A Splicing Regulator, SR45, Suppresses Plant Immunity via Regulating Salicylic Acid Pathway in Arabidopsis thaliana

Audrey Bui¹Arden Bui¹Iesh Gujral¹Serena Fan¹Anthony Long²Anna Hu¹Christopher Chin¹Jordan Powers¹Ronghui Yang³MIN GAO⁴Chong Zhang⁴Hua Lu⁴Bret Cooper³Xiao-Ning Zhang^5*

• ¹St Bonaventure University, Saint Bonaventure, United States
• ²Bronx High School Of Science The, New York, United States
• ³USDA-ARS Beltsville Agricultural Research Center, Beltsville, United States
• ⁴University of Maryland Baltimore County, Baltimore, United States
• ⁵St. Bonaventure University, Allegany, United States

Facing constant challenges from various pathogens and pests, plants have evolved different strategies to defend themselves both locally and systemically. A global change in RNA metabolism is one of the necessary steps to mount on a long-lasting immunity against present and future invasions. Arabidopsis Serine/Arginine-rich 45 (SR45) is an evolutionarily conserved RNA binding protein that regulates multiple steps of RNA metabolism. Out prior study suggests that SR45 acts as a negative regulator of plant immunity. To better understand the molecular mechanism for SR45's defense role, we examined the metabolic profile in both Col-0 and sr45-1. The results showed a significant accumulation of pipecolic acid (Pip), salicylic acid (SA) and other potential defense compounds in sr45-1, indicating an increased systemic immunity. The sr45-1 mutant exhibited an elevated resistance to a wide-range of biotrophic pathogen species and its insensitivity to Pip, SA and pathogen-pretreatment. Between the two alternatively spliced isoforms, SR45.1 and SR45.2, SR45.1 seemed to be the culprit for the observed immune suppression. Upon examinations of the transcriptome profile between Col-0 and sr45-1 either under mock or P. syringae PmaDG3 challenge, 1125 genes were identified as SR45-suppressed and PmaDG3-induced. Genes function in SA biosynthesis and systemic acquired resistance was overrepresented, including the ones coding for WRKY, RLK, RLP, protein kinases and TIR-NBS-LRR proteins. In addition, significant alternative splicing activity were identified in a list of genes either due to sr45-1 alone or both sr45-1 and PmaDG3 challenge. Among them, we characterized the effect of alternative splicing in two candidates, CBRLK1 and SRF1. Interestingly, alternative splicing in both exhibited a switch between receptor-like protein (RLP) and receptor-like kinase (RLK) in the predicted protein products. Overexpressing their sr45-1 dominant isoform in Col-0 led to a partial increase in immunity, suggesting the involvement of both alternative splicing events in SR45-conferred immune suppression. In summary, we hypothesize that SR45 regulates a subset of immune genes at either transcriptional or co-transcriptional pre-mRNA splicing levels to confer its function in a systemic immune suppression.