Abstract: Not Available
A continuous evolutionary process is the reason for existence of proliferating species in this changing ecosystem. Many species interact with each other in different ways and this interaction may have positive or negative consequences for all interacting species. Because agriculture has significant role in human life, so study based on effects or different factors responsible for damage to agriculture have always remained a hot issue among scientists. Plant pathogens are one of the important factors having significant negative effect on plants and as a result plants have evolved their own defense system against them (e.g., inducible resistance (R) gene mediated and basal defense mechanisms). Now-a-days a lot of research is being conducted at molecular level to employ the plant’s own defense system against plant pathogens. Pathogen can attack plant at any stage of life, so a constitutively active defense system is required for their effective control. But for continuously active system plant has to pay a fitness cost associated with other physiological functions like growth and reproduction (Karban, 2011). So balance is inevitable among defense and growth related metabolic activities of plants.
Recently Wang et al. (2011b) explained the regulation of R gene mediated and basal defense system genes against biotrophic pathogen with reference to circadian clock. They categorized the resistance of Arabidopsis against Hyaloperonospora arabidopsidis (Hpa) isolate Emwa1. Arabidopsis Columbia (Col-0) carrying the R gene “RPP4” responsible for resistance against Hpa was analyzed for differentially expressed genes. They identified 22 differentially expressed genes in wild type (RPP4) and mutant (rpp4). Individual mutants of these 22 genes were used to collect phenotypic data of different disease related parameters and then grouped into two clusters. Mutants of first cluster were found to have defects in R gene mediated Programmed Cell Death (PCD) resistance, while in second cluster mutants were having poor expression of genes related with basal defense mechanism. Some mutant plants (defective in PCD) were also identified that showed the susceptibility to gram negative bacterium which indicates the involvement of these genes in Microbial Associated Molecular Pattern (MAMP) triggered basal immunity. On further investigation some genes showed active role in both R gene mediated and basal defense system.
To get understanding that how a gene (involved in both R-mediated specific and non-specific basal resistance response) is regulated, Wang et al. (2011b) studied the promoter region of all mutant genes. These promoter regions were found to have evening elements that were regulated (both positively and negatively) by circadian regulator (CCA1). In order to confirm the role of CCA1 in plant defense regulation, mutants of cca1, ztl-4 (a mutant of ZEITLUPE) and a line over expressing the CCA1 were inoculated with Hpa Emwa1 at dawn. Both mutants (cca1 and ztl-4) showed compromised resistance but the line that was over expressing the CCA1 showed enhanced resistance indicating that it positively regulates the resistance mechanism. To further confirm the role of CCA1 in the regulation of R-mediated resistance response expression of cluster 1 genes was monitored in both wild type (CCA1) and mutant (cca1) plants. A rhythmic expression of these genes was observed with maximum expression peaks at every evening but in cca1 these genes showed diminished expression, confirming that cca1 is activator of these genes. Hpa sporulation mainly occur at dawn time and this rhythmic expression of defense genes in the absence of pathogen indicates that plants are programmed to anticipate infection according to a circadian schedule. It was further confirmed by infecting wild type and mutants at dusk, consequently both wild type and mutant showed enhanced susceptibility. All of these findings suggest potential role of circadian rhythm in the regulation of plant defense system.
A circadian clock regulates the plants activities and enhances the balanced interaction between plants and its environment. Hotta et al. (2007) concluded on the basis of reviewed literature that circadian clock modulates the plants metabolism depending upon its environmental signals, in addition plants may react differently for the same signal at different times of day. Circadian clock oscillates the plants metabolism according to the environmental signals to make the metabolism in a rhythmic motion of nearly 24 h. Plant defense response is a highly complicated process and circadian clock plays an important role in regulating these responses (Roden and Ingle, 2009; Wang et al., 2011a). Light possess significant role as a stimulus in defining the plant response towards a pathogen directly or indirectly (Chandra-Shekara et al., 2006; Griebel and Zeier, 2008). In study conducted by Wang et al. (2011b) circadian rhythm of gene expression may have evolved with continuous attack of Hpa on plant at dawn time and to anticipate the attack of pathogen plant follow a specific rhythmic activation of defense related genes. This study conducted by Wang et al. (2011b) is pioneering in its linking of plant defense responses with the circadian clock. Future work will improve the understanding of plant defense regulation at the molecular level in relation to circadian control and may open up new avenues for enhancing crop protection.