Balance Between Plant Growth and Defense: Transcriptional and Translational Control of Plant Immune System
The activation and maintenance of plant immune responses require a significant amount of energy because they are accompanied by massive transcriptional reprogramming. Spurious activation of plant defense machinery can lead to autoimmune diseases and growth inhibition. So it is important for plants to tightly regulate the immune system to ensure the balance between growth and defense. However, neither the molecular mechanisms nor the design principles of how plants reach this balance are understood.
In this dissertation work, I showed how transcriptional and translational control of plant immune system can help avoid the constant immune surveillance and elicit a proper level of defense responses when necessary. These fine tunings of the immune system ensure the balance between growth and defense.
My research on the transcriptional regulation of plant defense responses led to the surprising discovery that even without pathogen, plant can 'anticipate' potential infection according to a circadian schedule under conditions that favor the initiation of infection. Functional analysis of 22 novel immune components unveiled their transient expression at dawn, when the infection is most likely to happen. This pulse expression pattern was shown to be regulated by the central circadian oscillator, CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) since these 22 genes are no longer induced in the cca1 mutant. Moreover, the temporal control of the transcription level of these 22 immune genes by CCA1 also fine tunes their expression pattern according to the perceptions of different pathogenic signals. At the basal defense level, the expression of these genes can be transiently induced upon perceptions of critical infection stages of the pathogen. When an elevated level of defense response is needed, the high expression levels of these genes are maintained to confer a stronger immunity against pathogen. Since this stronger form of defense may also cause the suicidal death of the plant cells, the interplay between the circadian clock and defense allows a better decision on the proper level of the immunity to minimize the sacrificial death. The circadian clock is also known to regulate the growth-related cellular functions extensively. So the circadian clock can help to balance the energy used in growth and defense through transcriptional regulation on both sides.
Besides the integrated control by the circadian clock, the translational control on a key transcription factor involved in the growth-to-defense transition can also maintain the balance between growth and defense.TBF1 is a major transcription factor that can initiate the growth-to-defense transition through transcriptional repression of growth-associated cellular functions and induction of defense-related machinery. Bioinformatics studies identified 2 upstream open reading frames (uORFs) encoding multiple phenylalanine at 5' of the translation initiation codon of TBF1. Under normal conditions, these 2 uORFs can repress the translation of TBF1 to prevent accidental activation. However, pathogen infection may cause rapid and transient depletion of phenylalanine, a well-known precursor for cell wall components and the SAR signal SA. This depletion signal can be reflected by the increase of uncharged tRNAPhe, which subsequently leads to the phosphorylation of eIF2á and the release of uORFs' repression on TBF1. These findings provided the molecular details of how uORF-based translational control can couple transcriptional reprogramming with metabolic status to coordinately trigger the growth-to-defense transition.
In summary, my dissertation work has identified previously unrecognized regulatory mechanisms by which plant immune responses are balanced with growth. These new findings will further investigations into these novel interfaces between plants and pathogens. Future studies will definitely further improve our understandings of the plant-microbe interactions.
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