Porphyrin Biosynthesis Control under Water Stress: Sustained Porphyrin Status Correlates with Drought Tolerance in Transgenic Rice    

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, magnesiumprotoporphyrin 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.

Tetrapyrrole biosynthetic enzyme protoporphyrinogen IX oxidase 1 is required for plastid RNA editing

“…Several previous studies reported that PPO1 mediates the plant defense response, resistance to peroxidizing herbicide, and drought tolerance (29)(30)(31). These roles of PPO1 are directly linked to the photosensitizing and oxidizing properties of porphyrins, which are synthesized in the tetrapyrrole metabolic pathway.…”

Increased expression of Fe-chelatase leads to increased metabolic flux into heme and confers protection against photodynamically induced oxidative stress

“…FeCh induction plays a positive role in oxidative stress tolerance of transgenic rice Plants suffer severe photodynamic damage if the metabolic flux through the porphyrin biosynthetic pathway is deregulated, for example, in plants with treatment of peroxidizing herbicides (Lee and Duke 1994;Ha et al 2004;Jung et al 2004), under environmental stress (Phung et al 2011) or with deregulation of porphyrin biosynthetic genes (Lermontova and Grimm 2000;Jung et al 2008;Peter et al 2010). Transgenic tobacco plants expressing FeCh antisense RNA exhibited leaf necrosis, which was mainly due to accumulation of photoreactive Proto IX and partly to heme deficiency .…”

“…Similarly, the Arabidopsis mutant aci5-3, with no CHLI-1 RNA, accumulates less Proto IX but more heme in response to AF in comparison to WT, leading to resistance to AF (Soldatova et al 2005). WT and PPO-overexpressing rice respond to drought with a drastic reduction in Mg-porphyrins and more relevant, with a transient increase of the heme content, which, in analogy, may fulfill physiological purposes in response to the inevitable ROS production following water stress (Phung et al 2011). Our work substantiates the hypothesis that heme plays an essential part in the regulation of defense responses against oxidative stress.…”

“…7) since the unbound heme is used to increase the activity of heme-containing ROS scavengers (Phung et al 2011;, which are required for the cell to cope with the accompanying oxidative stress (Asada 1994). Unlike the decrease of APX in AF-treated WT plants, the increased heme content in transgenic plants was accompanied by sustained APX activity during AF-induced oxidative stress (Figs.…”

“…In addition, heme is required for the upregulation of ROS scavengers (catalase, peroxidases) during plant stress response . The strong expression of FeCh1, which plays an essential role in heme synthesis, ensures the supply of cofactors for hemoproteins that may be required in response to the stress conditions including those caused by norflurazon (NF), paraquat, wounding, ozone, and drought Ujwal et al 2002;Nagai et al 2007;Moulin et al 2008;Phung et al 2011;Avin-Wittenberg et al 2012). Stressinduced perturbations of the tetrapyrrole pathway trigger large changes in nuclear gene expression, and the specific accumulation of the chlorophyll intermediates MgProto IX and its monomethylester (ME) are proposed to coincide with these changes, suggesting that they may be important for plastid-to-nucleus signaling (Strand et al 2003;Alawady and Grimm 2005;Pontier et al 2007;Barajas-López et al 2013).…”