ABA Regulation of Plant Response to Biotic Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 408

So far, there has relatively less information related to the ABA-virus interaction
when compared with the interactions between ABA and non-virus pathogens, such
as bacteria, fungi, and oomycetes. Several researches have demonstrated that infec-
tion with plant virus can increase the ABA levels in host plants which indicates the
involvement of ABA in plant antivirus defense. For example, infection with banana
bunchy top virus (BBTV) or turnip mosaic virus (TuMV) can increase the accumu-
lation of ABA in banana and non-heading Chinese cabbage, respectively (Zhang
et al. 1997 ; Wang et al. 2011 ). Compared with both negative and positive roles of
ABA in plant defense against non-viral pathogens, previous studies mainly sup-
port a positive correlation between ABA levels and antiviral resistance. In tobacco,
exogenous application of ABA increases resistance to tobacco mosaic virus (TMV)
infection, and TMV infection also increases ABA concentrations (Whenham et al.
1986 ). Iriti and Faoro also demonstrated that exogenous application of ABA induces
a significant resistance to tobacco necrosis virus (Iriti and Faoro 2008 ). Our recent
results also suggest that ABA has a positive role in tobacco mosaic virus (TMV)-
cg infection. Exogenous application of ABA greatly inhibited the accumulation of
TMV-cg RNA in systemically infected leaves, and studies using both ABA-deficient
and ABA-insensitive mutants showed that the transportation of TMV-cg in system-
ically infected leaves was faster in these ABA-related mutants (Chen et al. 2013 ).
However, as to the bamboo mosaic virus (BaMV), the ABA pathway may have mul-
tifaceted effects on this virus accumulation in host plants. In this study, although
exogenous application of ABA enhances resistance to BaMV infection, mutations
in ABA biosynthesis genes, including NCED3 , ABA2, and AAO3 , differentially
affected BaMV accumulation. Their results demonstrated that ABA2 functions in
BaMV defense by dividing the ABA pathway into two parts, with upstream genes
(such NCED3 ) required for BaMV accumulation and downstream genes (such as
AAO3 , ABI1 , ABI3 , and ABI4 ) required for plant resistance (Alazem et al. 2014 ).
Thus, their finding is contrary to ours related to the role of ABA2 in plant antivirus
defense. This strengthens the probability that the role of ABA2 in plant virus defense
seems to vary among different plant-virus interactions. Furthermore, ABA also can
exert its positive effect on virus infection by downregulating the transcriptional level
of β - 1,3 - glucanase genes that can degrade the b-1,3-glucan callose, forming a physi-
cal barrier to viral spread through plasmodesmata (Rezzonico et al. 1998 ).
Despite the above observations, our knowledge about the role of ABA in anti-
viral defense is still limited and needs further investigation. As we know that each
plant virus encodes an average of 4-10 proteins required to coordinate the complex
biochemical and intermolecular interactions that facilitate infection. Really, several
studies have demonstrated that virus-encoded proteins, such as cucumber mosaic
virus-encoded 2b and beet severe curly top virus C2, can physically interact with
Argonaute1 and SAMDC1, respectively, to attenuate host antiviral RNA silencing
or interferes with DNA methylation-mediated gene silencing in Arabidopsis (Zhang
et al. 2006 , 2011 ). Thus, it would be interesting to further investigate how and where
these viral proteins interact with host factors and how various plant virus use these
interactions to modify the hormone-mediated signaling pathways, especially the
ABA pathway, to disturb host defense and finally promote infection and vice versa.