Rice TSV2 Encoding Threonyl-tRNA Synthetase is Needed for Early Chloroplast Development and Seedling Growth under Cold Stress

Threonyl-tRNA synthetase (ThrRS), one of aminoacyl-tRNA synthetases (AARSs), plays a crucial role in protein synthesis. However, the AARS functions on rice chloroplast development and growth were not fully appraised. In this study, a thermo-sensitive virescent mutant tsv2, which showed albino phenotype and lethal after the 4-leaf stage at 20 °C but recovered to normal when the temperatures rose, was identified and characterized. Map-based cloning and complementation tests showed that TSV2 encoded a chloroplast-located ThrRS protein in rice. The Lys-to-Arg mutation in the anticodon-binding domain hampered chloroplast development under cold stress, while the loss-of-function of the ThrRS core domain in TSV2 fatally led to seedling death regardless of growing temperatures. In addition, TSV2 had a specific expression in early leaves. Its disruption obviously resulted in down-regulation of certain genes associated with chlorophyll biosynthesis, photosynthesis and chloroplast development at cold conditions. Our observations revealed that rice nuclear-encoded TSV2 plays an important role in chloroplast development at the early leaf stage under cold stress.

Genome sequencing identified novel mechanisms underlying virescent mutation in upland cotton Gossypiuma hirsutum

Background Virescent mutation broadly exists in plants and is an ideal experimental material to investigate regulatory mechanisms underlying chlorophyll synthesis, photosynthesis and plant growth. Up to date, the molecular mechanisms in two virescent mutations have been clarified in cottons (Gossypiuma hirsutum). A virescent mutation has been found in the cotton strain Sumian 22, and the underlying molecular mechanisms have been studied. Methods The virescent mutant and wild type (WT) of Sumian 22 were cross-bred, and the F1 population were self-pollinated to calculate the segregation ratio. Green and yellow leaves from F2 populations were subjected to genome sequencing and bulked-segregant analysis was performed to screen mutations. Real-time quantitative PCR (RT-qPCR) were performed to identify genes in relations to chlorophyll synthesis. Intermediate products for chlorophyll synthesis were determined to validate the RT-qPCR results. Results The segregation ratio of green and virescent plants in F2 population complied with 3:1. Compared with WT, a 0.34 Mb highly mutated interval was identified on the chromosome D10 in mutant, which contained 31 genes. Among them, only ABCI1 displayed significantly lower levels in mutant than in WT. Meanwhile, the contents of Mg-protoporphyrin IX, protochlorophyllide, chlorophyll a and b were all significantly lower in mutant than in WT, which were consistent with the inhibited levels of ABCI1. In addition, a mutation from A to T at the -317 bp position from the start codon of ABCI1 was observed in the genome sequence of mutant. Conclusions Inhibited transcription of ABCI1 might be the mechanism causing virescent mutation in Sumian 22 cotton, which reduced the transportation of protoporphyrin IX to plastid, and then inhibited Mg-protoporphyrin IX, Protochlorophyllide and finally chlorophyll synthesis. These results provided novel insights into the molecular mechanisms underlying virescent mutation in cotton.

Rice TSV2 Encoding Threonyl-tRNA Synthetase is Needed for Early Chloroplast Development and Seedling Growth under Cold Stress

Background The chloroplast is a vital photosynthetic organelle for plant growth and development. However, the genetic factors involved in chloroplast development and its relationship with environment factors are largely unknown. Threonyl-tRNA synthetase (ThrRS), one of aminoacyl-tRNA synthetases (AARSs), plays a crucial role in protein synthesis. To date, there are few studies for AARS function on chloroplast development and plant growth, much less ThrRS in rice. Result In this paper, we characterized a thermo-sensitive virescent mutant tsv2, which showed albino phenotype and could not survive after the 4-leaf stage when grown at 20 °C, but recovered the normal phenotype when the temperature rose. Map-based cloning and complementation tests showed that TSV2 encoded a chloroplast-located ThrRS protein in rice and the Lys-to-Arg mutation in the anticodon-binding domain affected chloroplast development under cold stress. Furthermore, the loss-of-function of the core domain in TSV2 led to seedling death regardless of temperatures. In addition, TSV2 had a tissue-specific expression, and its disruption resulted in an evidently down-regulation of certain genes associated with chlorophyll biosynthesis, photosynthesis and chloroplast development at cold stress. Conclusion The TSV2 encodes a rice threonyl-tRNA synthetase, located in chloroplasts, which is essential for cold-responsive regulation for chloroplast development and plant growth and closely related to the assembly of chloroplast ribosomes and functions at the first step of chloroplast differentiation.

