and ecologically important group of
symbiotic fungi that provide varied benefits to plants including enhanced phosphorus nutrition and tolerance towards metal toxicity and drought. Besides, AMF are
considered important BCA of pathogens in the natural agriculture systems for
Aphanomyces, Fusarium, Phytophthora, and Sclerotium (Zambolim and Schenck
1983; Rosendahl 1985; Mark and Cassells 1996; Cordier et al. 1998). The several
mechanisms explained for biocontrol by AMF include stress alleviation, along with
alterations of rhizosphere, root system, nutrient availability status, other biochemical
and anatomical aspects of plants cells, and induction of ISR (Singh and Giri 2017).
In addition, isolates of Trichoderma have the ability to induce plant growth by direct
and indirect mechanisms, enhance photosynthetic activity, and to reduce disease
severity in plants by resilient antagonistic and mycoparasitic effects against
phytopathogens, and inducing systemic resistance by releasing proteins and secondary metabolites (Keswani et al. 2016; Zachow et al. 2016). Moreover, endophytes
produce antimicrobial, insecticidal, antioxidant, anti-tumor, and anti-viral
metabolites. For instance, endophytic fungi can produce alkaloid (perfumoid,
phomoenamide,
joxysporidinone,
alantrypinene,
alantryleunone,
anhydrooxysporidinone, and deoxyoxysporidinone) and other cytotoxic compounds
(nidurufin,
sterigmatocystin,
averantin,
11a-methoxycurvularin
10 Microbe-Mediated Biotic Stress Signaling and Resistance Mechanisms in Plants
313