Abstract
Some chloroplast proteins are known to serve as messengers to transmit retrograde signals from chloroplasts to the nuclei in response to environmental stresses. However, whether particular chloroplast proteins respond to drought stress and serve as messengers for retrograde signal transduction are unclear. Here, we used isobaric tags for relative and absolute quantitation (iTRAQ) to monitor the proteomic changes in tobacco (Nicotiana benthamiana) treated with drought stress/re-watering. We identified 3936 and 1087 differentially accumulated total leaf and chloroplast proteins, respectively, which were grouped into 16 categories. Among these, one particular category of proteins, that includes carbonic anhydrase 1 (CA1), exhibited a great decline in chloroplasts, but a remarkable increase in leaves under drought stress. The subcellular localizations of CA1 proteins from moss (Physcomitrella patens), Arabidopsis thaliana and rice (Oryza sativa) in P. patens protoplasts consistently showed that CA1 proteins gradually diminished within chloroplasts but increasingly accumulated in the cytosol under osmotic stress treatment, suggesting that they could be translocated from chloroplasts to the cytosol and act as a signal messenger from the chloroplast. Our results thus highlight the potential importance of chloroplast proteins in retrograde signaling pathways and provide a set of candidate proteins for further research.
Highlights
Drought stress, which adversely affects plant growth and causes substantial losses in crop production [1], has been exacerbated by climate change
Of particular interest was one category of proteins that declined in abundance in the chloroplasts under drought stress, but significantly increased in the total leaf proteins, which we called PSAPs. These proteins represent potential retrograde signal messengers since they were induced by drought and transferred into the cytosol or nucleus for potential signal transduction. To further explore their function, we investigated the subcellular localization of a moss (Physcomitrella patens), a A. thaliana and a rice (Oryza sativa) homolog of one of these proteins, carbonic anhydrase 1 (CA1), in P. patens protoplasts under osmotic stress
To examine the physiological changes of N. benthamiana plants in response to drought stress, 40 day-old N. benthamiana plants were subjected to drought stress, and their relative water content (RWC), and phytohormone were determined
Summary
Drought stress, which adversely affects plant growth and causes substantial losses in crop production [1], has been exacerbated by climate change. Plants have developed various mechanisms to respond to stress, involving changes in gene expression, metabolism, and physiology [2,3,4]. Cells 2020, 9, 259 plants have complex networks for sensing and signaling environmental stress using different hierarchies of sensors including those in organelles such as chloroplasts [5]. In one stress-sensing/signaling pathway, perturbation of cellular homeostasis due to environmental stress is reported to the nucleus from organelles by so-called retrograde signals [5]. These organelle-derived signals regulate transcriptional activities in the nuclei through communication pathways whose mechanism is unclear [6]. Given that chloroplasts are major sites for the biosynthesis of amino acids, secondary metabolites, and phytohormones [7,8], it poses the possibility that some of these molecules may serve as signals for communication from the plastids to the nuclei; for example, 2-c-methyl-d-erythritol-2,4-cyclo-diphosphate (MEcPP) and phosphor-nucleotide 3-phosphoadenosine
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