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.