Serine Hydroxymethyltransferase 6 is involved in growth and resistance against pathogens via ethylene and lignin production in Arabidopsis
Photorespiratory serine hydroxymethyltransferases (SHMTs) are important enzymes of cellular one-carbon metabolism. In this work, we investigated the potential role of SHMT6 in Arabidopsis thaliana. We found that SHMT6 is localized in the nucleus and expressed in different tissues during development. Interestingly the gene is inducible in response to avirulent, virulent Pseudomonas syringae and to Fusarium oxysporum infection. Overexpression of SHMT6 leads to large flowers, siliques, seeds, roots and enhanced biomass. This enhanced growth was accompanied by increased stomatal conductance and photosynthetic capacity as well as ATP, protein, and chlorophyll levels. By contrast, a shmt6 knockout mutant displayed reduced growth. When challenged with Pseudomonas syringae pv tomato (Pst) DC3000 expressing AvrRpm1, SHMT6 overexpression lines displayed a clear hypersensitive response which was characterized by enhanced electrolyte leakage and reduced bacterial growth. In response to virulent Pst DC3000, the shmt6 mutant developed severe disease symptoms and becomes very susceptible, whereas SHMT6 overexpression lines showed enhanced resistance with increased expression of defense pathways. In response to Fusarium oxysporum, overexpression lines showed reduction in symptoms. Moreover, SHMT6 overexpression lead to enhanced production of ethylene and lignin, which are important components of the defense response. Transcriptome analysis of shmt6 mutant and WT showed that several genes involved in defense, ethylene and lignin pathways are suppressed in the mutant. Collectively, our data reveal that SHMT6 plays an important role in development and defense against pathogens. Overall design: Pseudomonas syringae pv. Tomato (Pst) DC3000 was grown at 28°C in King's B (KB) medium supplemented with 50 mg/ml rifampicin. After overnight cultivation, a bacterial suspension was prepared in 10 mM MgCl2 at OD600 0.2 (Pst AvrRpm1). Four-week-old soilrite and agropeat grown fully expanded Arabidopsis leaves were infiltrated with suspension of avirulent Pseudomonas syringae pv tomato (Pst) DC3000 in 10 mM MgCl2 at OD600 = 0.04. Control (10 mM MgCl2) and pathogen infiltrated samples were harvested at 6 hours of infection.