Exploring the cross-talk among SA, JA and ABA at molecular level in response to Alternaria brassicae in Brassica and its wild relatives
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Date
2019
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Publisher
National Institute for Plant Biotechnology ICAR-Indian Agricultural Research Institute New Delhi
Abstract
Brassica juncea is a globally cultivated oilseed crop which accounts for approximately
28.6% share of world’s oilseed production. During its growth period B. juncea crop is
challenged by multiple abiotic and biotic stresses. Alternaria blight caused by
Alternaria brassicae is one of the most pressing challenge of all because it causes
almost 40-60% yield loss. B. juncea is susceptible to this pathogen however, variable
level of resistance have been reported in some wild relatives of B. juncea like Sinapis
alba and Camelina sativa which exhibit moderate and high resistance towards A.
brassicae respectively. Phytohormones like SA and JA play lead role as signal
molecules mediating the defence responses in plants by triggering expression of
respective downstream responsive genes. Role of ABA in this intricate signal
mechanism is still not fully understood however, the recent reports have suggested that
ABA acts as a positive as well as a negative regulator of resistance depending upon the
patho-phytosystem. In this context, we designed an experiment involving three
genotypes- B. juncea, S. alba and C. sativa. All three genotypes were characterized for
pathogenic, histological, biochemical and molecular response towards A. brassicae.
Further, the response of all three species to exogenous application of SA, MeJA and
ABA towards pathogen infection was studied under in vitro and in vivo conditions.
Prior to target gene expression analysis, normalization analysis was performed in all
three genotypes for identifying the two most stable reference genes out of seven
candidate reference genes. Selected target genes were biosynthesis genes and
downstream responsive genes of SA, JA and ABA signalling pathways. Finally, a
quantitative estimation of SA, JA and ABA levels was performed in treated samples of
all three genotypes. In the pathogenicity assay it was found that the A. brassicae highly
virulent isolate causes higher foliar damage and yield reduction in susceptible genotype
however, the damage recorded in both the wild genotypes were significantly lower as
compared to B. juncea. High rate of disease progression and higher H2O2 accumulation
was noticed in B. juncea in response to the virulent isolate of A. brassicae. Higher
expression of PDF1.2, a JA-induced defence related gene was noticed in resistant
genotypes in response to virulent A. brassicae isolate. In the normalization analysis it
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was found that TIPS41 and PP2A exhibited an overall stable expression in all three
genotypes however, different genes were found best for each group of treatments. In
the target gene expression analysis it was found that JA biosynthesis genes like LOX2
and AOC were downregulated in response to SA and ABA and variable expression level
in response to A. brassicae was seen in susceptible and resistant genotypes. Similarly,
ABA related genes were found to be downregulated in response of JA. This indicates
an antagonistic relation between JA and ABA in A. brassicae-Brassica phytopathosystem. In response to A. brassicae, ABA and SA pathway related genes also
exhibited significantly variable expression levels in the susceptible genotype B. juncea
and both the resistant genotypes S. alba and C. sativa. HPLC quantification of ABA
shows higher ABA accumulation in SA and A. brassicae treated samples in all three
genotypes however the ABA levels were significantly higher in B. juncea. A significant
spike in free SA quantity was noticed in response to ABA and A. brassicae treatment
in susceptible genotype B. juncea. GC-Ms quantification of JA shows a higher JA
accumulation in resistant genotypes in response to A. brassicae in both the resistant
genotypes. Overall analysis shows an antagonistic relation between SA and JA, ABA
and JA at gene expression level as well as at the level of hormone accumulation and
disease assessment. High levels of JA accumulated in the resistant genotypes shows an
activated JA-induced response to the pathogen in resistant genotypes which was found
supressed in B. juncea.
Keywords: Brassica juncea, Alternaria brassicae, hormone cross-talk, disease
tolerance and hormone biosynthesis
Description
T-10190
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