Accumulation of the NON-YELLOW COLORING 1 protein of the chlorophyll cycle requires chlorophyll b in Arabidopsis thaliana

Effect of the Enhanced Production of Chlorophyll b on the Light Acclimation of Tomato

Tomato (Solanum lycopersicum Mill.) is one of the widely cultured vegetables under protected cultivation, in which insufficient light is one of the major factors that limit its growth, yield, and quality. Chlorophyll b (Chl b) is exclusively present in the light-harvesting complex (LHC) of photosystems, while its synthesis is strictly regulated in response to light conditions in order to control the antenna size. Chlorophyllide a oxygenase (CAO) is the sole enzyme that converts Chl a to Chl b for Chl b biosynthesis. Previous studies have shown that overexpressing CAO without the regulating domain (A domain) in Arabidopsis overproduced Chl b. However, the growth characteristics of the Chl b overproduced plants under different light environmental conditions are not well studied. Considering tomatoes are light-loving plants and sensitive to low light stress, this study aimed to uncover the growth character of tomatoes with enhanced production of Chl b. The A domain deleted Arabidopsis CAO fused with the FLAG tag (BCF) was overexpressed in tomatoes. The BCF overexpressed plants accumulated a significantly higher Chl b content, resulting in a significantly lower Chl a/b ratio than WT. Additionally, BCF plants possessed a lower maximal photochemical efficiency of photosystem II (Fv/Fm) and anthocyanin content than WT plants. The growth rate of BCF plants was significantly faster than WT plants under low-light (LL) conditions with light intensity at 50-70 µmol photons m^-2 s^-1, while BCF plants grew slower than WT plants under high-light (HL) conditions. Our results revealed that Chl b overproduced tomato plants could better adapt to LL conditions by absorbing more light for photosynthesis but adapt poorly to excess light conditions by accumulating more ROS and fewer anthocyanins. Enhanced production of Chl b is able to improve the growth rate of tomatoes that are grown under LL conditions, indicating the prospect of employing Chl b overproduced light-loving crops and ornamental plants for protected or indoor cultivation.

The Mixing Ratio and Filling-Amount Affect the Tissue Browning and Antioxidant Properties of Fresh-Cut Baby Leaf Lettuce (Lactuca sativa L.) and Rocket (Eruca sativa Mill.) Grown in Floating Growing Systems

Different types of baby leaf vegetables (BLV) are often mixed and packaged as salad mixes. This work has evaluated the effects of BLV mixing ratios (100% lettuce ‘Lollo Bionda’, 100 LB; 75% lettuce + 25% rocket, 75 LB; 50% lettuce + 50% rocket, 50 LB) and the weight filling amount (125 g filling amount, 125F; 250 g, 250F) on the antioxidant properties and browning potential (BP) of lettuce and rocket baby leaves during storage for 9 days at 4 °C in the dark. The samples were packaged in thermos-sealed bags previously prepared using polypropylene film. The results showed that the 50 LB mix had preserved high amounts of chlorophylls and internal nutrients on d9, regardless of the filling amount. No visible browning symptoms were detected in the 50 LB samples. The 50 LB × 125F mix was found to be the most efficient strategy to maintain the antioxidant property of BLV. Thus, the optimisation of the mixing ratio and its combination with an appropriate filling amount could represent an effective postharvest practice.

Expression of ZjPSY, a Phytoene Synthase Gene from Zoysia japonica Affects Plant Height and Photosynthetic Pigment Contents

Phytoene synthase (PSY) is a key limiting enzyme in the carotenoid biosynthesis pathway for regulating phytoene synthesis. In this study, ZjPSY was isolated and identified from Zoysia japonica, an important lawn grass species. ZjPSY cDNA was 1230 bp in length, corresponding to 409 amino acids. ZjPSY showed higher expression in young leaves and was downregulated after GA₃, ABA, SA, and MeJA treatments, exhibiting a sensitivity to plant hormones. Regulatory elements of light and plant hormone were found in the upstream of ZjPSY CDS. Expression of ZjPSY in Arabidopsis thaliana protein led to carotenoid accumulation and altered expression of genes involved in the carotenoid pathway. Under no-treatment condition, salt treatment, and drought treatment, transgenic plants exhibited yellowing, dwarfing phenotypes. The carotenoid content of transgenic plants was significantly higher than that of wild-type under salt stress and no-treatment condition. Yeast two-hybrid screening identified a novel interacting partner ZjJ2 (DNAJ homologue 2), which encodes heat-shock protein 40 (HSP40). Taken together, this study suggested that ZjPSY may affect plant height and play an important role in carotenoid synthesis. These results broadened the understanding of carotenoid synthesis pathways and laid a foundation for the exploration and utilization of the PSY gene.

