Date Published: January 20, 2017
Publisher: Public Library of Science
Author(s): Eve Kaurilind, Mikael Brosché, Jinxing Lin.
Plants are exposed to abiotic and biotic stress conditions throughout their lifespans that activates various defense programs. Programmed cell death (PCD) is an extreme defense strategy the plant uses to manage unfavorable environments as well as during developmentally induced senescence. Here we investigated the role of leaf age on the regulation of defense gene expression in Arabidopsis thaliana. Two lesion mimic mutants with misregulated cell death, catalase2 (cat2) and defense no death1 (dnd1) were used together with several double mutants to dissect signaling pathways regulating defense gene expression associated with cell death and leaf age. PCD marker genes showed leaf age dependent expression, with the highest expression in old leaves. The salicylic acid (SA) biosynthesis mutant salicylic acid induction deficient2 (sid2) had reduced expression of PCD marker genes in the cat2 sid2 double mutant demonstrating the importance of SA biosynthesis in regulation of defense gene expression. While the auxin- and jasmonic acid (JA)- insensitive auxin resistant1 (axr1) double mutant cat2 axr1 also led to decreased expression of PCD markers; the expression of several marker genes for SA signaling (ISOCHORISMATE SYNTHASE 1, PR1 and PR2) were additionally decreased in cat2 axr1 compared to cat2. The reduced expression of these SA markers genes in cat2 axr1 implicates AXR1 as a regulator of SA signaling in addition to its known role in auxin and JA signaling. Overall, the current study reinforces the important role of SA signaling in regulation of leaf age-related transcript signatures.
Plants are sessile organisms and typically experience altered environmental conditions throughout their life cycle. Plant survival depends on their ability to acclimate to the surrounding environment and requires systemic signaling from mature to young developing leaves [1–4]. Reactive oxygen species (ROS) are produced during cell metabolism and production rates increase under stress conditions leading to plant damage [5,6]. However, ROS are not only damaging agents, they are actively produced by the plant and used as signaling molecules both in development and in response to abiotic and biotic stress [7–9]. Hydrogen peroxide (H2O2) is the most stable ROS and an important signaling molecule involved in triggering tolerance to various abiotic and biotic stresses at low concentrations; high concentrations lead directly to programmed cell death (PCD) . The life-time of ROS signals is controlled by antioxidants and ROS scavenging enzymes. About 70% of H2O2 is produced during photorespiration  which may help protect the cell and provide adaption to unfavorable conditions . Catalases are the main enzymes detoxifying H2O2 to H2O and O2 in the peroxisome . However, catalases can also be involved in the removal of H2O2 from other subcellular compartments and thus function as a sink for cellular H2O2 .
Systemic signaling is under intense study in plants. From a mechanistic perspective systemic signaling studies provide information on how cells and organs communicate with each other; from a practical perspective these studies aim to identify signals or even molecules that induce defense responses that could be of agricultural importance such as providing more rapid activation of defense after pathogen attack . Here we used two different mutants that display increased defense gene expression and spontaneous cell death: cat2 where cell death develops as a result of increased H2O2 production  and dnd1 where cell death develops due to misregulated CYCLIC NUCLEOTIDE GATED CHANNEL 2 function and SA signaling . The five marker genes (FMO1, PLA2A, WRKY75, WRKY40 and GLTP) enabled evaluation of the role of leaf age on the expression of cell death related marker genes. In Col-0 and cat2, four of the genes (FMO1, PLA2A, WRKY75 and GLTP) had lowest expression in young leaves, increasing in mature leaves and highest in old leaves. Similarly, PLA2A and WRKY75 increased with leaf age in dnd1. Overall, this suggests that the youngest leaves have not yet entered into a strong defense or cell death program, and may instead be under regulation by a developmental program. Furthermore, the youngest leaves are likely to be sink leaves that receive their photosynthates from the mature and old leaves . Especially in cat2 this means that the photorespiration is lower in the youngest leaves and would not accumulate as high levels of H2O2 as the mature and old leaves. Consistent with this observation, when cat2 is put into conditions of severe high light stress the youngest leaves do not develop cell death . Similarly, in dnd1, spontaneous cell death is more prominent in the mature and old leaves . High expression of defense genes in old leaves may be an adaptive response, since older leaves are more resistant to pathogens through e.g. accumulation of SA [48,49].