Brchli1 mutation induces bright yellow leaves by disrupting magnesium chelatase I subunit function in Chinese cabbage (Brassica rapa L. ssp. pekinensis)

Magnesium chelatase (MgCh) plays a pivotal role in photosynthesis, catalyzing the insertion of magnesium into protoporphyrin IX (Proto IX), a key intermediate in chlorophyll (Chl) biosynthesis. MgCh is a heteromeric complex composed of the MgCh D subunit (CHLD), the MgCh H subunit (CHLH), and the MgCh I subunit (CHLI). The bright yellow leaves (byl) mutant was obtained through ethyl methanesulfonate (EMS) mutagenesis of the 'FT' Chinese cabbage (Brassica rapa L. ssp. pekinensis) doubled haploid line, whose Chl content, net photosynthetic rate (Pn), and non-photochemical quenching coefficient (NPQ) were decreased, and whose chloroplast development was incomplete. byl recovered to a light green phenotype under weak light conditions. Genetic analysis revealed that the bright yellow leaves phenotype of byl was caused by a single recessive nuclear gene. Using Mutmap sequencing and Kompetitive allele-specific PCR (KASP) identification, BraA01g010040.3.5C, encoding the CHLI subunit of MgCh, was identified as the candidate gene and named Brchli1. A nonsynonymous G-to-A mutation in the Brchli1 exon resulted in the substitution of aspartic acid with asparagine. Brchli1-silenced Chinese cabbage displayed bright yellow leaves with decreased Brchli1 expression. Transiently overexpressed Brchli1 in the byl mutant restored the green leaf phenotype and significantly increased relative Brchli1 expression levels. Both BrCHLI1 and its mutated variant were localized in chloroplasts. Yeast two-hybrid and luciferase complementation imaging assays demonstrated that BrCHLI1 interacted with both BrCHLD and itself. BrCHLI1 mutations did not affect its interaction with BrCHLD. Together, Brchli1 mutations impaired the function of MgCh, providing insights into the molecular mechanism of leaf coloration.

A reference-guided TILLING by amplicon-sequencing platform supports forward and reverse genetics in barley

Barley is a diploid species with a genome smaller than those of other members of the Triticeae tribe, making it an attractive model for genetic studies in Triticeae crops. The recent development of barley genomics has created a need for a high-throughput platform to identify genetically uniform mutants for gene function investigations. In this study, we report an ethyl methanesulfonate (EMS)-mutagenized population consisting of 8525 M₃ lines in the barley landrace "Hatiexi" (HTX), which we complement with a high-quality de novo assembly of a reference genome for this genotype. The mutation rate within the population ranged from 1.51 to 4.09 mutations per megabase, depending on the treatment dosage of EMS and the mutation discrimination platform used for genotype analysis. We implemented a three-dimensional DNA pooling strategy combined with multiplexed amplicon sequencing to create a highly efficient and cost-effective TILLING (targeting induced locus lesion in genomes) platform in barley. Mutations were successfully identified from 72 mixed amplicons within a DNA pool containing 64 individual mutants and from 56 mixed amplicons within a pool containing 144 individuals. We discovered abundant allelic mutants for dozens of genes, including the barley Green Revolution contributor gene Brassinosteroid insensitive 1 (BRI1). As a proof of concept, we rapidly determined the causal gene responsible for a chlorotic mutant by following the MutMap strategy, demonstrating the value of this resource to support forward and reverse genetic studies in barley.

A BIN2-GLK1 Signaling Module Integrates Brassinosteroid and Light Signaling to Repress Chloroplast Development in the Dark

Arabidopsis GLYCOGEN SYNTHASE KINASE 3 (GSK3)-like kinases play various roles in plant development, including chloroplast development, but the underlying molecular mechanism is not well defined. Here, we demonstrate that transcription factors GLK1 and GLK2 interact with and are phosphorylated by the BRASSINOSTEROID insensitive2 (BIN2). The loss-of-function mutant of BIN2 and its homologs, bin2-3 bil1 bil2, displays abnormal chloroplast development, whereas the gain-of-function mutant, bin2-1, exhibits insensitivity to BR-induced de-greening and reduced numbers of thylakoids per granum, suggesting that BIN2 positively regulates chloroplast development. Furthermore, BIN2 phosphorylates GLK1 at T175, T238, T248, and T256, and mutations of these phosphorylation sites alter GLK1 protein stability and DNA binding and impair plant responses to BRs/darkness. On the other hand, BRs and darkness repress the BIN2-GLK module to enhance BR/dark-mediated de-greening and impair the formation of the photosynthetic apparatus. Our results thus provide a mechanism by which BRs modulate photomorphogenesis and chloroplast development.