Research Progress in the Interconversion, Turnover and Degradation of Chlorophyll

Chlorophylls (Chls, Chl a and Chl b) are tetrapyrrole molecules essential for photosynthetic light harvesting and energy transduction in plants. Once formed, Chls are noncovalently bound to photosynthetic proteins on the thylakoid membrane. In contrast, they are dismantled from photosystems in response to environmental changes or developmental processes; thus, they undergo interconversion, turnover, and degradation. In the last twenty years, fruitful research progress has been achieved on these Chl metabolic processes. The discovery of new metabolic pathways has been accompanied by the identification of enzymes associated with biochemical steps. This article reviews recent progress in the analysis of the Chl cycle, turnover and degradation pathways and the involved enzymes. In addition, open questions regarding these pathways that require further investigation are also suggested.

NOL-mediated functional stay-green traits in perennial ryegrass (Lolium perenne L.) involving multifaceted molecular factors and metabolic pathways regulating leaf senescence

Loss of chlorophyll (Chl) is a hallmark of leaf senescence, which may be regulated by Chl catabolic genes, including NON-YELLOW COLORING 1 (NYC1)-like (NOL). The objective of this study was to determine molecular factors and metabolic pathways underlying NOL regulation of leaf senescence in perennial grass species. LpNOL was cloned from perennial ryegrass (Lolium perenne L.) and found to be highly expressed in senescent leaves. Transient overexpression of LpNOL accelerated leaf senescence and Chl b degradation in Nicotiana benthamiana. LpNOL RNA interference (NOLi) in perennial ryegrass not only significantly blocked Chl degradation in senescent leaves, but also delayed initiation and progression of leaf senescence. This study found that NOL, in addition to functioning as a Chl b reductase, could enact the functional stay-green phenotype in perennial grass species, as manifested by increased photosynthetic activities in NOLi plants. Comparative transcriptomic analysis revealed that NOL-mediated functional stay-green in perennial ryegrass was mainly achieved through the modulation of Chl catabolism, light harvesting for photosynthesis, photorespiration, cytochrome respiration, carbohydrate catabolism, oxidative detoxification, and abscisic acid biosynthesis and signaling pathways.

HCAR Is a Limitation Factor for Chlorophyll Cycle and Chlorophyll b Degradation in Chlorophyll-b-Overproducing Plants

The chlorophyll (Chl) cycle is the metabolic pathway for Chl a and Chl b inter-conversion. In this pathway, Chl b is synthesized from Chl a by the catalyzing action of chlorophyllide a oxygenase (CAO). In contrast, Chl b is firstly reduced to produce 7-hydroxymethyl Chl (HMChl) a, which is catalyzed by two isozymes of Chl b reductase (CBR), non-yellow coloring 1 (NYC1) and NYC1-like (NOL). Subsequently, HMChl a is reduced to Chl a by HMChl a reductase (HCAR). CAO plays a pivotal role in Chl a/b ratio regulation and plants over-accumulate Chl b in CAO-overexpressing plants. NYC1 is more accumulated in Chl-b-overproducing plants, while HCAR is not changed. To investigate the role of HCAR in Chl cycle regulation, the Chl metabolites of Chl-b-overproducing plants were analyzed. The results showed that HMChl a accumulated in these plants, and it decreased and the Chl a/b ratio increased by overexpressing HCAR, implying HCAR is insufficient for Chl cycle in Chl-b-overproducing plants. Furthermore, during dark-induced senescence, the non-programmed cell death symptoms (leaves dehydrated with green color retained) of Chl-b-overproducing plants were obviously alleviated, and the content of HM pheophorbide (HMPheide) a and Pheide b were sharply decreased by overexpressing HCAR. These results imply that HCAR is also insufficient for Chl degradation in Chl-b-overproducing plants during senescence, thus causing the accumulation of Chl metabolites and non-programmed cell death of leaves. With these results taken together, we conclude that HCAR is not well regulated and it is a limiting factor for Chl cycle and Chl b degradation in Chl-b-overproducing plants.

