including organelles and various membrane compartments [33]. A large number of
effectors accumulate in the plant nucleus [36]. In systemically infected plants,
phytoplasmas secret effector protein that trigger Ca
2+ influx into the sieve elements
detected as early as 24 h, conferring forisome dispersion, callose deposition, and
probably cell wall thickening [37].
2.5
Early Response to Symbiotic Microbes
Beneficial microbes such as soil bacteria, epiphyte bacteria and arbuscular mycorrhiza provides beneficial effects on plant growth or/and stress resistance against plant
pathogen and insect pests [38, 39]. Pathogenic as well as beneficial bacteria are
initially recognized as harmful invaders in order to limit the microbe spread [2]
(Scheme 1). FLS2 in plants can also detect flagellins of beneficial microbes to initiate
plant-induced systemic resistances (ISR). For example, flagellin extracted from plant
growth-promoting rhizobacteria (PGPR) Pseudomonas putida (KT2440) induces
transcriptional and metabolic changes and systemic resistance in maize plants
[40]. Arabidopsis fls2 mutant plant failed to close their stomata when treated with
flagellin from PGPR Bacillus subtilis [41]. In the early symbiotic establishment
between Lotus japonicus and Sinorhizobium meliloti, Flg22 induces defense-caused
inhibition of rhizobial infection and delay nodule organogenesis [42]. Upon interaction of host plant and rhizobia, rhizobia secrete lipochitooligosaccharidic (LCO)
nodulation (Nod) factors (NFs). NF recognition is crucial for the establishment of
symbiosis between a host plant and rhizobia [43]. Indeed, PRR families are predicted
to bind chitin-based molecules, including the bacterial NFs [44]. They all possess
conserved LysM, chitin-binding LysM domains, but the specificity of each receptor
for each chitin oligomer remains unclear.
LysM-PRRs might be involved in the perception of fungal lipochitooligosaccharides (Myc-LCOs), which are symbiosis-mediating signals in the arbuscular
mycorrhiza (AM) [45]. The formation of AM symbiosis is initiated when
strigolactone hormones, secreted from host plant roots that stimulate hyphal
branching and fungal metabolism, fungal short-chain chitin oligomers, as well as
Myc-LCOs, elicit pre-symbiosis responses in the host plant [46]. Fungal LCOs have
a striking structural similarity to rhizobial Nod-factor LCOs. Genome-wide expression studies demonstrated that defined sets of genes were induced by Nod- and
Myc-LCOs, indicating LCO specific in early symbiosis perception [47].
Early in the molecular study of plant genes that regulate the transduction of
signals associated with the presence of a microbe on or in plant tissues, some
genes were found to be required for activating both fungal and bacterial symbiotic
interactions [48]. This pathway called the common symbiotic signaling pathway
(CSSP) has been extensively reviewed in [49]. This pathway includes RLKs and
co-receptor proteins that perceive the presence of rhizobial bacteria or AM fungi and
a series of relay signaling proteins that enter into the nucleus of the plant via
nucleoporins, where they induce regular calcium spiking. Oscillations in Ca
2+
activate a nuclear calciumÀ/calmodulin-dependent protein kinase (CCaMK) that
24 Decrypting Early Perception of Biotic Stress on Plants
583
effectors accumulate in the plant nucleus [36]. In systemically infected plants,
phytoplasmas secret effector protein that trigger Ca
2+ influx into the sieve elements
detected as early as 24 h, conferring forisome dispersion, callose deposition, and
probably cell wall thickening [37].
2.5
Early Response to Symbiotic Microbes
Beneficial microbes such as soil bacteria, epiphyte bacteria and arbuscular mycorrhiza provides beneficial effects on plant growth or/and stress resistance against plant
pathogen and insect pests [38, 39]. Pathogenic as well as beneficial bacteria are
initially recognized as harmful invaders in order to limit the microbe spread [2]
(Scheme 1). FLS2 in plants can also detect flagellins of beneficial microbes to initiate
plant-induced systemic resistances (ISR). For example, flagellin extracted from plant
growth-promoting rhizobacteria (PGPR) Pseudomonas putida (KT2440) induces
transcriptional and metabolic changes and systemic resistance in maize plants
[40]. Arabidopsis fls2 mutant plant failed to close their stomata when treated with
flagellin from PGPR Bacillus subtilis [41]. In the early symbiotic establishment
between Lotus japonicus and Sinorhizobium meliloti, Flg22 induces defense-caused
inhibition of rhizobial infection and delay nodule organogenesis [42]. Upon interaction of host plant and rhizobia, rhizobia secrete lipochitooligosaccharidic (LCO)
nodulation (Nod) factors (NFs). NF recognition is crucial for the establishment of
symbiosis between a host plant and rhizobia [43]. Indeed, PRR families are predicted
to bind chitin-based molecules, including the bacterial NFs [44]. They all possess
conserved LysM, chitin-binding LysM domains, but the specificity of each receptor
for each chitin oligomer remains unclear.
LysM-PRRs might be involved in the perception of fungal lipochitooligosaccharides (Myc-LCOs), which are symbiosis-mediating signals in the arbuscular
mycorrhiza (AM) [45]. The formation of AM symbiosis is initiated when
strigolactone hormones, secreted from host plant roots that stimulate hyphal
branching and fungal metabolism, fungal short-chain chitin oligomers, as well as
Myc-LCOs, elicit pre-symbiosis responses in the host plant [46]. Fungal LCOs have
a striking structural similarity to rhizobial Nod-factor LCOs. Genome-wide expression studies demonstrated that defined sets of genes were induced by Nod- and
Myc-LCOs, indicating LCO specific in early symbiosis perception [47].
Early in the molecular study of plant genes that regulate the transduction of
signals associated with the presence of a microbe on or in plant tissues, some
genes were found to be required for activating both fungal and bacterial symbiotic
interactions [48]. This pathway called the common symbiotic signaling pathway
(CSSP) has been extensively reviewed in [49]. This pathway includes RLKs and
co-receptor proteins that perceive the presence of rhizobial bacteria or AM fungi and
a series of relay signaling proteins that enter into the nucleus of the plant via
nucleoporins, where they induce regular calcium spiking. Oscillations in Ca
2+
activate a nuclear calciumÀ/calmodulin-dependent protein kinase (CCaMK) that
24 Decrypting Early Perception of Biotic Stress on Plants
583