RiceTSV3Encoding Obg-like GTPase Protein is Essential for Chloroplast Development during the Early Leaf Stage under Cold Stress

ABSTRACTThe Spo0B-associated GTP-binding (Obg) proteins occupy a wide variety of roles in the viability of nearly all bacteria. Its detailed roles in higher plants have not yet been elucidated. A novel rice thermo-sensitive virescent mutanttsv3was identified in this study that displayed albino phenotype at 20°C before the 3-leaf stage while being normal green at 32°C or even at 20°C after the 4-leaf stage. The mutant phenotype was aligned with altered chlorophyll (Chl) content and chloroplast development. Map-based cloning and complementation test showed thatTSV3encoded a kind of small GTP binding protein. Subcellular localization revealed that TSV3 was in chloroplast.TSV3transcripts were highly expressed in leaves and weak or undetectable in other tissues, suggesting the tissue-specific expression. Intsv3mutant, the transcriptional levels of certain genes associated with biogenesis of chloroplast ribosomes 50S subunit were severely decreased at the 3-leaf-stage under cold stress, but could be recovered to normal levels at a higher temperature (32°C). The observations from this study indicated that the rice nuclear-encodedTSV3plays important roles in chloroplast development at early leaf stage under cold stress.

Assimilatory pigments, photosynthetic activity and ultrastructure of chloroplasts of the variegated-leaf chimera of Acer platanoides L.

The assimilatory pigment composition, photosynthetic activity and ultrastructure of chloroplasts were studied in the chlorophyll - deficient chimera of <i>Acer platanoides</i> L. Part of the crown of this chimera is a virescent mutant with variegated leaves. It was found that there exists no qualitative difference in the pigment composition between normal and variegated leaves. The accumulation of chlorophyll in the mutated part is more delayed that the accumulation of carotenoids. The photosynthetic rate on a chlorophyll basis is much higher in variegated than in green leaves. This difference gradually falls off with development. In the early spring, chloroplasts from the yellow spots of leaf blade have no lamellar system but only many vesicles are dispersed in the stroma. Occasionally also a single granum consisting of a few thylakoids occurs in the stroma. At the end of summer chloroplasts from yellow spots of variegated leaves possess a poorly developed lamellar system.

Phenotype, Inheritance, and Regulation of Expression of a New Virescent Mutant in Watermelon: Juvenile Albino

Juvenile albino, gene symbol ja, is a spontaneous virescent mutant, first observed in `Dixielee' and an F2 population of `G17AB' (msms) × `Dixielee' in 1992. Hypocotyls, new young leaves, shoot tips, tendrils and flowers on the main shoot of the ja mutant are all albino during early spring. The interior portions of albino leaves gradually become green, while the margins remain albino. Fruit rind color of the mutant is variegated. Growth of the ja mutant is severely impaired in the early spring. However, the mutant grows at a rate comparable to wild-type in the summer, and produces fruit of almost normal size. Genetic analysis of F1, F2, and BC1 populations derived from the ja mutant showed that the gene for the ja mutant is inherited as a single, recessive, nuclear gene. Segregation ratios in the F2 and BC1 progenies derived from the cross between the previously reported delayed green virescent mutant and the ja mutant indicate independent inheritance of the genes dg and ja. Temperature and red/far-red light had no differential effect on mutant and the wild-type plants. An increase of daylength from 8 to 15 hours increased fresh weight and chlorophyll content more in the ja mutant than in the wild-type. The mutant had a higher chlorophyll a: b ratio than the wild-type under long days. Chlorophyll synthesis or accumulation in the mutant is severely impaired under short days. This is the only virescent mutant in the family Cucurbitaceae whose expression is regulated by daylength.