Temporal activation of different stress responses in a 24 h diurnal cycle
Circadian control of plant responses to drought
drought responses rely on ABA- dependent and ABA-independent pathways.
- One of the main regulators of drought-stress signaling, cytosolic free [Ca2+], is regulated by the clock and integrates oscillator time with the envi- ronmental signals.
- The stomatal aperture is able to anticipate the dawn and dusk signals
- TOC1 and PRR7 are clock components that might be related to drought responses;GRP7 is highly expressed in guard cells and has a key role in the regulation of stomatal opening.
Role of the circadian clock regulating plant responses to high salinity
- The molecular connection between the clock and plant responses to high salinity seems to rely on the circadian component GIGANTEA (GI), which confers high-salt tolerance through the control of the SALT OVERLY SENSITIVE (SOS) pathway.
- Salt stresses also feed back to regulate the amplitude, period, and phase of the circadian clock
Circadian timing and cold-stress responses in plants Adaptation
The canonical cold-response pathway involves the CBF proteins which function as transcriptional activators of the so-called COLD REGULATED (COR) targets
- CCA1 and LHY positively regulate CBF gene expression by binding directly to their promoters
- The PRR9/7/5 proteins bind to the promoters of CBF genes and repress their transcription in a circadian-dependent manner
- CBF1 regulates LUX transcription by binding directly to its promoter. The lux mutant seedlings are also sensitive to freezing stress
- Alternative splicing is one mechanism by which temperature signals are transmitted to the clock by the cold-induced alternative splicing of LHY, PRR3, 5, 7, 9, and TOC1
Circadian control of plant immunity
the circadian clock has a key role dictating pathogen–host interactions and thereby contributing to enhanced plant fitness by balancing immune responses with cellular metabolism
- arrhythmic plants with a dysfunctional clock such as CCA1 overexpressing plants (CCA1-ox) and elf3-1 mutants do not show the temporal variation in pathogen susceptibility
- The expression of a large number of genes involved in PAMP perception is under the control of the circadian clock even in the absence of pathogen attacks.
- The expres- sion of many R genes and their regulatory targets is also circadian-regulated
- the RPP4 and a majority of its putative target genes have clock-related motifs in their promoters, and accordingly display rhythmic expression that largely over-laps with that of CCA1
- CCA1 directly activates RPP4 to facilitate a gradual and sustained expression of the target defense genes involved in PCD.
- CCA1 can also activate, via a distinctive diurnal phase, some RPP4-target genes independently of RPP4 for basal defense.
- CCA1 and LHY contribute to plant defense through the circadian regulation of stomatal aperture
Defense hormone signaling
The accumulation of salicylic acid (SA) and jasmonic acid (JA) is clock-regulated: SA has a maximum accumulation at midnight, whereas JA peaks around midday, in tune with the corresponding pathogen behaviors.
- The hormone JA is synthesized following pathogen perception.
DOI: 10.1016/j.tplants.2015.01.001
DOI: 10.3389/fpls.2015.00648
Genetic architecture of plant stress resistance: multi-trait genome-wide association mapping
DOI: 10.1111/nph.14220