Beyond pathways: genetic dissection of tocopherol content in maize kernels by combining linkage and association analyses

Although tocopherols play an important role in plants and animals, the genetic architecture of tocopherol content in maize kernels has remained largely unknown. In this study, linkage and association analyses were conducted to examine the genetic architecture of tocopherol content in maize kernels. Forty-one unique quantitative trait loci (QTLs) were identified by linkage mapping in six populations of recombinant inbred lines (RILs). In addition, 32 significant loci were detected via genome-wide association study (GWAS), 18 of which colocalized with the QTLs identified by linkage mapping. Fine mapping of a major QTL validated the accuracy of GWAS and QTL mapping results and suggested a role for nontocopherol pathway genes in the modulation of natural tocopherol variation. We provided genome-wide evidence that genes involved in fatty acid metabolism, chlorophyll metabolism and chloroplast function may affect natural variation in tocopherols. These findings were confirmed through mutant analysis of a particular gene from the fatty acid pathway. In addition, the favourable alleles for many of the significant SNPs/QTLs represented rare alleles in natural populations. Together, our results revealed many novel genes that are potentially involved in the variation of tocopherol content in maize kernels. Pyramiding of the favourable alleles of the newly elucidated genes and the well-known tocopherol pathway genes would greatly improve tocopherol content in maize.

Mg-dechelation of chlorophyll a by Stay-Green activates chlorophyll b degradation through expressing Non-Yellow Coloring 1 in Arabidopsis thaliana

The first step in chlorophyll a degradation is the extraction of the central Mg. This reaction is catalyzed by Mg-dechelatase encoded by Stay-Green (SGR) in land plants. SGR extracts Mg from chlorophyll a but not from chlorophyll b, and chlorophyll b must be converted to chlorophyll a before degradation. The first reaction of the chlorophyll b to chlorophyll a conversion is catalyzed by chlorophyll b reductase. Non-Yellow Coloring 1 (NYC1) and NYC1 like (NOL) are isozymes of chlorophyll b reductase. When SGR was transiently overexpressed in Arabidopsis, both chlorophyll a and b were degraded, suggesting that the chlorophyll b to chlorophyll a conversion is activated by SGR overexpression. To examine the involvement of chlorophyll b reductases in SGR-induced chlorophyll b degradation, SGR was transiently overexpressed in nyc1, nol, and nyc1 nol double mutants by dexamethasone treatment. It was found that in the wild type and nol mutant, chlorophyll a and b were degraded and all the chlorophyll-binding proteins decreased. Meanwhile, in nyc1 and nyc1 nol mutants, chlorophyll b degradation was suppressed and the light-harvesting complex of photosystem II remained. The mRNA and protein levels of NYC1 increased after SGR overexpression in wild type plants. These results suggest that Mg-dechelation of chlorophyll a by SGR activates chlorophyll b degradation by inducing the expression of NYC1. This is an effective regulation of a metabolic pathway.

Genetic and Hormonal Regulation of Chlorophyll Degradation during Maturation of Seeds with Green Embryos

The embryos of some angiosperms (usually referred to as chloroembryos) contain chlorophylls during the whole period of embryogenesis. Developing embryos have photochemically active chloroplasts and are able to produce assimilates, further converted in reserve biopolymers, whereas at the late steps of embryogenesis, seeds undergo dehydration, degradation of chlorophylls, transformation of chloroplast in storage plastids, and enter the dormancy period. However, in some seeds, the process of chlorophyll degradation remains incomplete. These residual chlorophylls compromise the quality of seed material in terms of viability, nutritional value, and shelf life, and represent a serious challenge for breeders and farmers. The mechanisms of chlorophyll degradation during seed maturation are still not completely understood, and only during the recent decades the main pathways and corresponding enzymes could be characterized. Among the identified players, the enzymes of pheophorbide a oxygenase pathway and the proteins encoded by STAY GREEN (SGR) genes are the principle ones. On the biochemical level, abscisic acid (ABA) is the main regulator of seed chlorophyll degradation, mediating activity of corresponding catabolic enzymes on the transcriptional level. In general, a deep insight in the mechanisms of chlorophyll degradation is required to develop the approaches for production of chlorophyll-free high quality seeds.

