Chloroembryos: a unique photosynthesis system

Physiology, genomics, and evolutionary aspects of desert plants

BACKGROUND

Despite the exposure to arid environmental conditions across the globe ultimately hampering the sustainability of the living organism, few plant species are equipped with several unique genotypic, biochemical, and physiological features to counter such harsh conditions. Physiologically, they have evolved with reduced leaf size, spines, waxy cuticles, thick leaves, succulent hydrenchyma, sclerophyll, chloroembryo, and photosynthesis in nonfoliar and other parts. At the biochemical level, they are evolved to perform efficient photosynthesis through Crassulacean acid metabolism (CAM) and C4 pathways with the formation of oxaloacetic acid (Hatch-Slack pathway) instead of the C3 pathway. Additionally, comparative genomics with existing data provides ample evidence of the xerophytic plants' positive selection to adapt to the arid environment. However, adding more high-throughput sequencing of xerophyte plant species is further required for a comparative genomic study toward trait discovery related to survival. Learning from the mechanism to survive in harsh conditions could pave the way to engineer crops for future sustainable agriculture.

AIM OF THE REVIEW

The distinct physiology of desert plants allows them to survive in harsh environments. However, the genomic composition also contributes significantly to this and requires great attention. This review emphasizes the physiological and genomic adaptation of desert plants. Other important parameters, such as desert biodiversity and photosynthetic strategy, are also discussed with recent progress in the field. Overall, this review discusses the different features of desert plants, which prepares them for harsh conditions intending to translate knowledge to engineer plant species for sustainable agriculture.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This review comprehensively presents the physiology, molecular mechanism, and genomics of desert plants aimed towards engineering a sustainable crop.

The light and hypoxia induced gene ZmPORB1 determines tocopherol content in the maize kernel

Tocopherol is an important lipid-soluble antioxidant beneficial for both human health and plant growth. Here, we fine mapped a major QTL-qVE1 affecting γ-tocopherol content in maize kernel, positionally cloned and confirmed the underlying gene ZmPORB1 (por1), as a protochlorophyllide oxidoreductase. A 13.7 kb insertion reduced the tocopherol and chlorophyll content, and the photosynthetic activity by repressing ZmPORB1 expression in embryos of NIL-K22, but did not affect the levels of the tocopherol precursors HGA (homogentisic acid) and PMP (phytyl monophosphate). Furthermore, ZmPORB1 is inducible by low oxygen and light, thereby involved in the hypoxia response in developing embryos. Concurrent with natural hypoxia in embryos, the redox state has been changed with NO increasing and H2O2 decreasing, which lowered γ-tocopherol content via scavenging reactive nitrogen species. In conclusion, we proposed that the lower light-harvesting chlorophyll content weakened embryo photosynthesis, leading to fewer oxygen supplies and consequently diverse hypoxic responses including an elevated γ-tocopherol consumption. Our findings shed light on the mechanism for fine-tuning endogenous oxygen concentration in the maize embryo through a novel feedback pathway involving the light and low oxygen regulation of ZmPORB1 expression and chlorophyll content.

Impact of pod and seed photosynthesis on seed filling and canopy carbon gain in soybean

There is a limited understanding of the carbon assimilation capacity of nonfoliar green tissues and its impact on yield and seed quality since most photosynthesis research focuses on leaf photosynthesis. In this study, we investigate the photosynthetic efficiency of soybean (Glycine max) pods and seeds in a field setting and evaluate its effect on mature seed weight and composition. We demonstrate that soybean pod and seed photosynthesis contributes 13% to 14% of the mature seed weight. Carbon assimilation by soybean pod and seed photosynthesis can compensate for 81% of carbon loss through the respiration of the same tissues, and our model predicts that soybean pod and seed photosynthesis contributes up to 9% of the total daily carbon gain of the canopy. Chlorophyll fluorescence (CF) shows that the operating efficiency of photosystem II in immature soybean seeds peaks at the 10 to 100 mg seed weight stage, while that of immature pods peaks at the 75 to 100 mg stage. This study provides quantitative information about the efficiency of soybean pod and seed photosynthesis during tissue development and its impact on yield.