Nitric oxide regulates chlorophyllide biosynthesis and singlet oxygen generation differently between Arabidopsis and barley

Nitric oxide (NO) has a general inhibitory effects on chlorophyll biosynthesis, especially to the step of 5-aminolevulinic acid (ALA) biosynthesis and protochlorophyllide (Pchlide) to chlorophyllide (Chlide) conversion (responsible by the NADPH:Pchlide oxidoreductase POR). Previous study suggested that barley large POR aggregates may be generated by dithiol oxidation of cysteines of two POR monomers, which can be disconnected by some reducing agents. POR aggregate assembly may be correlated with seedling greening in barley, but not in Arabidopsis. Thus, NO may affect POR activity and seedling greening differently between Arabidopsis and barley. We proved this assumption by non-denaturing gel-analysis and reactive oxygen species (ROS) monitoring during the greening. NO treatments cause S-nitrosylation to POR cysteine residues and disassembly of POR aggregates. This modification reduces POR activity and induces Pchlide accumulation and singlet oxygen generation upon dark-to-high-light shift (and therefore inducing photobleaching lesions) in barley leaf apex, but not in Arabidopsis seedlings. ROS staining and ROS-related-gene expression detection confirmed that superoxide anion and singlet oxygen accumulated in barley etiolated seedlings after the NO treatments, when exposed to a fluctuating light. The data suggest that POR aggregate assembly may be correlated with barley chlorophyll biosynthesis and redox homeostasis during greening. Cysteine S-nitrosylation may be one of the key reasons for the NO-induced inhibition to chlorophyll biosynthetic enzymes.

Characterization and Complementation of a Chlorophyll-Less Dominant Mutant GL1 in Lagerstroemia indica

Crape myrtle (Lagerstroemia indica) is a woody ornamental plant popularly grown because of its long-lasting, midsummer blooms and beautiful colors. The GL1 dominant mutant is the first chlorophyll-less mutant identified in crape myrtle. It was obtained from a natural yellow leaf bud mutation. We previously revealed that leaf color of the GL1 mutant is affected by light intensity. However, the mechanism of the GL1 mutant on light response remained unclear. The acclimation response of mutant and wild-type (WT) plants was assessed in a time series after transferring from low light (LL) to high light (HL) by analyzing chlorophyll synthesis precursor content, photosynthetic performance, and gene expression. In LL conditions, coproporphyrinogen III (Coprogen III) content had the greatest amount of accumulation in the mutant compared with WT, increasing by 100%. This suggested that the yellow leaf phenotype of the GL1 dominant mutant might be caused by disruption of coproporphyrinogen III oxidase (CPO) biosynthesis. Furthermore, the candidate gene, oxygen-independent CPO (HEMN), might only affect expression of upstream genes involved in chlorophyll metabolism in the mutant. Moreover, two genes, photosystem II (PSII) 10 kDa protein (psbR) and chlorophyll a/b binding protein gene (CAB1), had decreased mRNA levels in the GL1 mutant within the first 96 h following LL/HL transfer compared with the WT. Hierarchical clustering revealed that these two genes shared a similar expression trend as the oxygen-dependent CPO (HEMF). These findings provide evidence that GL1 is highly coordinated with PSII stability and chloroplast biogenesis.

The roles of tetrapyrroles in plastid retrograde signaling and tolerance to environmental stresses