BrWRKY65, a WRKY Transcription Factor, Is Involved in Regulating Three Leaf Senescence-Associated Genes in Chinese Flowering Cabbage

Plant-specific WRKY transcription factors (TFs) have been implicated to function as regulators of leaf senescence, but their association with postharvest leaf senescence of economically important leafy vegetables, is poorly understood. In this work, the characterization of a Group IIe WRKY TF, BrWRKY65, from Chinese flowering cabbage (Brassica rapa var. parachinensis) is reported. The expression of BrWRKY65 was up-regulated following leaf chlorophyll degradation and yellowing during postharvest senescence. Subcellular localization and transcriptional activation assays showed that BrWRKY65 was localized in the nucleus and exhibited trans-activation ability. Further electrophoretic mobility shift assay (EMSA) and transient expression analysis clearly revealed that BrWRKY65 directly bound to the W-box motifs in the promoters of three senescence-associated genes (SAGs) such as BrNYC1 and BrSGR1 associated with chlorophyll degradation, and BrDIN1, and subsequently activated their expressions. These findings demonstrate that BrWRKY65 may be positively associated with postharvest leaf senescence, at least partially, by the direct activation of SAGs. Taken together, these findings provide new insights into the transcriptional regulatory mechanism of postharvest leaf senescence in Chinese flowering cabbage.

Simultaneous regulation of antenna size and photosystem I/II stoichiometry in Arabidopsis thaliana

The photosystem I/II ratio increased when antenna size was enlarged by transient induction of CAO in chlorophyll b -less mutants, thus indicating simultaneous regulation of antenna size and photosystem I/II stoichiometry. Regulation of antenna size and photosystem I/II stoichiometry is an indispensable strategy for plants to acclimate to changes to light environments. When plants grown in high-light conditions are transferred to low-light conditions, the peripheral antennae of photosystems are enlarged. A change in the photosystem I/II ratio is also observed under the same light conditions. However, our knowledge of the correlation between antenna size modulation and variation in photosystem I/II stoichiometry remains limited. In this study, chlorophyll a oxygenase was transiently induced in Arabidopsis thaliana chlorophyll b-less mutants, ch1-1, to alter the antenna size without changing environmental conditions. In addition to the accumulation of chlorophyll b, the levels of the peripheral antenna complexes of both photosystems gradually increased, and these were assembled to the core antenna of both photosystems. However, the antenna size of photosystem II was greater than that of photosystem I. Immunoblot analysis of core antenna proteins showed that the number of photosystem I increased, but not that of photosystem II, resulting in an increase in the photosystem I/II ratio. These results clearly indicate that antenna size adjustment was coupled with changes in photosystem I/II stoichiometry. Based on these results, the physiological importance of simultaneous regulation of antenna size and photosystem I/II stoichiometry is discussed in relation to acclimation to light conditions.

ELS1, a novel MATE transporter related to leaf senescence and iron homeostasis in Arabidopsis thaliana

The multidrug and toxic compound extrusion (MATE) transporters mediate the coupled exchange of organic substrates and monovalent cations have been recently implicated in various plant biological activities. In this work, we isolated a dominant mutant from an Arabidopsis activation-tagging mutant pool. This mutant exhibits pleiotropic phenotype including early flowering, dwarf and bushy architecture, minified lateral organs and early leaf senescence, and is therefore designated early leaf senescence 1-Dominaint (els1-D). Genotyping assays showed that els1-D is a gain-of-function mutant of a novel MATE transporter gene, ELS1, which encodes a close homolog of the previously reported ADP1, BCD1 and DTX50. Further investigations revealed that the overexpression of ELS1 reduces iron content in els1-D, and the accelerated senescence of the detached els1-D leaves can be recovered by exogenous iron supply. In addition, we also found that ELS1 is an iron responsive gene. Based on these findings, we proposed that ELS1 is related to leaf senescence and iron homeostasis in Arabidopsis.

