Charting S-nitrosothiol function during the plant defence response

Plant diseases are responsible for about ~25% of total world crop losses per annum. Therefore, plant disease resistance is an important trait to develop in crop plants. Fortunately, plants have evolved a relatively robust defence system to protect themselves from diseases caused by micro-organisms. Understanding how this defence system works will lead to crops that are able to resist microbial infection more effectively, this is especially important at this time of food shortages. Previously, we have demonstrated that S-nitrosylation is a molecular switch that controls the expression of disease resistance against a broad spectrum of potential microbial pathogens in Arabidopsis, a model plant species. Increased S-nitrosylation leads to pathogen susceptibility while decreased S-nitrosylation results in pathogen resistance. Therefore, understanding how plants can regulate the extent of this protein modification may provide us with insights to develop disease resistant crops. By employing a genetics-based strategy, we have identified three genes, SPL1, SPL2 and SPL3 that appear to control the levels of S-nitrosylation during the plant defence response. Utilising standard molecular biology procedures we cloned SPL1 and found it encoded a protein that mediated ubiquintination, another key mechanism to regulate cellular processes in plants. Also, SPL1 contained motifs that suggested it physically interacted with a number of other proteins. Using a variety of molecular and biochemical experimental procedures we will characterise the role of SPL1 in the regulation of S-nitrosylation during the establishment of disease resistance. In a complementary approach, we utilised technology that enabled us to monitor the expression of all known genes in Arabidopsis plant lines that exhibit either increased or decreased amounts of S-nitrosylation. In this way we have identified genes that are directly regulated by changes in the levels of S-nitrosothiols (SNOs), the products of S-nitrosylation. Loss-of-function mutations in one of these genes, SRG1, which encodes a zinc finger protein that controls gene expression, results in hypersensitivity to SNOs. Furthermore, SRG1 is S-nitrosylated. These findings suggest that SRG1 senses increasing SNO levels by becoming S-nitrosylated and then subsequently drives the expression of hitherto unidentified genes that protect plants against high levels of SNOs, termed nitrosative stress. By employing state-of-the-art genomics technologies we will identify the DNA binding sites of SRG1 and determine the consequences of this binding on the expression of adjacent genes / non-coding RNAs. We will also explore if prior S-nitrosylation of SRG1 is required to initiate this process. Collectively, this work will provide major insights how S-nitrosylation controls the expression of plant disease resistance. In the long term the results from this project may aid the development of disease resistant crops.

Loss-of-function mutations in AtGSNOR1 result in increased S-nitrosothiol (SNO) levels in both naïve and pathogen challenged plants and compromise multiple modes of disease resistance. To explore the mechanisms underpinning SNO formation and turnover in plants, we carried out a forward genetic screen to identify suppressors of atgsnor1-3, a mutation that abolishes AtGSNOR1 function. Three activation tagged lines were identified in which atgsnor1-3-mediated enhanced disease susceptibility was suppressed, without constitutively activating defence responses. One of the corresponding tagged genes, SPL1, encodes a RING domain E3 ligase with ankyrin repeats and a leucine zipper. We will determine SPL1 function with particular reference to: which domains are required to suppress atgsnor1-3; its ability to act as an E3 ligase; how S-nitrosylation regulates E3 ligase activity and the biological implications of SNO-SRG1 formation. Previous gene expression profiling identified SNO regulated genes. Reverse genetics revealed that mutations in SNO Regulated Gene 1 (SRG1), a predicted zinc finger transcription factor (ZFTF), resulted in SNO hypersensitivity and compromised AtGSNOR1 expression. SRG1 was also S-nitrosylated during nitrosative stress. Together, these findings are consistent with SRG1 operating as a SNO sensor and regulator. By employing an Illumina Solexa chromatin immunoprecipitation (ChIP)-sequencing (Seq) approach we will identify genome-wide SRG1 DNA binding sites. This will be combined with Illumina Solexa-based gene expression profiling to determine the impact on transcription of adjacent genes / RNAs resulting from SRG1 occupancy of any given binding site. Collectively, this work will provide significant insights into SNO signalling and turnover during the establishment of plant disease resistance.