Metabolic and transcriptomic study of pennycress natural variation identifies targets for oil improvement

Pennycress (Thlaspi arvense L.), a member of the Brassicaceae family, produces seed oil high in erucic acid, suitable for biodiesel and aviation fuel. Although pennycress, a winter annual, could be grown as a dedicated bioenergy crop, an increase in its seed oil content is required to improve its economic competitiveness. The success of crop improvement relies upon finding the right combination of biomarkers and targets, and the best genetic engineering and/or breeding strategies. In this work, we combined biomass composition with metabolomic and transcriptomic studies of developing embryos from 22 pennycress natural variants to identify targets for oil improvement. The selected accession collection presented diverse levels of fatty acids at maturity ranging from 29% to 41%. Pearson correlation analyses, weighted gene co-expression network analysis and biomarker identifications were used as complementary approaches to detect associations between metabolite level or gene expression and oil content at maturity. The results indicated that improving seed oil content can lead to a concomitant increase in the proportion of erucic acid without affecting the weight of embryos. Processes, such as carbon partitioning towards the chloroplast, lipid metabolism, photosynthesis, and a tight control of nitrogen availability, were found to be key for oil improvement in pennycress. Besides identifying specific targets, our results also provide guidance regarding the best timing for their modification, early or middle maturation. Thus, this work lays out promising strategies, specific for pennycress, to accelerate the successful development of lines with increased seed oil content for biofuel applications.

Plastid 2-Cys peroxiredoxins are essential for embryogenesis in Arabidopsis

The redox couple formed by NADPH-dependent thioredoxin reductase C (NTRC) and 2-Cys peroxiredoxins (Prxs) allows fine-tuning chloroplast performance in response to light intensity changes. Accordingly, the Arabidopsis 2cpab mutant lacking 2-Cys Prxs shows growth inhibition and sensitivity to light stress. However, this mutant also shows defective post-germinative growth, suggesting a relevant role of plastid redox systems in seed development, which is so far unknown. To address this issue, we first analyzed the pattern of expression of NTRC and 2-Cys Prxs in developing seeds. Transgenic lines expressing GFP fusions of these proteins showed their expression in developing embryos, which was low at the globular stage and increased at heart and torpedo stages, coincident with embryo chloroplast differentiation, and confirmed the plastid localization of these enzymes. The 2cpab mutant produced white and abortive seeds, which contained lower and altered composition of fatty acids, thus showing the relevance of 2-Cys Prxs in embryogenesis. Most embryos of white and abortive seeds of the 2cpab mutant were arrested at heart and torpedo stages of embryogenesis suggesting an essential function of 2-Cys Prxs in embryo chloroplast differentiation. This phenotype was not recovered by a mutant version of 2-Cys Prx A replacing the peroxidatic Cys by Ser. Neither the lack nor the overexpression of NTRC had any effect on seed development indicating that the function of 2-Cys Prxs at these early stages of development is independent of NTRC, in clear contrast with the operation of these regulatory redox systems in leaves chloroplasts.

Progress in understanding and improving oil content and quality in seeds

The world's population is projected to increase by two billion by 2050, resulting in food and energy insecurity. Oilseed crops have been identified as key to address these challenges: they produce and store lipids in the seeds as triacylglycerols that can serve as a source of food/feed, renewable fuels, and other industrially-relevant chemicals. Therefore, improving seed oil content and composition has generated immense interest. Research efforts aiming to unravel the regulatory pathways involved in fatty acid synthesis and to identify targets for metabolic engineering have made tremendous progress. This review provides a summary of the current knowledge of oil metabolism and discusses how photochemical activity and unconventional pathways can contribute to high carbon conversion efficiency in seeds. It also highlights the importance of ¹³C-metabolic flux analysis as a tool to gain insights on the pathways that regulate oil biosynthesis in seeds. Finally, a list of key genes and regulators that have been recently targeted to enhance seed oil production are reviewed and additional possible targets in the metabolic pathways are proposed to achieve desirable oil content and quality.

Transcriptomic comparison of seeds and silique walls from two rapeseed genotypes with contrasting seed oil content

Silique walls play pivotal roles in contributing photoassimilates and nutrients to fuel seed growth. However, the interaction between seeds and silique walls impacting oil biosynthesis is not clear during silique development. Changes in sugar, fatty acid and gene expression during Brassica napus silique development of L192 with high oil content and A260 with low oil content were investigated to identify key factors affecting difference of their seed oil content. During the silique development, silique walls contained more hexose and less sucrose than seeds, and glucose and fructose contents in seeds and silique walls of L192 were higher than that of A260 at 15 DAF, and sucrose content in the silique walls of L192 were lower than that of A260 at three time points. Genes related to fatty acid biosynthesis were activated over time, and differences on fatty acid content between the two genotypes occurred after 25 DAF. Genes related to photosynthesis expressed more highly in silique walls than in contemporaneous seeds, and were inhibited over time. Gene set enrichment analysis suggested photosynthesis were activated in L192 at 25 and 35 DAF in silique walls and at both 15 and 35 DAF in the seed. Expressions of sugar transporter genes in L192 was higher than that in A260, especially at 35 DAF. Expressions of genes related to fatty acid biosynthesis, such as BCCP2s, bZIP67 and LEC1s were higher in L192 than in A260, especially at 35 DAF. Meanwhile, genes related to oil body proteins were expressed at much lower levels in L192 than in A260. According to the WGCNA results, hub modules, such as ME.turquoise relative to photosynthesis, ME.green relative to embryo development and ME.yellow relative to lipid biosynthesis, were identified and synergistically regulated seed development and oil accumulation. Our results are helpful for understanding the mechanism of oil accumulation of seeds in oilseed rape for seed oil content improvement.