Chlorophyll b degradation by chlorophyll b reductase under high-light conditions

The light-harvesting chlorophyll a/b binding protein complex of photosystem II (LHCII) is the main antenna complex of photosystem II (PSII). Plants change their LHCII content depending on the light environment. Under high-light conditions, the content of LHCII should decrease because over-excitation damages the photosystem. Chlorophyll b is indispensable for accumulating LHCII, and chlorophyll b degradation induces LHCII degradation. Chlorophyll b degradation is initiated by chlorophyll b reductase (CBR). In land plants, NON-YELLOW COLORING 1 (NYC1) and NYC1-Like (NOL) are isozymes of CBR. We analyzed these mutants to determine their functions under high-light conditions. During high-light treatment, the chlorophyll a/b ratio was stable in the wild-type (WT) and nol plants, and the LHCII content decreased in WT plants. The chlorophyll a/b ratio decreased in the nyc1 and nyc1/nol plants, and a substantial degree of LHCII was retained in nyc1/nol plants after the high-light treatment. These results demonstrate that NYC1 degrades the chlorophyll b on LHCII under high-light conditions, thus decreasing the LHCII content. After the high-light treatment, the maximum quantum efficiency of the PSII photochemistry was lower in nyc1 and nyc1/nol plants than in WT and nol plants. A larger light-harvesting system would damage PSII in nyc1 and nyc1/nol plants. The fluorescence spectroscopy of the leaves indicated that photosystem I was also damaged by the excess LHCII in nyc1/nol plants. These observations suggest that chlorophyll b degradation by NYC1 is the initial reaction for the optimization of the light-harvesting capacity under high-light conditions.

Oryza sativa Chloroplast Signal Recognition Particle 43 (OscpSRP43) Is Required for Chloroplast Development and Photosynthesis

A rice chlorophyll-deficient mutant w67 was isolated from an ethyl methane sulfonate (EMS)-induced IR64 (Oryza sativa L. ssp. indica) mutant bank. The mutant exhibited a distinct yellow-green leaf phenotype in the whole plant growth duration with significantly reduced levels of chlorophyll and carotenoid, impaired chloroplast development and lowered capacity of photosynthesis compared with the wild-type IR64. Expression of a number of genes associated with chlorophyll metabolism, chloroplast biogenesis and photosynthesis was significantly altered in the mutant. Genetic analysis indicated that the yellow-green phenotype was controlled by a single recessive nuclear gene located on the short arm of chromosome 3. Using map-based strategy, the mutation was isolated and predicted to encode a chloroplast signal recognition particle 43 KD protein (cpSRP43) with 388 amino acid residuals. A single base substitution from A to T at position 160 resulted in a premature stop codon. OscpSRP43 was constitutively expressed in various organs with the highest level in the leaf. Functional complementation could rescue the mutant phenotype and subcellular localization showed that the cpSRP43:GFP fusion protein was targeted to the chloroplast. The data suggested that Oryza sativa cpSRP43 (OscpSRP43) was required for the normal development of chloroplasts and photosynthesis in rice.

Chlorophyll b in angiosperms: Functions in photosynthesis, signaling and ontogenetic regulation

Chlorophyll b (Chlb) is an antenna chlorophyll. The binding of Chlb by antenna proteins is crucial for the correct assembly of the antenna complexes in thylakoid membranes. Since the levels of the proteins of major and minor antenna are affected to different extents by Chlb binding, the availability of Chlb influences the composition and the size of antenna complexes which in turn determine the supramolecular organization of the thylakoid membranes in grana. Therefore, Chlb synthesis levels have a major impact on lateral mobility and diffusion of membrane molecules, and thus affect not only light harvesting and thermal energy dissipation processes, but also linear electron transport and repair processes in grana. Furthermore, in angiosperms Chlb synthesis affects plant functions beyond chloroplasts. First, the stability of pigment-protein complexes in the antennae, which depends on Chlb, is an important factor in the regulation of plant ontogenesis, and Chlb levels were recently shown to influence plant ontogenetic signaling. Second, the amounts of minor antenna proteins in chloroplasts, which depend on the availability of Chlb, were recently shown to affect ABA levels and signaling in plants. These mechanisms can be examined in mutants where Chlb synthesis is reduced or abolished. The dramatic effects caused by the lack of Chlb on plant productivity are interpreted in this review in light of the pleiotropic effects on photosynthesis and signaling, and the potential to manipulate Chlb biosynthesis for the improvement of crop production is discussed.