ABA Regulation of Plant Response to Biotic Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 400

pathway core components (from ABA synthesis, metabolism, transport, to signal
transduction) were responded to pathogen infection. For example, the expression
of three key ABA biosynthesis enzymes, namely NCED3 , ABA3 / LOS5 and AAO3 ,
was all induced by Pseudomonas syringae pv. Tomato DC3000 , Pseudomonas
syringae pv. maculicola ES4326, and Botrytis Cinerea infection; similarly, the
expression of two ABA catabolism genes, CYP707A1 and CYP707A4 , was also
slightly induced in a long-term manner by the infection of the three pathogens
(Chan 2012 ). Thus, homeostasis of endogenous ABA is important for the plant to
properly respond to pathogen infection. Recently, PYR/PYLs, a family of novel
START domain proteins, were identified as ABA receptors (Ma et al. 2009 ; Park
et al. 2009 ). The ABA-bound receptors are able to inactivate the PP2Cs and dis-
rupt or decrease the physical interaction between the PP2Cs and the SnRK2 s
and finally transduce the signal to downstream targets (Fujii et al. 2009 ). Chan's
results also showed that the expression levels of most PYR / PYL s were inhibited
while the expression levels of most PP2C s were induced by pathogen infection
(Chan 2012 ). Therefore, there has an increased ratio of PP2Cs:PYR/PYLs upon
pathogen infection which may be required for activation of the downstream ABA
signal pathway. Consistent with the these findings, one recent research has demon-
strated that Arabidopsis plants impaired in ABA biosynthesis, such as aba1 - 6 , or
in ABA signaling, like the quadruple pyr/pyl mutant ( pyr1pyl1pyl2pyl4 ), showed
enhanced resistance to the fungus Plectosphaerella cucumerina while the hab1 -
1abi1 - 2abi2 - 2 mutant impaired in three phosphatases that negatively regulate
ABA signaling, are more susceptible to this fungus (Sánchez-Vallet et al. 2012 ).
In tomato, the expression of ABA pathway core components was also affected in
B.cinerea -infected fruits (Blanco-Ulate et al. 2013 ). Based on both the expression
pattern and function of ABA pathway core components under pathogen infec-
tion, we can deduce that ABA may play important roles during plant-pathogen
interactions.
20.5 ABA and Stomatal Innate Immunity
Plants use innate physical and biochemical barriers to protect themselves from a
variety of pathogens. Thus, to successfully infect a host plant, pathogens need to
evolve mechanisms that allow them to circumvent plant mechanical barriers, such
as cell walls and waxy epidermal cuticles. Some fungal pathogens penetrate plant
tissue using mechanical force or by secreting cuticle- and cell-wall-degrading
enzymes (Mendgen et al. 1996 ; van Kan J.A. 2006 ). But unlike fungal pathogens,
bacteria pathogens cannot directly penetrate the leaf epidermis and instead enter
leaf tissues using preexisting openings, such as stomata, hydathodes, nectarthodes,
and wounds (Melotto et al. 2008 ). Among them, stomata represent a major route
for many phytopathogens to enter the plant tissues (Zeng et al. 2010 ). In order to
prevent pathogen entry into host tissue through the stomata, plants can actively
close the stomata by activating stomatal innate immunity, which usually occurs