Abstract
Screening of the expressed sequence tag library of the wild rice species Oryza minuta revealed an unknown gene that was rapidly and strongly induced in response to attack by a rice fungal pathogen (Magnaporthe oryzae) and an insect (Nilaparvata lugens) and by wounding, abscisic acid (ABA), and methyl jasmonate treatments. Its recombinant protein was identified as a bifunctional nuclease with both RNase and DNase activities in vitro. This gene was designated OmBBD (for O. minuta bifunctional nuclease in basal defense response). Overexpression of OmBBD in an Arabidopsis (Arabidopsis thaliana) model system caused the constitutive expression of the PDF1.2, ABA1, and AtSAC1 genes, which are involved in priming ABA-mediated callose deposition. This activation of defense responses led to an increased resistance against Botrytis cinerea. atbbd1, the knockout mutant of the Arabidopsis ortholog AtBBD1, was susceptible to attack by B. cinerea and had deficient callose deposition. Overexpression of either OmBBD or AtBBD1 in atbbd1 plants complemented the susceptible phenotype of atbbd1 against B. cinerea as well as the deficiency of callose deposition. We suggest that OmBBD and AtBBD1 have a novel regulatory role in ABA-mediated callose deposition.
Highlights
Screening of the expressed sequence tag library of the wild rice species Oryza minuta revealed an unknown gene that was rapidly and strongly induced in response to attack by a rice fungal pathogen (Magnaporthe oryzae) and an insect (Nilaparvata lugens) and by wounding, abscisic acid (ABA), and methyl jasmonate treatments
We have demonstrated the potential role of OmBBD and AtBBD1 in a host defense system against a necrotrophic pathogen, B. cinerea, via jasmonic acid (JA)- and ABAdependent plant regulatory pathways and callose deposition
We were unable to determine whether or not the nuclease activity of OmBBD is essential for the defense responses in Arabidopsis
Summary
Screening of the expressed sequence tag library of the wild rice species Oryza minuta revealed an unknown gene that was rapidly and strongly induced in response to attack by a rice fungal pathogen (Magnaporthe oryzae) and an insect (Nilaparvata lugens) and by wounding, abscisic acid (ABA), and methyl jasmonate treatments. Overexpression of OmBBD in an Arabidopsis (Arabidopsis thaliana) model system caused the constitutive expression of the PDF1.2, ABA1, and AtSAC1 genes, which are involved in priming ABAmediated callose deposition This activation of defense responses led to an increased resistance against Botrytis cinerea. In the callose synthase-deficient mutant pmr, which shows impaired pathogen-induced callose deposition, SA-dependent defense responses are strongly induced to augment the resistance to powdery mildew (Nishimura et al, 2003), whereas JAdependent defense responses are down-regulated, resulting in its susceptibility to A. brassicicola (Flors et al, 2008). These findings indicate that pathogeninduced callose deposition plays an important role in resistance to necrotroph fungus and is closely related. The molecular and cellular action mechanisms involved in the regulation of these responses of plant bifunctional nucleases (BFNs) remain largely unknown
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