Identification and Phenotypic Characterization of ZEBRA LEAF16 Encoding a β-Hydroxyacyl-ACP Dehydratase in Rice

Fatty acid de novo biosynthesis in plastids: Key enzymes and their critical roles for male reproduction and other processes in plants

Fatty acid de novo biosynthesis in plant plastids is initiated from acetyl-CoA and catalyzed by a series of enzymes, which is required for the vegetative growth, reproductive growth, seed development, stress response, chloroplast development and other biological processes. In this review, we systematically summarized the fatty acid de novo biosynthesis-related genes/enzymes and their critical roles in various plant developmental processes. Based on bioinformatic analysis, we identified fatty acid synthase encoding genes and predicted their potential functions in maize growth and development, especially in anther and pollen development. Finally, we highlighted the potential applications of these fatty acid synthases in male-sterility hybrid breeding, seed oil content improvement, herbicide and abiotic stress resistance, which provides new insights into future molecular crop breeding.

Morphological Analyses and QTL Mapping of Mottled Leaf in Zucchini (Cucurbita pepo L.)

The mottled leaf is one of the agronomic traits of zucchini and can be applied as a marker trait in aggregation breeding. However, the genetic mechanism responsible for mottled leaf has yet to be elucidated. In the present study, we used two inbred lines (line '19': silver mottled leaf; line '113': normal leaf) as parents for the physiological and genetic analysis of mottled leaf. The synthesis and net photosynthetic rate of chlorophyll were not significantly affected in the mottled areas of leaves. However, we detected a large space between the palisade parenchyma in the leaf mottle area of line '19', which may have caused the mottled leaf phenotype. Light also plays an important role in the formation of mottled leaf, and receiving light during the early stages of leaf development is a necessary factor. Genetic analysis has previously demonstrated that mottled leaf is a quantitative trait that is controlled by multiple genes. Based on the strategy of quantitative trait locus sequencing (QTL-seq), two QTLs were identified on chromosomes 1 and 17, named CpML1.1 and CpML17.1, respectively. Two major loci were identified using R/qtl software version 1.66 under greenhouse conditions in April 2019 (2019A) and April 2020 (2020A) and under open cultivation conditions in May 2020 (2020M). The major QTL, CpML1.1, was located in a 925.2-kb interval on chromosome 1 and explained 10.51%-24.15% of the phenotypic variation. The CpML17.1 was located in a 719.7-kb interval on chromosome 17 and explained 16.25%-38.68% of the phenotypic variation. Based on gene annotation, gene sequence alignment, and qRT-PCR analysis, the Cp4.1LG01g23790 at the CpML1.1 locus encoding a protein of the TPX2 family (target protein of Xklp2) may be a candidate gene for mottled leaf in zucchini. Our findings may provide a theoretical basis for the formation of mottled leaf and provide a foundation for the fine mapping of genes associated with mottled leaf. Molecular markers closely linked to mottled leaf can be used in molecular-assisted selection for the zucchini mottled leaf breeding.

Genetic mapping and molecular characterization of the delayed green gene dg in watermelon (Citrullus lanatus)

Leaf color mutants are common in higher plants that can be used as markers in crop breeding and are important tools in understanding regulatory mechanisms of chlorophyll biosynthesis and chloroplast development. Genetic analysis was performed by evaluating F₁, F₂ and BC₁ populations derived from two parental lines (Charleston gray with green leaf color and Houlv with delayed green leaf color), suggesting that a single recessive gene controls the delayed green leaf color. In this study, the delayed green mutant showed a conditional pale green leaf color at the early leaf development but turned to green as the leaf development progressed. Delayed green leaf plants showed reduced pigment content, photosynthetic, chlorophyll fluorescence parameters, and impaired chloroplast development compared with green leaf plants. The delayed green (dg) locus was mapped to 7.48 Mb on chromosome 3 through bulk segregant analysis approach, and the gene controlling delayed green leaf color was narrowed to 53.54 kb between SNP130 and SNP135 markers containing three candidate genes. Sequence alignment of the three genes indicated that there was a single SNP mutation (G/A) in the coding region of ClCG03G010030 in the Houlv parent, which causes an amino acid change from Arginine to Lysine. The ClCG03G010030 gene encoded FtsH extracellular protease protein family is involved in early delayed green leaf development. The expression level of ClCG03G010030 was significantly reduced in delayed green leaf plants than in green leaf plants. These results indicated that the ClCG03G010030 might control watermelon green leaf color and the single SNP variation in ClCG03G010030 may result in early delayed green leaf color development during evolutionary process.

