Abstract
Drought is the most serious abiotic stress, which significantly reduces crop productivity. The phytohormone ABA plays a pivotal role in regulating stomatal closing upon drought stress. Here, we characterized the physiological function of AtBBD1, which has bifunctional nuclease activity, on drought stress. We found that AtBBD1 localized to the nucleus and cytoplasm, and was expressed strongly in trichomes and stomatal guard cells of leaves, based on promoter:GUS constructs. Expression analyses revealed that AtBBD1 and AtBBD2 are induced early and strongly by ABA and drought, and that AtBBD1 is also strongly responsive to JA. We then compared phenotypes of two AtBBD1-overexpression lines (AtBBD1-OX), single knockout atbbd1, and double knockout atbbd1/atbbd2 plants under drought conditions. We did not observe any phenotypic difference among them under normal growth conditions, while OX lines had greatly enhanced drought tolerance, lower transpirational water loss, and higher proline content than the WT and KOs. Moreover, by measuring seed germination rate and the stomatal aperture after ABA treatment, we found that AtBBD1-OX and atbbd1 plants showed significantly higher and lower ABA-sensitivity, respectively, than the WT. RNA sequencing analysis of AtBBD1-OX and atbbd1 plants under PEG-induced drought stress showed that overexpression of AtBBD1 enhances the expression of key regulatory genes in the ABA-mediated drought signaling cascade, particularly by inducing genes related to ABA biosynthesis, downstream transcription factors, and other regulatory proteins, conferring AtBBD1-OXs with drought tolerance. Taken together, we suggest that AtBBD1 functions as a novel positive regulator of drought responses by enhancing the expression of ABA- and drought stress-responsive genes as well as by increasing proline content.
Highlights
Plants are constantly challenged by various abiotic stresses, including UV irradiation, high temperature, cold, high salinity, and drought, and they exhibit rapid changes at both physiological and molecular levels to adapt to changing environmental conditions.Of these abiotic stresses, drought is the most serious abiotic stress
We recently reported that the DUF151 domain-containing Arabidopsis BBD (Bifunctional nucleases in Basal Defense response) proteins, AtBBD1 and AtBBD2, exhibit nonsubstrate-specific DNase and RNase activity [21], similar to OmBBD in Oryza minuta, possibly involving an ABA-mediated synergistic interaction between ABA and Jasmonic acid (JA) against biotic stresses [17]
We found that the expression of AtBBD1 was much higher than that of AtBBD2 in all tissues except in root, and that AtBBD1 was expressed more than three times higher in the rosette and cauline leaves compared to the other tissues (Figure 1B)
Summary
Plants are constantly challenged by various abiotic stresses, including UV irradiation, high temperature, cold, high salinity, and drought, and they exhibit rapid changes at both physiological and molecular levels to adapt to changing environmental conditions. The phytohormone ABA is a key regulator for drought stress responses in plants. Another study reported that a direct association of drought stress-induced PCD, with nucleic acid degradation by the enhanced hydrolase activity, indicates the possible involvement of nucleases in drought stress responses [20]. We recently reported that the DUF151 domain-containing Arabidopsis BBD (Bifunctional nucleases in Basal Defense response) proteins, AtBBD1 and AtBBD2, exhibit nonsubstrate-specific DNase and RNase activity [21], similar to OmBBD in Oryza minuta, possibly involving an ABA-mediated synergistic interaction between ABA and JA against biotic stresses [17]. We confirmed that AtBBD1 is as a positive regulator in ABA-mediated drought response
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