The nuclear-encoded plastid ribosomal protein L18s are essential for plant development

Plastid ribosomal proteins (PRPs) are necessary components for plastid ribosome biogenesis, playing essential roles in plastid development. The ribosomal protein L18 involved in the assemble of 5S rRNA and 23S rRNA, is vital for E. coli viability, but the functions of its homologs in plant plastid remain elusive. Here, we characterized the functions of the plant plastid ribosomal protein L18s (PRPL18s) in Arabidopsis and rice. AtPRPL18 was ubiquitously expressed in most of the plant tissues, but with higher expression levels in seedling shoots, leaves, and flowers. AtPRPL18 was localized in chloroplast. Genetic and cytological analyses revealed that a loss of function of AtPRPL18 resulted in embryo development arrest at globular stage. However, overexpression of AtPRPL18 did not show any visible phenotypical changes in Arabidopsis. The rice OsPRPL18 was localized in chloroplast. In contrast to AtPRPL18, knockout of OsPRPL18 did not affect embryo development, but led to an albino lethal phenotype at the seedling stage. Cytological analyses showed that chloroplast development was impaired in the osprpl18-1 mutant. Moreover, a loss-function of OsPRPL18 led to defects in plastid ribosome biogenesis and a serious reduction in the efficiency of plastid intron splicing. In all, these results suggested that PRPL18s play critical roles in plastid ribosome biogenesis, plastid intron splicing, and chloroplast development, and are essential for plant survival.

Rice LEAFY COTYLEDON1 Hinders Embryo Greening During the Seed Development

LEAFY COTYLEDON1 (LEC1) is the central regulator of seed development in Arabidopsis, while its function in monocots is largely elusive. We generated Oslec1 mutants using CRISPR/Cas9 technology. Oslec1 mutant seeds lost desiccation tolerance and triggered embryo greening at the early development stage. Transcriptome analysis demonstrated that Oslec1 mutation altered diverse hormonal pathways and stress response in seed maturation, and promoted a series of photosynthesis-related genes. Further, genome-wide identification of OsLEC1-binding sites demonstrated that OsLEC1 bound to genes involved in photosynthesis, photomorphogenesis, as well as abscisic acid (ABA) and gibberellin (GA) pathways, involved in seed maturation. We illustrated an OsLEC1-regulating gene network during seed development, including the interconnection between photosynthesis and ABA/GA biosynthesis/signaling. Our findings suggested that OsLEC1 acts as not only a central regulator of seed maturation but also an inhibitor of embryo greening during rice seed development. This study would provide new understanding for the OsLEC1 regulatory mechanisms on photosynthesis in the monocot seed development.

Plastid-Localized EMB2726 Is Involved in Chloroplast Biogenesis and Early Embryo Development in Arabidopsis

Embryogenesis is a critical developmental process that establishes the body organization of higher plants. During this process, the biogenesis of chloroplasts from proplastids is essential. A failure in chloroplast development during embryogenesis can cause morphologically abnormal embryos or embryonic lethality. In this study, we isolated a T-DNA insertion mutant of the Arabidopsis gene EMBRYO DEFECTIVE 2726 (EMB2726). Heterozygous emb2726 seedlings produced about 25% albino seeds with embryos that displayed defects at the 32-cell stage and that arrested development at the late globular stage. EMB2726 protein was localized in chloroplasts and was expressed at all stages of development, such as embryogenesis. Moreover, the two translation elongation factor Ts domains within the protein were critical for its function. Transmission electron microscopy revealed that the cells in emb2726 embryos contained undifferentiated proplastids and that the expression of plastid genome-encoded photosynthesis-related genes was dramatically reduced. Expression studies of DR5:GFP, pDRN:DRN-GFP, and pPIN1:PIN1-GFP reporter lines indicated normal auxin biosynthesis but altered polar auxin transport. The expression of pSHR:SHR-GFP and pSCR:SCR-GFP confirmed that procambium and ground tissue precursors were lacking in emb2726 embryos. The results suggest that EMB2726 plays a critical role during Arabidopsis embryogenesis by affecting chloroplast development, possibly by affecting the translation process in plastids.

