ABA Regulation of Plant Response to Biotic Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 402

20.6 ABA and Callose Deposition
Upon the recognition of a successful invasion, the host plant would activate post-
invasive resistance responses, such as rapid callose deposition and generation of
reactive oxygen species (ROS), during which ABA involved. Compared with its
positive roles in stomatal innate immunity, the role of ABA in post-invasive resist-
ance responses is controversial and seems to vary among different plant-pathogen
interactions.
Accumulating studies have demonstrated that both ABA and callose can
affect the consequence of many plant-pathogen interactions, and ABA can both
negatively and positively affect the deposition of callose which is dependent on
the plant-pathogen combination (Flors et al. 2001 ). However, the exact molecu-
lar mechanism behind the modulation of callose by ABA upon pathogen attack
is unclear.
In plant-bacteria interactions, several studies have demonstrated that ABA gen-
erally functions as a negative regulator in the post-invasive defense by suppress-
ing bacteria-induced callose formation. When challenged with Pst DC3000 , ABA
hyper-responsive abi2 mutants strongly decreased callose deposition accompa-
nied by enhanced susceptibility, while ABA-insensitive abi1 - 1 and abi2 - 1 mutants
deposit augmented levels of callose which coincided with enhanced resistance (de
Torres-Zabala et al. 2007 ; Goritschnig et al. 2008 ). Furthermore, pre-treatment
with ABA also strongly reduced callose deposition in subsequently Pst DC3000
inoculated plants (de Torres-Zabala et al. 2007 ).
Contrarily, in some plant-fungal-oomycetes interactions, ABA can exert an
positive role by stimulating callose deposition or by modulating the priming of
callose deposition (Ton and Mauch-Mani 2004 ; Flors et al. 2001 ). Treatment
with exogenous ABA can mimic the effect of ʲ -aminobutyric acid (BABA)
and result in priming for pathogen-inducible callose deposition, coinciding
with enhanced resistance against P. cucumerina and Alternaria brassicicola .
It was also demonstrated that both the intact ABA signaling and callose for-
mation are crucial for BABA-induced resistance against P. cucumerina and A.
brassicicola in Arabidopsis , since the ABA-deficient mutant aba1 - 5 , the ABA-
insensitive mutant abi4 - 1 and the callose-deficient mutant pmr4 - 1 failed to
express the BABA-induced resistance. The reduction or absence of primed cal-
lose deposition in BABA-treated aba1/ibs3 and abi4 plants upon infection by
Hyaloperonospora parasitica and P. cucumerina, respectively, further provides
a causal link between ABA signaling and callose deposition (Ton and Mauch-
Mani 2004 ; Ton et al. 2005 ). Thus, these studies clearly showed that ABA
functions in the BABA-induced augmentation of pathogen-induced callose dep-
osition. However, it should be noted that besides of its role in callose deposition,
ABA-dependent resistance can also to be exerted in a callose-independent man-
ner, such as activation of JA biosynthesis, as reflected in Arabidopsis - Pythium
irregulare interaction (Adie et al. 2007 ).