A β-ketoacyl carrier protein reductase confers heat tolerance via the regulation of fatty acid biosynthesis and stress signaling in rice

Heat stress is a major environmental threat affecting crop growth and productivity. However, the molecular mechanisms associated with plant responses to heat stress are poorly understood. Here, we identified a heat stress-sensitive mutant, hts1, in rice. HTS1 encodes a thylakoid membrane-localized β-ketoacyl carrier protein reductase (KAR) involved in de novo fatty acid biosynthesis. Phylogenetic and bioinformatic analysis showed that HTS1 probably originated from streptophyte algae and is evolutionarily conserved in land plants. Thermostable HTS1 is predominantly expressed in green tissues and strongly induced by heat stress, but is less responsive to salinity, cold and drought treatments. An amino acid substitution at A254T in HTS1 causes a significant decrease in KAR enzymatic activity and, consequently, impairs fatty acid synthesis and lipid metabolism in the hts1 mutant, especially under heat stress. Compared to the wild-type, the hts1 mutant exhibited heat-induced higher H₂ O₂ accumulation, a larger Ca²⁺ influx to mesophyll cells, and more damage to membranes and chloroplasts. Also, disrupted heat stress signaling in the hts1 mutant depresses the transcriptional activation of HsfA2s and the downstream target genes. We suggest that HTS1 is critical for underpinning membrane stability, chloroplast integrity and stress signaling for heat tolerance in rice.

Enhanced SA and Ca2+ signaling results in PCD-mediated spontaneous leaf necrosis in wheat mutant wsl

Leaf is the major photosynthesis organ and the key source of wheat (Triticum aestivum L.) grain. Spotted leaf (spl) mutant is a kind of leaf lesion mimic mutants (LMMs) in plants, which is an ideal material for studying the mechanisms of leaf development. In this study, we report the leaf abnormal development molecular mechanism of a spl mutant named white stripe leaf (wsl) derived from wheat cultivar Guomai 301 (WT). Histochemical observation indicated that the leaf mesophyll cells of the wsl were destroyed in the necrosis regions. To explore the molecular regulatory network of the leaf development in mutant wsl, we employed transcriptome analysis, histochemistry, quantitative real-time PCR (qRT-PCR), and observations of the key metabolites and photosynthesis parameters. Compared to WT, the expressions of the chlorophyll synthesis and photosynthesis-related homeotic genes were repressed; many genes in the WRKY transcription factor (TF) families were highly expressed; the salicylic acid (SA) and Ca²⁺ signal transductions were enhanced in wsl. Both the chlorophyll contents and the photosynthesis rate were lower in wsl. The contents of SA and reactive oxygen species (ROS) were significantly higher, and the leaf rust resistance was enhanced in wsl. Based on the experimental data, a primary molecular regulatory model for leaf development in wsl was established. The results indicated that the SA accumulation and enhanced Ca²⁺ signaling led to programmed cell death (PCD), and ultimately resulted in spontaneous leaf necrosis of wsl. These results laid a solid foundation for further research on the molecular mechanism of leaf development in wheat.

Identification and fine mapping of a recessive gene controlling zebra leaf phenotype in maize