Comparative analysis of the plastid conversion, photochemical activity and chlorophyll degradation in developing embryos of green-seeded and yellow-seeded pea (Pisum sativum) cultivars

Developing seeds of some higher plants are photosynthetically active and contain chlorophylls (Chl), which are typically destroyed at the late stages of seed maturation. However, in some crop plant cultivars, degradation of embryonic Chl remains incomplete, and mature seeds preserve green colour, as it is known for green-seeded cultivars of pea (Pisum sativum L.). The residual Chl compromise seed quality and represent a severe challenge for farmers. Hence, comprehensive understanding of the molecular mechanisms, underlying incomplete Chl degradation is required for maintaining sustainable agriculture. Therefore, here we address dynamics of plastid conversion and photochemical activity alterations, accompanying degradation of Chl in embryos of yellow- and green-seeded cultivars Frisson and Rondo respectively. The yellow-seeded cultivar demonstrated higher rate of Chl degradation at later maturation stage, accompanied with termination of photochemical activity, seed dehydration and conversion of green plastids into amyloplasts. In agreement with this, expression of genes encoding enzymes of Chl degradation was lower in the green seeded cultivar, with the major differences in the levels of Chl b reductase (NYC1) and pheophytinase (PPH) transcripts. Thus, the difference between yellow and green seeds can be attributed to incomplete Chl degradation in the latter at the end of maturation period.

Photosynthesis in non-foliar tissues: implications for yield

Photosynthesis is currently a focus for crop improvement. The majority of this work has taken place and been assessed in leaves, and limited consideration has been given to the contribution that other green tissues make to whole-plant carbon assimilation. The major focus of this review is to evaluate the impact of non-foliar photosynthesis on carbon-use efficiency and total assimilation. Here we appraise and summarize past and current literature on the substantial contribution of different photosynthetically active organs and tissues to productivity in a variety of different plant types, with an emphasis on fruit and cereal crops. Previous studies provide evidence that non-leaf photosynthesis could be an unexploited potential target for crop improvement. We also briefly examine the role of stomata in non-foliar tissues, gas exchange, maintenance of optimal temperatures and thus photosynthesis. In the final section, we discuss possible opportunities to manipulate these processes and provide evidence that Triticum aestivum (wheat) plants genetically manipulated to increase leaf photosynthesis also displayed higher rates of ear assimilation, which translated to increased grain yield. By understanding these processes, we can start to provide insights into manipulating non-foliar photosynthesis and stomatal behaviour to identify novel targets for exploitation in continuing breeding programmes.

Combinatorial interactions of the LEC1 transcription factor specify diverse developmental programs during soybean seed development

The LEAFY COTYLEDON1 (LEC1) transcription factor is a central regulator of seed development, because it controls diverse biological programs during seed development, such as embryo morphogenesis, photosynthesis, and seed maturation. To understand how LEC1 regulates different gene sets during development, we explored the possibility that LEC1 acts in combination with other transcription factors. We identified and compared genes that are directly transcriptionally regulated by ABA-RESPONSIVE ELEMENT BINDING PROTEIN3 (AREB3), BASIC LEUCINE ZIPPER67 (bZIP67), and ABA INSENSITIVE3 (ABI3) with those regulated by LEC1. We showed that LEC1 operates with specific sets of transcription factors to regulate different gene sets and, therefore, distinct developmental processes. Thus, LEC1 controls diverse processes through its combinatorial interactions with other transcription factors. DNA binding sites for the transcription factors are closely clustered in genomic regions upstream of target genes, defining cis-regulatory modules that are enriched for DNA sequence motifs that resemble sequences known to be bound by these transcription factors. Moreover, cis-regulatory modules for genes regulated by distinct transcription factor combinations are enriched for different sets of DNA motifs. Expression assays with embryo cells indicate that the enriched DNA motifs are functional cis elements that regulate transcription. Together, the results suggest that combinatorial interactions between LEC1 and other transcription factors are mediated by cis-regulatory modules containing clustered cis elements and by physical interactions that are documented to occur between the transcription factors.

