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Pathway details: proline biosynthesis III


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  Pathway was created on Mon Jul 29, 2013.
 Contributed by junyoni:
General Background: In addition to be a major constituent of proteins, L-proline also acts as an osmotic protectant in bacteria, plants and animals that are under osmotic stress. In Arabidopsis, proline can account for up to 20% of the free amino acid pool after salt stress . There are two alternative routes in proline biosynthesis in higher plants: the L-ornithine and the L-glutamate pathways. It is also known that, as in plants, both ornithine and glutamate are precursors of proline biosynthesis in microorganisms and mammals. The plant glutamate pathway differs from that in bacteria and human. In bacteria and human, the conversion of glutamate to glutamate-5-semialdehyde (GSA) is catalyzed by two enzymes via two consecutive reactions, whereas, in higher plants the conversion is catalyzed by a bi-functional enzyme in a single reaction . Many research activities have been devoted to understand the relative contributions of the two alternative pathways to the increased proline accumulations under stress. Measurement of free proline content and gene expression and enzyme activity levels from salt-stress treated plants showed that in young Arabidopsis plants, both the ornithine and the glutamate pathways contribute to the increase of proline level during osmotic stress, whereas, in adult plants, only the glutamate pathway plays an important role in proline accumulation. The decreased ornithine aminotransferase (OAT) and increased pyrroline-5-carboxylate synthetase (P5CS) transcripts level in salt-stressed moth bean plants suggested that the glutamate pathway is the major contributor in proline accumulation upon osmotic stress . The same study also found that the glutamate pathway is upregulated under nitrogen limitation, whereas, the ornithine pathway is prominent under high nitrogen input. Studies of transcripts level of P5CS and δ-OAT and their correlations with the free proline content in Medicago, however, suggested that both the ornithine and the glutamate pathways contribute to the osmotic stress-induced proline accumulation . Abscisic acid (ABA) and salt stress stimulate proline biosynthesis in plants . On the other hand, phospholipase D (PLD) was shown a negative regulator of proline biosynthesis in Arabidopsis thaliana . The first reaction catalyzed by P5CS is the rate-limiting step. P5CS is primarily regulated at the transcriptional level.
  Parts of this pathway occur in: multiple locations     cytosol     nucleus     mitochondrion     plastid  


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