As Y. Shen et al. [25] extensively reviewed about early signaling responses to
fungal pathogens. Barley cultivars in North America were highly susceptible to the
most devastating barley stem rust disease caused by Puccinia graminis f. sp. tritici
(Pgt). In 1942, new resistant Rpg1 gene discovered has protected barley cultivars
from severe stem rust losses for over 70 years [26]. The Rpg1 gene encodes a
constitutively expressed protein containing two tandem kinase domains: the protein
kinase 1 (pK1) domain and protein kinase 2 (pK2) domain. The pK1 is a pseudo
kinase, whereas the pK2 domain is catalytically active, and both domains are
required for stem rust resistance. The pseudo kinase pK1 domain is associated
with disease resistance, and the pK2 domain is involved in protein phosphorylation
[27, 28]. The RPG1 protein is a functional kinase located in the plasma membrane,
endomembranes, and cytosol. The resistance protein RPG1 disappeared rapidly
(within 5 min) when barley seedling leaves were inoculated by avirulent and viable
stem rust P. graminis pathotype MCCF. The disappearance of the RPG1 protein is
due to phosphorylation and the phosphorylated status sustained for 20 h after
inoculation. It is suggested that RPG1 protein phosphorylation is essential for
disease resistance. The reciprocal responses of barley and stem rust belong to ETI
of plant [25]. Interestingly, in the incompatible combination of wheat and leaf rust
(Lovrin 10 and leaf rust race 260), the expression of the TaCDPK2 gene was
obviously increased at the levels of mRNA and protein, while the TaCAMTA4
gene expression level started to decrease gradually after wheat leaves inoculated
with leaf rust after 4 h [25].
2.3
Early Response to Virus
Unlike bacteria and fungi, viruses are not commonly regarded as encoding PRRsPAMPs/MAMPs [29]. However, it was recently found that Arabidopsis mutants for
LRR-RLK BAK1 are more susceptible to viral infection [30], suggesting that PRRs
and unknown PAMP or DAMP might be involved in early virus perception. RNA
silencing clearly represents a major plant immune strategy against viruses
[31]. Thus, future research is required to understand systematically how these
PRRs are involved in virus perception.
2.4
Early Response to Phytoplasmas
Phytoplasmas are plant pathogenic bacteria deviated from Gram-positive bacteria
without cell wall, and they have significant genome reductions and are obligate
intracellular pathogens of plants [32]. Phytoplasmas, unlike extracellular bacterial
phytopathogens, do not require specialized secretory systems for pathogenesis;
instead, they directly introduce their effector proteins inside host plant cytoplasm
[33]. Phytoplasmas, transmitted by insect vectors to plants, have distinctly regulated
effector genes for insect and plant colonization [34, 35]. Once translocated into the
plant cytoplasm, effectors can traffic to different subcellular compartments,
582
S. A. Zebelo
fungal pathogens. Barley cultivars in North America were highly susceptible to the
most devastating barley stem rust disease caused by Puccinia graminis f. sp. tritici
(Pgt). In 1942, new resistant Rpg1 gene discovered has protected barley cultivars
from severe stem rust losses for over 70 years [26]. The Rpg1 gene encodes a
constitutively expressed protein containing two tandem kinase domains: the protein
kinase 1 (pK1) domain and protein kinase 2 (pK2) domain. The pK1 is a pseudo
kinase, whereas the pK2 domain is catalytically active, and both domains are
required for stem rust resistance. The pseudo kinase pK1 domain is associated
with disease resistance, and the pK2 domain is involved in protein phosphorylation
[27, 28]. The RPG1 protein is a functional kinase located in the plasma membrane,
endomembranes, and cytosol. The resistance protein RPG1 disappeared rapidly
(within 5 min) when barley seedling leaves were inoculated by avirulent and viable
stem rust P. graminis pathotype MCCF. The disappearance of the RPG1 protein is
due to phosphorylation and the phosphorylated status sustained for 20 h after
inoculation. It is suggested that RPG1 protein phosphorylation is essential for
disease resistance. The reciprocal responses of barley and stem rust belong to ETI
of plant [25]. Interestingly, in the incompatible combination of wheat and leaf rust
(Lovrin 10 and leaf rust race 260), the expression of the TaCDPK2 gene was
obviously increased at the levels of mRNA and protein, while the TaCAMTA4
gene expression level started to decrease gradually after wheat leaves inoculated
with leaf rust after 4 h [25].
2.3
Early Response to Virus
Unlike bacteria and fungi, viruses are not commonly regarded as encoding PRRsPAMPs/MAMPs [29]. However, it was recently found that Arabidopsis mutants for
LRR-RLK BAK1 are more susceptible to viral infection [30], suggesting that PRRs
and unknown PAMP or DAMP might be involved in early virus perception. RNA
silencing clearly represents a major plant immune strategy against viruses
[31]. Thus, future research is required to understand systematically how these
PRRs are involved in virus perception.
2.4
Early Response to Phytoplasmas
Phytoplasmas are plant pathogenic bacteria deviated from Gram-positive bacteria
without cell wall, and they have significant genome reductions and are obligate
intracellular pathogens of plants [32]. Phytoplasmas, unlike extracellular bacterial
phytopathogens, do not require specialized secretory systems for pathogenesis;
instead, they directly introduce their effector proteins inside host plant cytoplasm
[33]. Phytoplasmas, transmitted by insect vectors to plants, have distinctly regulated
effector genes for insect and plant colonization [34, 35]. Once translocated into the
plant cytoplasm, effectors can traffic to different subcellular compartments,
582
S. A. Zebelo