Salt‐stress induced modulation of chlorophyll biosynthesis during de‐etiolation of rice seedlings

Abstract

Chlorophyll biosynthesis in plants is subjected to modulation by various environmental factors. To understand the modulation of the chlorophyll (Chl) biosynthesis during greening process by salt, 100-200 mM NaCl was applied to the roots of etiolated rice seedlings 12 h prior to the transfer to light. Application of 200 mM NaCl to rice seedlings that were grown in light for further 72 h resulted in reduced dry matter production (-58%) and Chl accumulation (-66%). Ionic imbalance due to salinity stress resulted in additional downregulation (41-45%) of seedling dry weight, Chl and carotenoid contents over and above that of similar osmotic stress induced by polyethylene glycol. Downregulation of Chl biosynthesis may be attributed to decreased activities of Chl biosynthetic pathway enzymes, i.e. 5-aminolevulinic acid (ALA) dehydratase (EC-2.4.1.24), porphobilinogen deaminase (EC-4.3.1.8), coproporphyrinogen III oxidase (EC-1.3.3.3), protoporphyrinogen IX oxidase (EC-1.3.3.4), Mg-protoporphyrin IX chelatase (EC-6.6.1.1) and protochlorophyllide oxidoreductase (EC-1.3.33.1). Reduced enzymatic activities were due to downregulation of their protein abundance and/or gene expression in salt-stressed seedlings. The extent of downregulation of ALA biosynthesis nearly matched with that of protochlorophyllide and Chl to prevent the accumulation of highly photosensitive photodynamic tetrapyrroles that generates singlet oxygen under stress conditions. Although, ALA synthesis decreased, the gene/protein expression of glutamyl-tRNA reductase (EC-1.2.1.70) increased suggesting it may play a role in acclimation to salt stress. The similar downregulation of both early and late Chl biosynthesis intermediates in salt-stressed seedlings suggests a regulatory network of genes involved in tetrapyrrole biosynthesis.

Transcriptomic analysis of salt stress responsive genes in Rhazya stricta

“…The regulated key enzymes in this pathway are 5-aminolevulinic acid (ALA) dehydratase (EC-2.4.1.24), porphobilinogen deaminase (EC-2.5.1.61), coproporphyrinogen III oxidase (EC-1.3.3.3), protoporphyrinogen IX oxidase (EC-1.3.3.4), Mg-protoporphyrin IX chelatase (EC-6.6.1.1) and ferrochelatase (EC-4.99.1.1). These enzymes are nuclear-encoded, but the pathway is located in chloroplasts and mitochondria, and they are important for biosynthesizing chlorophyll and heme in plant leaves [8]. Genes encoding these six enzymes are alaD , pbgD , cpo , ppx , chlD and fch , respectively.…”

“…The first four genes encode enzymes involved in biosynthesis in chloroplasts, while last two enzymes act in directing the pathway towards heme biosynthesis in both chloroplasts and mitochondria. The two tetrapyrroles (chloroplast and heme) are important for plant growth and development, especially under abiotic stresses [8]. The Mg-chelatase enzyme was also reported to respond to changing environments [8].…”

“…The two tetrapyrroles (chloroplast and heme) are important for plant growth and development, especially under abiotic stresses [8]. The Mg-chelatase enzyme was also reported to respond to changing environments [8]. This important pathway is usually severely affected by salt stress in plants.…”

“…Processes like plant growth and photosynthesis are greatly affected by salt stress due to the reduction of chlorophyll biosynthesis, which has a major effect on crop productivity [7]. Strategies of salt tolerance in plants include the maintenance of tetrapyrrole levels (e.g., siroheme, chlorophyll, heme and phytochromobilin) under stress [8]. Tetrapyrroles are mostly synthesized in the chloroplast, with the last two steps of heme synthesis occurring in mitochondria [9].…”

Biochemical and molecular changes induced by salinity stress in Oryza sativa L.

“…3a). The lowering of chlorophyll content is possibly due to the disrupted photosynthetic machinery and decreased activities of different enzymes reported by Turan and Tripathy (2015) in rice seedlings. Amirjani (2011) and Rao et al (2013) also observed the degradation of leaf chlorophyll a, b and total chlorophyll content after NaCl treatment in rice.…”