This review provides new insights that tetrapyrrole signals play important roles in nuclear gene expression, chloroplast development and plant's resistance to environmental stresses. Higher plants contain many tetrapyrroles, including chlorophyll (Chl), heme, siroheme, phytochromobilin and some of their precursors, all of which have important biological functions. Genetic and physiological studies indicated that tetrapyrrole (mainly Mg-protoporphyrin IX) retrograde signals control photosynthesis-associated nuclear gene (PhANG) expression. Recent studies have shown that tetrapyrrole-derived signals may correlate with plant resistance to environmental stresses such as drought, high-light stress, water stress, osmotic stress, salinity and heavy metals. Signaling and physiological roles of Mg-protoIX-binding proteins (such as PAPP5, CRD and HSP90) and heme-binding proteins (such as HO and TSPO) and tetrapyrrole-signaling components (such as GUN1, ABI4 and CBFA) are summarized. Some of them positively regulate plant development and response to environmental stresses. The intermediate signaling components (such as PTM, HSP70-HSP90-HAP1 complex and PAPP5) between the nucleus and the plastid also positively regulate plant resistance to environmental stresses. This review provides new insights that genetically modified plants with enhanced tetrapyrrole levels have improved resistance to environmental stresses.

Assembly of NADPH: protochlorophyllide oxidoreductase complex is needed for effective greening of barley seedlings

NADPH:protochlorophyllide (Pchlide) oxidoreductase (POR) is the key enzyme in the light-induced greening of higher plants. A unique light-harvesting POR:Pchlide complexes (LHPP) has been found in barley etioplasts, but not in other plant species. Why PORs from barley, but not from other plants, can form LHPP? And its function is not well understood. We modeled the barley and Arabidopsis POR proteins and compared molecular surface. The results confirm the idea that barley PORA can form a five-unit oligomer that interacts with a single PORB. Chemical treatment experiments indicated that POR complex may be formed by dithiol oxidation of cysteines of two adjacent proteins. We further showed that LHPP assembly was needed for barley POR functions and seedling greening. On the contrary, Arabidopsis POR proteins only formed dimers, which were not related to the functions or the greening. Finally, POR complex assembly (including LHPP and POR dimers) did not affect the formation of prolamellar bodies (PLBs) that function for efficient capture of light energy for photo conversion in etioplasts.

A single leaf of Camellia oleifera has two types of carbon assimilation pathway, C(3) and crassulacean acid metabolism

The C(4) plants, whose first product of photosynthetic CO(2) fixation is a four-carbon acid, have evolved independently many times. Crassulacean acid metabolism (CAM) is a biological mechanism known to exhibit some C(4) characteristics such as the C(3) cycle during daylight and demonstrates the C(4) cycle at night. There are also various C(3)-CAM intermediates, whose CAM pathway can be induced by environmental changes. However, neither fungus-induced CAM nor Theaceae CAM have been reported previously. Here, we show that CAM could be generated by fungus infection in Camellia oleifera Abel. young leaves, even at a location of a single leaf where the upper part had been transformed into a succulent one, while the lower part remained unchanged. The early photosynthetic products of dark-grown C. oleifera succulent leaves were malate, whereas C. oleifera normal leaves and light-grown succulent leaves incorporated most of (14)C into the primary photosynthetic product 3-phosphoglycerate. Camellia oleifera succulent leaves have a lower absolute δ(13)C value, much lower photorespiration rates and lower transpiration rates during the day than those of C. oleifera normal leaves. Like a typical CAM plant, stomata of C. oleifera succulent leaves closed during the daylight, but opened at night, and therefore had a detectable CO(2) compensation point in darkness. Net photosynthetic rates (P(n)) fluctuated diurnally and similarly with stomatal aperture. No light intensity saturation could be defined for C. oleifera succulent leaves. C(4) key enzymes in C. oleifera succulent leaves were increased at both the transcriptional/translational levels as well as at the enzyme activity level.

Mg-protoporphyrin, haem and sugar signals double cellular total RNA against herbicide and high-light-derived oxidative stress

Cellular total RNA level is usually stable, although it may increase gradually during growth or seed germination, or decrease gradually under environmental stresses. However, we found that plant cell RNA could be doubled within 48 h in response to herbicide-induced Mg-protoporphyrin and heme accumulation or a high level of sugar treatment. This rapid RNA multiplication is important for effective cellular resistance to oxidative stress, such as high-light and herbicide co-stress conditions, where the plastid-signalling defective mutant gun1 shows an apparent phenotype (more severe photobleaching). Hexokinase is required for sugar-induced RNA multiplication. While both sugar and Mg-protoporphyrin IX require plastid protein GUN1 and a nuclear transcription factor ABI4, haem appears to function through an independent pathway to control RNA multiplication. The transcription co-factor CAAT binding protein mediates the rapid RNA multiplication in plant cells in all the cases.