Abstract
SmD3 is a core component of the small nuclear ribonucleoprotein (snRNP) that is essential for pre-mRNA splicing. The role of Arabidopsis SmD3 in plant immunity was assessed by testing sensitivity of smd3a and smd3b mutants to Pseudomonas syringae pv. tomato (Pst) DC3000 infection and its pathogenesis effectors flagellin (flg22), EF-Tu (elf18) and coronatine (COR). Both smd3 mutants exhibited enhanced susceptibility to Pst accompanied by marked changes in the expression of key pathogenesis markers. mRNA levels of major biotic stress response factors were also altered upon treatment with Pseudomonas effectors. Our genome-wide transcriptome analysis of the smd3b-1 mutant infected with Pst, verified by northern and RT-qPCR, showed that lack of SmD3-b protein deregulates defense against Pst infection at the transcriptional and posttranscriptional levels including defects in splicing and an altered pattern of alternative splicing. Importantly, we show that SmD3-b dysfunction impairs mainly stomatal immunity as a result of defects in stomatal development. We propose that it is the malfunction of the stomata that is the primary cause of an altered mutant response to the pathogen. Other changes in the smd3b-1 mutant involved enhanced elf18- and flg22-induced callose deposition, reduction of flg22-triggered production of early ROS and boost of secondary ROS caused by Pst infection. Together, our data indicate that SmD3 contributes to the plant immune response possibly via regulation of mRNA splicing of key pathogenesis factors.
Highlights
Plants are challenged by numerous phytopathogens such as bacteria, fungi, and viruses (Muthamilarasan and Prasad, 2013)
To assess cellular defense to the pathogen in smd3b and smd3a plants we investigated changes in mRNA levels of key pathogenesis markers: PR1, PR2, PR4, PR5, PDF1.2 (PLANT DEFENSIN 1.2) and GSTF6 that are involved in the salicylic acid (SA) response, and two JASMONATE-ZIM-DOMAIN PROTEINS JAZ1 and JAZ9 from the jasmonic acid (JA) pathway, which are induced by coronatine in a specific mechanism used by the pv. tomato strain DC3000 (Pst) DC3000 strain to manipulate jasmonate signaling (Lieberherr et al, 2003; Demianski et al, 2011; Barah et al, 2013)
Expression of other pathogen response-related factors, SGT1 (SALICYLIC ACID GLUCOSYLTRANSFERASE 1), NPR1 (NON-EXPRESSER OF PR GENES 1), NPR3 (NPR1-LIKE PROTEIN 3) was not significantly altered in the smd3b-1 mutant (Supplementary Figure 1E). These results provide evidence that lack of SmD3 protein dysregulates the response to Pst DC3000 infection
Summary
Plants are challenged by numerous phytopathogens such as bacteria, fungi, and viruses (Muthamilarasan and Prasad, 2013). Flg mimics pathogen appearance and causes oxidative stress, callose deposition and ethylene production, leading to the induction of resistance genes [e.g., PR1 and PR5 (PATHOGENESISRELATED GENES), PAL1 (PHE AMMONIA LYASE 1) and GSTF6 (GLUTATHIONE S-TRANSFERASE 6)], but in contrast to the pathogen does not produce the hypersensitive response (HR) type of necrosis (Maleck et al, 2000; Asai et al, 2002; GomezGomez and Boller, 2002) Another well-known PRR is the receptor kinase EFR (ELONGATION FACTOR Tu RECEPTOR), which recognizes an 18 amino-acid (elf18) fragment of the bacterial translation elongation factor EF-Tu. Another well-known PRR is the receptor kinase EFR (ELONGATION FACTOR Tu RECEPTOR), which recognizes an 18 amino-acid (elf18) fragment of the bacterial translation elongation factor EF-Tu These PRRs initiate immune signaling by heterodimerization with the LRR-RLK family co-receptor BAK1 (BRI1-ASSOCIATED RECEPTOR KINASE) and recruitment of BIK1 (BOTRYTIS-INDUCED KINASE 1) kinase (Chinchilla et al, 2007; Macho and Zipfel, 2014; Couto and Zipfel, 2016; Yeh et al, 2016)
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