Thioredoxins (TRXs) are a large family of small proteins that function in multiple metabolic processes in all living organisms. They are a good example of proteins that have diversified throughout evolution, probably from one ancestral protein that arose in ancient cyanobacteria.
TRXs are ubiquitous cellular workhorses that can alter the catalytic properties of specific enzymes. In these so-called redox-regulated reactions, TRXs can transfer hydrogen to convert a disulfide bridge formed between two cysteine amino acids to their unbridged thiol forms. These TRX-responsive disulfide bridges have regulatory functions in numerous cellular processes, including regulatory cascades related to enzyme regulation, stress responses, DNA translation and transcription, protein folding and repair, cell death, and gene expression.
Plants have the largest number of thioredoxins of all organisms and are found in all of the major compartments of the plant cell - cytosol, mitochondria, and chloroplasts. In chloroplasts, TRXs regulate numerous enzymes involved in the transformation of carbon into sugars. They also function to regulate the photosynthetic production of ATP, the energy currency of all living organisms and in fatty acid synthesis.
In a report to be published in the November issue of Plant Physiology, researchers have found that the f and m type plant thioredoxins previously thought to be localized only in chloroplasts are also found in other, nonphotosynthetic, tissues, where they may have multiple functions. A team from the laboratory of Dr. Mariam Sahrawy of the Estaci¨®n Experimental del Zaidin in Granada, Spain, established the presence of these redox proteins in tissues other than the chloroplast by fusing the promoters of these genes to a visible reporter molecule, ¦Â-glucuronidase (GUS), and by expression analysis, in situ hybridization, and immunological analysis.
All the different types of plant TRXs are encoded in the nucleus. The f and m types were isolated from chloroplasts and function there in light-regulated reactions related to photosynthesis. h-type TRXs are localized in the cytosol and are thought to function in seeds in maturation and germination, in plant reproduction, and in other signaling pathways. Thioredoxin h is also thought to be a mobile element in plants transported through the phloem or directly from cell to cell through plasmodesmata and thus may act as a messenger molecule in signaling. Several other types of TRXs have also been found in plants.
The scientists identified regions in the control sequences of the two TRX genes, which specify whether the genes will be regulated by light, where they will be expressed, and the metabolic pathway in which they may function. Different metabolic pathways that were identified as TRX regulated include embryo and seed formation, protein storage, the circadian clock, pollen localization, and hormonal regulation.
Dr.Sahrawy and her collaborators Juan de Dios Barajas-Lopez, Antonio Jesus Serrato, Adela Olmedilla and Ana Chueca found that the f and m type TRX genes were expressed in early development of seedlings and in stems, leaves, roots, and flowers, and identified potential new functions in cell division, germination, and plant reproduction. Whereas the light regulated expression of these genes in the chloroplast is well known, the discovery by Sahrawy and her team that their expression in nonphotosynthetic tissues is also regulated by light, albeit indirectly, is a new and potentially far reaching insight into the regulation of plant metabolism.
In addition, their finding that f and m TRXs occur in the vascular tissues of both leaves and roots, suggests that these TRX types may function as messengers or in the regulation of transporter molecules. The proteins were also found in pea seeds and in the ovules and pollen grains of flowers in regions of active cell division, where the f and m TRX types may function in redox regulation or in protection against oxidative damage. Other studies have shown that TRXs can function directly as antioxidants or in signaling pathways that sense reactive oxygen species and can thus protect plants against oxidative damage.
Sahrawy and her team are now concentrating on analyzing the specific functions of these TRXs in different stages of plant development and under different physiological conditions. Knowledge of the diverse functions of these versatile proteins may be important in engineering drought and heat tolerant plants and in helping to protect plants against oxidative damage under stress conditions.
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