Abstract
A controlled flow of porphyrin metabolites is critical for organisms, but little is known about the control of porphyrin biosynthesis under environmental stress. We monitored transgenic rice (Oryza sativa) plants expressing Myxococcus xanthus protoporphyrinogen oxidase (PPO) for their response to drought stress. Transgenic plants showed significantly improved drought tolerance, as indicated by a higher shoot water potential, less oxidative damage, and a more favorable redox balance compared with wild-type plants. Both transgenic and wild-type plants responded to the onset of drought stress, even prior to changes in shoot water potential and oxidative metabolism, by drastically scavenging porphyrin intermediates in leaves, which was crucial for alleviating reactive oxygen species-induced stress. Protoporphyrin IX, protochlorophyllide, magnesium-protoporphyrin IX, and its methyl ester were absent or hardly detected with the intensification of water stress (-3.1 MPa) in the wild type, whereas transgenic plants retained these intermediates to some extent. Additionally, the expression and activity of most enzymes involved in porphyrin biosynthesis, particularly in the chlorophyll branch, were primarily down-regulated under dehydrating conditions, with stronger repression in the wild type than in transgenic plants. There was up-regulation of Glutamate 1-Semialdehyde Aminotransferase, PPO1, and Fe Chelatase2 transcripts in drought-stressed transgenic plants, enabling the transgenic plants to make larger pools of 5-aminolevulinic acid and protoporphyrin IX available for subsequent steps in the heme branch. Overexpression of PPO ultimately protected the transgenic plants from drought-induced cytotoxicity, demonstrating clearly that manipulation of porphyrin biosynthesis can produce drought-tolerant plants. Our results support a possible role for tetrapyrroles in signaling their metabolic state and in plant protection under drought stress conditions.
Highlights
A controlled flow of porphyrin metabolites is critical for organisms, but little is known about the control of porphyrin biosynthesis under environmental stress
To assess the relationship between porphyrin biosynthesis and drought tolerance as well as the metabolic activities in the branched pathway of Mg- and Fe-porphyrin synthesis, we examined the effects of drought stress on the regulation of porphyrin biosynthetic intermediates in leaves
As drought stress markedly affected the levels of Mg-tetrapyrrole intermediates, we considered whether the change in the chlorophyll-heme ratio could be explained by a regulatory switch in Proto IX distribution effected through regulating the genes encoding two enzymes at the branch point, Fe- and Mg-chelatase
Summary
A controlled flow of porphyrin metabolites is critical for organisms, but little is known about the control of porphyrin biosynthesis under environmental stress. Porphyrin Biosynthesis Control under Water Stress porphyrin(ogen)s per light interval, below which plastids with their highly efficient antioxidative defense system can keep these metabolites in their reduced, and nontoxic, state as well as sufficiently detoxify basal levels of photosensitization products (Keetman et al, 2002). Once this limit is exceeded, excited porphyrin(ogen)s tend to spread into other cellular compartments that are less well protected against their photodynamic action
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