The role of two NAC transcription factors during age-related resistance in Arabidopsis

Abstract

As Arabidopsis thaliana ages it becomes more resistant to virulent Pseudomonas syringae pv. tomato (Pst) bacteria. This is known as age-related resistance (ARR). ARR is associated with flowering and intercellular accumulation of salicylic acid (SA). A microarray experiment identified a number of jasmonic acid/ethylene (JA/ET)-associated genes whose expression was up-regulated during ARR. This thesis explores the role of JA/ET signaling during ARR by characterizing the role of two JA/ET-associated No Apical Meristem Cup-shaped Cotyledons (NAC) transcription factors: ANAC055 and ANAC092. Analysis of nae single and double mutants suggests that the NACs play nonredundant roles during ARR. The partial ARR defect of anac092 is followed one week later by an enhanced ARR response, and this is associated with a delay in flowering. Furthermore, mature 35S:ANAC092 exhibits increased susceptibility to Pst. Collectively, this data suggests that ANAC092 is a negative regulator of ARR and it contributes to positive regulation of flowering and the onset of ARR. The late flowering mutant luminidependens1 also exhibits a partial ARR defect, suggesting that the autonomous flowering pathway contributes to ARR. Gene expression data suggests that ANAC055 and ANAC092 regulate expression of some JA/ET-associated genes during ARR. The JA/ET signaling mutant ethylene insensitive2 (ein2) exhibits a partial ARR defect and reduced expression of ANAC055 and ANAC092, suggesting that EIN2 is a positive regulator of expression of ANAC055 and ANAC092 during ARR. Phytohormone analyses reveal that JA accumulates to similar levels in young and mature wild-type plants after inoculation with Pst, suggesting that increased expression of some JA/ET-associated genes in mature compared to young plants after inoculation with Pst is not associated with elevated levels of JA. This thesis contributes to our understanding of ARR by identifying some components of the NAC pathway, exploring the relationship between flowering and ARR, and conducting some phytohormone analysis.