Abstract
Abscisic acid (ABA) is a well-characterized plant hormone, known to mediate developmental aspects as well as both abiotic and biotic stress responses. Notably, the exogenous application of ABA has recently been shown to increase susceptibility to the fungal pathogen Fusarium graminearum, the causative agent of Fusarium head blight (FHB) in wheat and other cereals. However roles and mechanisms associated with ABA’s modulation of pathogen responses remain enigmatic. Here the identification of putative ABA receptors from available genomic databases for Triticum aestivum (bread wheat) and Brachypodium distachyon (a model cereal) are reported. A number of these were cloned for recombinant expression and their functionality as ABA receptors confirmed by in vitro assays against protein phosphatases Type 2Cs. Ligand selectivity profiling of one of the wheat receptors (Ta_PYL2DS_FL) highlighted unique activities compared to Arabidopsis AtPYL5. Mutagenic analysis showed Ta_PYL2DS_FL amino acid D180 as being a critical contributor to this selectivity. Subsequently, a virus induced gene silencing (VIGS) approach was used to knockdown wheat Ta_PYL4AS_A (and similar) in planta, yielding plants with increased early stage resistance to FHB progression and decreased mycotoxin accumulation. Together these results confirm the existence of a family of ABA receptors in wheat and Brachypodium and present insight into factors modulating receptor function at the molecular level. That knockdown of Ta_PYL4AS_A (and similar) leads to early stage FHB resistance highlights novel targets for investigation in the future development of disease resistant crops.
Highlights
Abscisic acid (ABA) is a phytohormone that plays well-established roles in plant development and abiotic stress responses [1]
Structure-function studies have shown that ABA binding in a cavity in the receptor leads to a gate-latch conformational closure of the pocket, yielding a surface on the receptor that has high affinity for the catalytic site on Protein Phosphatase 2C proteins (PP2Cs) [4,5,6,7,8]
In the case of T. aestivum, initially the Arabidopsis AtPYL5 cDNA sequence was used as a blast query against all available wheat sequences in the National Center for Biotechnology Information (NCBI) database where one hit was subsequently used as a template to search the wheat (T. aestivum cv ‘Chinese Spring’), International Wheat Genomics Sequencing Consortium (IWGSC) survey sequence and Wheat reference sequence chromosome 3B databases by blastn and tblastn.Blast hits were aligned using Clustal Omega [40] and their evolutionary relationship investigated using Mega6 [41]
Summary
Abscisic acid (ABA) is a phytohormone that plays well-established roles in plant development (seed dormancy, germination, seedling growth and seed deposition) and abiotic stress (drought, high salt, high temperature) responses [1]. One of the most significant findings was the identification of a family of soluble ABA receptors [2, 3]. These are small START (steroidogenic acute regulatory-related lipid transfer) domain proteins comprising a portion of the larger Betv superfamily. Binding of the ABA-stimulated receptor to the PP2C blocks substrate access, yielding a non-competitive inhibition of the phosphatase, releasing the downstream positive transcriptional effectors Sucrose-Non fermenting Kinase1-related protein kinases (e.g. SnRK2; reviewed in [9, 10])
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