Leaf color mutant is an important resource for studying chlorophyll biosynthesis and chloroplast development in maize. Here, a novel mutant zebra crossband 9 (zb9) with transverse green-/yellow-striped leaves appeared from ten-leaf stage until senescence was identified from mutant population derived from the maize inbred line RP125. The yellow section of the zb9 mutant displays a reduction of chlorophyll and carotenoid contents, as well as impaired chloroplast structure. Genetic analysis showed that the zb9 mutant phenotype was caused by a single recessive gene. Map-based cloning demonstrated that the zb9 locus was delimited into a 648 kb region on chromosome 1 covering thirteen open reading frames (ORFs). Among them, a point mutation (G to A) in exon 2 of the gene Zm00001d029151, named Zmzb9, was identified based on sequencing analysis. The causal gene Zmzb9 encodes UDP-glucose-4-epimerase 4 (UGE4), a key enzyme involved in chloroplast development and was considered as the only candidate gene controlling the mutant phenotype. Expression patterns indicated that the causal gene was abundantly expressed in the leaves and sheaths, as well as significantly downregulated in the mutant compared to that in the wild type. Subcellular localization showed that ZmZB9 was localized in chloroplasts and implied the putative gene involved in chloroplast development. Taken together, we propose that the causal gene Zmzb9 tightly associated with the zebra leaf phenotype, and the obtained gene here will help to uncover the regulatory mechanism of pigment biosynthesis and chloroplast development in maize.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-021-01202-7.

WHITE STRIPE LEAF8, encoding a deoxyribonucleoside kinase, is involved in chloroplast development in rice

WSL8 encoding a deoxyribonucleoside kinase (dNK) that catalyzes the first step in the salvage pathway of nucleotide synthesis plays an important role in early chloroplast development in rice. The chloroplast is an organelle that converts light energy into chemical energy; therefore, the normal differentiation and development of chloroplast are pivotal for plant survival. Deoxyribonucleoside kinases (dNKs) play an important role in the salvage pathway of nucleotides. However, the relationship between dNKs and chloroplast development remains elusive. Here, we identified a white stripe leaf 8 (wsl8) mutant that exhibited a white stripe leaf phenotype at seedling stage (before the four-leaf stage). The mutant showed a significantly lower chlorophyll content and defective chloroplast morphology, whereas higher reactive oxygen species than the wild type. As the leaf developed, the chlorotic mutant plants gradually turned green, accompanied by the restoration in chlorophyll accumulation and chloroplast ultrastructure. Map-based cloning revealed that WSL8 encodes a dNK on chromosome 5. Compared with the wild type, a C-to-G single base substitution occurred in the wsl8 mutant, which caused a missense mutation (Leu 349 Val) and significantly reduced dNK enzyme activity. A subcellular localization experiment showed the WSL8 protein was targeted in the chloroplast and its transcripts were expressed in various tissues, with more abundance in young leaves and nodes. Ribosome and RNA-sequencing analysis indicated that some components and genes related to ribosome biosynthesis were down-regulated in the mutant. An exogenous feeding experiment suggested that the WSL8 performed the enzymic activity of thymidine kinase, especially functioning in the salvage synthesis of thymidine monophosphate. Our results highlight that the salvage pathway mediated by the dNK is essential for early chloroplast development in rice.

Zebra leaf 15, a receptor-like protein kinase involved in moderate low temperature signaling pathway in rice

BACKGROUND

Zebra leaf mutants are an important resource for studying leaf colour in rice. In most such mutants, the zebra leaf phenotype results from defective chloroplast biogenesis. The molecular mechanism by which zebra leaves develop remains unclear, so additional zebra-leaf mutants need to be identified.

RESULTS

We isolated a novel rice zebra-leaf mutant, named zebra leaf 15 (z15), which showed transversely striped leaves with yellow-green or white-green sectors, in which chloroplast structure was disturbed. Transmission electron microscopy revealed that the structure of various organelles was impaired in yellow/white sectors. Z15, a single-copy gene in the rice genome, encodes a receptor-like protein kinase. Subcellular localization analysis indicates that Z15 and z15 are localized on the plasma membrane. The expression of Z15 is induced by moderate low temperature (18 °C). The mutation of Z15 influenced the expression of two downstream genes, OsWRKY71 and OsMYB4, that were responsive to moderate low temperature. The results show that Z15 plays a crucial role in the early stages of the response to moderate low temperature in rice.

CONCLUSIONS

We identified a novel zebra-leaf mutant (z15) that impaired chloroplast structure in rice, LOC_Os05g12680, encoding a receptor-like protein kinase. Further study indiceted that Z15 plays a crucial role in the early stages of the response to moderate low temperature in rice.