Abstract
Cassava (Manihot esculenta) is a major staple food, animal feed and energy crop in the tropics and subtropics. It is one of the most drought-tolerant crops, however, the mechanisms of cassava drought tolerance remain unclear. Abscisic acid (ABA)-responsive element (ABRE)-binding factors (ABFs) are transcription factors that regulate expression of target genes involved in plant tolerance to drought, high salinity, and osmotic stress by binding ABRE cis-elements in the promoter regions of these genes. However, there is little information about ABF genes in cassava. A comprehensive analysis of Manihot esculenta ABFs (MeABFs) described the phylogeny, genome location, cis-acting elements, expression profiles, and regulatory relationship between these factors and Manihot esculenta betaine aldehyde dehydrogenase genes (MeBADHs). Here we conducted genome-wide searches and subsequent molecular cloning to identify seven MeABFs that are distributed unevenly across six chromosomes in cassava. These MeABFs can be clustered into three groups according to their phylogenetic relationships to their Arabidopsis (Arabidopsis thaliana) counterparts. Analysis of the 5′-upstream region of MeABFs revealed putative cis-acting elements related to hormone signaling, stress, light, and circadian clock. MeABF expression profiles displayed clear differences among leaf, stem, root, and tuberous root tissues under non-stress and drought, osmotic, or salt stress conditions. Drought stress in cassava leaves and roots, osmotic stress in tuberous roots, and salt stress in stems induced expression of the highest number of MeABFs showing significantly elevated expression. The glycine betaine (GB) content of cassava leaves also was elevated after drought, osmotic, or salt stress treatments. BADH1 is involved in GB synthesis. We show that MeBADH1 promoter sequences contained ABREs and that MeBADH1 expression correlated with MeABF expression profiles in cassava leaves after the three stress treatments. Taken together, these results suggest that in response to various dehydration stresses, MeABFs in cassava may activate transcriptional expression of MeBADH1 by binding the MeBADH1 promoter that in turn promotes GB biosynthesis and accumulation via an increase in MeBADH1 gene expression levels and MeBADH1 enzymatic activity. These responses protect cells against dehydration stresses by preserving an osmotic balance that enhances cassava tolerance to dehydration stresses.
Highlights
Cassava (Manihot esculenta) is a major staple food, animal feed and energy crop in the tropics and subtropics
ABI5 can directly activate its own expression, whereas B-box 21 (BBX21) negatively regulates this activity through interactions with ABI5. These results indicate that BBX21 coordinates with HYPOCOTYL 5 (HY5) and ABI5 on the ABI5 promoter and that these transcriptional regulators work in concert to integrate light and Abscisic acid (ABA) signaling in Arabidopsis[60]
We identified seven Manihot esculenta ABFs (MeABFs) that were distributed unevenly across six chromosomes, and clustered into three groups according to their phylogenetic relationships to Arabidopsis counterparts
Summary
Cassava (Manihot esculenta) is a major staple food, animal feed and energy crop in the tropics and subtropics. Abscisic acid (ABA)-responsive element (ABRE)-binding factors (ABFs) are transcription factors that regulate expression of target genes involved in plant tolerance to drought, high salinity, and osmotic stress by binding ABRE cis-elements in the promoter regions of these genes. We show that MeBADH1 promoter sequences contained ABREs and that MeBADH1 expression correlated with MeABF expression profiles in cassava leaves after the three stress treatments Taken together, these results suggest that in response to various dehydration stresses, MeABFs in cassava may activate transcriptional expression of MeBADH1 by binding the MeBADH1 promoter that in turn promotes GB biosynthesis and accumulation via an increase in MeBADH1 gene expression levels and MeBADH1 enzymatic activity. These three factors have been shown to be key regulators of ABA signaling in response to osmotic stress[20], as was found in Gossypium hirsutum in which overexpression of exogenous and endogenous ABFs substantially increase drought tolerance, primarily through stomatal regulation that reduces transpiration and photosynthetic productivity[21]
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