The circadian clock components constitute DNA‐binding transcription factors. In the model plant, Arabidopsis thaliana, the circadian clock framework is made up of various transcriptional–translational feedback loops (TTFLs). It has been documented that several abiotic factors, such as light and temperature, provide an input signal to the clock in turn, the clock provides various output signals that influence the downstream processes (Kinmonth‐Schultz et al., 2013). Since 1985, when the molecular study of the plant circadian clock began, the clock network has widened, and we are still trying to delineate it (Kloppstech, 1985 McClung, 2014). Many of the plant physiological and developmental processes are regulated by this endogenous regulator (Yakir et al., 2007). Plants, being sessile in nature, are dependent on this endogenous clock machinery to acclimatize themselves with environmental variations and to prepare for future changes. It is well documented that a diverse range of organisms, including cyanobacteria, algae, fungi, plants, flies, birds and mammals, possess their own circadian clock (Bell‐Pedersen et al., 2005). The circadian clock temporally coordinates biological processes with diurnal environmental changes, thus enhancing overall fitness. The circadian rhythms are maintained by a well‐defined molecular machinery-the circadian clock. These circadian rhythms persist even in the absence of external cues for a definite time period, known as the free running time (Jones, 2009). An endogenous biological rhythm with a period of approximately 24 h is known as the circadian rhythm (McClung, 2001). The biological processes of an organism change greatly during the day and night, and continuous entrainment by environmental factors results in the establishment of a biological rhythm. An understanding of this dynamic relationship between clock and stress will open up new avenues in the understanding of endogenous mechanisms of defence signalling in plants. We focus on how the circadian clock gates the expression of various stress‐related transcripts in a prolific manner to enhance plant fitness. In this review, we highlight the studies carried out so far on two model plant pathogens, namely Pseudomonas syringae and Hyaloperonospora arabidopsidis, and the involvement of the circadian clock in gating effector‐triggered immunity and pathogen‐associated molecular pattern‐triggered immunity. It has been proposed that the circadian clock shapes the outcome of plant–pathogen interactions. Recent studies on circadian clock genes and genes involved in defence signalling have indicated a possible reciprocal interaction between the two. Cross‐talk between the circadian clock and a diverse range of physiological processes in plants, including stress acclimatization, hormone signalling, photomorphogenesis and defence signalling, is currently being explored. The circadian clock is the internal time‐keeping machinery in higher organisms.
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