Ascorbic Acid Deficiency Activates Cell Death and Disease Resistance Responses in Arabidopsis

Abstract

Programmed cell death, developmental senescence, and responses to pathogens are linked through complex genetic controls that are influenced by redox regulation. Here we show that the Arabidopsis (Arabidopsis thaliana) low vitamin C mutants, vtc1 and vtc2, which have between 10% and 25% of wild-type ascorbic acid, exhibit microlesions, express pathogenesis-related (PR) proteins, and have enhanced basal resistance against infections caused by Pseudomonas syringae. The mutants have a delayed senescence phenotype with smaller leaf cells than the wild type at maturity. The vtc leaves have more glutathione than the wild type, with higher ratios of reduced glutathione to glutathione disulfide. Expression of green fluorescence protein (GFP) fused to the nonexpressor of PR protein 1 (GFP-NPR1) was used to detect the presence of NPR1 in the nuclei of transformed plants. Fluorescence was observed in the nuclei of 6-to 8-week-old GFP-NPR1 vtc1 plants, but not in the nuclei of transformed GFP-NPR1 wild-type plants at any developmental stage. The absence of senescence-associated gene 12 (SAG12) mRNA at the time when constitutive cell death and basal resistance were detected confirms that elaboration of innate immune responses in vtc plants does not result from activation of early senescence. Moreover, H 2 O 2 -sensitive genes are not induced at the time of systemic acquired resistance execution. These results demonstrate that ascorbic acid abundance modifies the threshold for activation of plant innate defense responses via redox mechanisms that are independent of the natural senescence program.The complex relationships between programmed cell death (PCD) and natural senescence observed during leaf development are far from understood. However, one clear distinction is that senescence in leaves is essentially reversible, but PCD is not (Thomas et al., 2003). The genetically programmed cell suicide events that comprise PCD are triggered by enhanced levels of reactive oxygen species (ROS; Chen and Dickman, 2004;Laloi et al., 2004;Wagner et al., 2004). However, senescence-enhanced genes are also expressed in response to ROS (Navabpour et al., 2003).While the chemical nature of ROS dictates that they are potentially harmful to cells, plants use ROS as second messengers in signal transduction cascades regulating diverse processes such as mitosis, tropisms, and cell death. It is now well accepted that ROS accumulation is crucial to plant development as well as defense (Foyer and Noctor, 2005a). ROS signal transduction will ensue only if ROS escape destruction by cellular antioxidants that determine the lifetime and specificity of the signal. Ascorbic acid (AA) and glutathione are the major redox buffers of the plant cells, and they themselves are also signal-transducing molecules that can either signal independently or further transmit ROS signals (Fig. 1). They are thus intrinsic to redox homeostasis and redox-signaling events (Foyer and Noctor, 2005b).ROS production is often genetically programmed, for example, during the hypersen...

Modulation of host plant immunity by Tobamovirus proteins

“…gmp1 silenced plants produced enhanced levels of ROS and reduced TMV accumulation. In agreement with this, previous reports had shown that reduced levels of GMP1 were associated with augmented basal defences mediated by SA and pathogenesis-related (PR) protein expression in A. thaliana (Barth et al, 2004;Pavet et al, 2005). Conti et al (2012) demonstrated that gmp1 silenced N. benthamiana plants also displayed an increased accumulation of PR proteins.…”

Mitochondrial Reactive Oxygen Species. Contribution to Oxidative Stress and Interorganellar Signaling

“…Another example is the biosynthesis of ascorbate, the last step of which involves the mtETC (Siendones et al, 1999;Bartoli et al, 2000;Millar et al, 2003). Stresses on the mtETC could perturb ascorbate synthesis with drastic effects on mitochondrial and cellular redox balance , as exemplified by ascorbatedeficient Arabidopsis mutants (Pavet et al, 2005).…”

Mitogen-Activated Protein Kinases and Reactive Oxygen Species Signaling in Plants

“…Overexpression of ascorbate peroxidase can suppress H 2 O 2 -induced cell death (Murgia et al, 2004), and silencing of thylakoidal ascorbate peroxidase enhances methyl viologeninduced photooxidative stress (Tarantino et al, 2005). Ascorbate-deficient mutants exhibit microlesions and constitutive PR gene expression, lack induction of H 2 O 2 -sensitive genes, and display higher tolerance to infection by P. syringae when compared to wild type (Pavet et al, 2005). Silencing of the tobacco thioredoxin CITRX resulted in enhanced pathogen-induced ROS accumulation, calcium-dependent protein kinase activation, and defense gene expression (Rivas et al, 2004).…”