Mutant-Based Model of Two Independent Pathways for Carotenoid-Mediated Chloroplast Biogenesis in Arabidopsis Embryos

Chloroplasts are essential for autonomous plant growth, and their biogenesis is a complex process requiring both plastid and nuclear genome. One of the essential factors required for chloroplast biogenesis are carotenoids. Carotenoids are synthesized in plastids, and it was shown that plastid localized methylerythritol 4-phosphate (MEP) pathway provides substrates for their biosynthesis. Here, we propose a model, using results of our own mutant analysis combined with the results of others, that a MEP-independent pathway, likely a mevalonate (MVA)-dependent pathway, provides intermediates for chloroplast biogenesis in Arabidopsis embryos. The pattern of this chloroplast biogenesis differs from the MEP-dependent chloroplast biogenesis. In MEP-dependent chloroplast biogenesis, chloroplasts are formed rather uniformly in the whole embryo, with stronger chlorophyll accumulation in cotyledons. In a MEP-independent pathway, chloroplasts are formed predominantly in the hypocotyl and in the embryonic root. We also show that this pattern of chlorophyll accumulation is common to MEP pathway mutants as well as to the mutant lacking geranylgeranyl diphosphate synthase 11 (GGPPS11) activity in plastids but expressing it in the cytosol (GGPPS11cyt). It was recently described that shorter GGPPS11 transcripts are present in Arabidopsis, and they can be translated into active cytosolic proteins. We therefore propose that the MEP-independent pathway for chloroplast biogenesis in Arabidopsis embryos is an MVA pathway that provides substrates for the synthesis of GGPP via GGPPS11cyt and this is then transported to plastids, where it is used for carotenoid biosynthesis and subsequently for chloroplast biogenesis mainly in the hypocotyl and in the embryonic root.

Central role of the LEAFY COTYLEDON1 transcription factor in seed development

Seed development is a complex period of the flowering plant life cycle. After fertilization, the three main regions of the seed, embryo, endosperm and seed coat, undergo a series of developmental processes that result in the production of a mature seed that is developmentally arrested, desiccated, and metabolically quiescent. These processes are highly coordinated, both temporally and spatially, to ensure the proper growth and development of the seed. The transcription factor, LEAFY COTYLEDON1 (LEC1), is a central regulator that controls several aspects of embryo and endosperm development, including embryo morphogenesis, photosynthesis, and storage reserve accumulation. Thus, LEC1 regulates distinct sets of genes at different stages of seed development. Despite its critical importance for seed development, an understanding of the mechanisms underlying LEC1's multifunctionality is only beginning to be obtained. Recent studies describe the roles of specific transcription factors and the hormones, gibberellic acid and abscisic acid, in controlling the activity and transcriptional specificity of LEC1 across seed development. Moreover, studies indicate that LEC1 acts as a pioneer transcription factor to promote epigenetic reprogramming during embryogenesis. In this review, we discuss the mechanisms that enable LEC1 to serve as a central regulator of seed development.

A Protochlorophyllide (Pchlide) a Oxygenase for Plant Viability

Higher plants contain a small, 5-member family of Rieske non-heme oxygenases that comprise the inner plastid envelope protein TIC55, phaeophorbide a oxygenasee (PAO), chlorophyllide a oxygenase (CAO), choline monooxygenase, and a 52 kDa protein (PTC52) associated with the precursor NADPH:protochlorophyllide (Pchlide) oxidoreductase A (pPORA) A translocon (PTC). Some of these chloroplast proteins have documented roles in chlorophyll biosynthesis (CAO) and degradation (PAO and TIC55), whereas the function of PTC52 remains unresolved. Biochemical evidence provided here identifies PTC52 as Pchlide a oxygenase of the inner plastid envelope linking Pchlide b synthesis to pPORA import. Protochlorophyllide b is the preferred substrate of PORA and its lack no longer allows pPORA import. The Pchlide b-dependent import pathway of pPORA thus operates in etiolated seedlings and is switched off during greening. Using dexamethasone-induced RNA interference (RNAi) we tested if PTC52 is involved in controlling both, pPORA import and Pchlide homeostasis in planta. As shown here, RNAi plants deprived of PTC52 transcript and PTC52 protein were unable to import pPORA and died as a result of excess Pchlide a accumulation causing singlet oxygen formation during greening. In genetic studies, no homozygous ptc52 knock-out mutants could be obtained presumably as a result of embryo lethality, suggesting a role for PTC52 in the initial greening of plant embryos. Phylogenetic studies identified PTC52-like genes amongst unicellular photosynthetic bacteria and higher plants, suggesting that the biochemical function associated with PTC52 may have an ancient evolutionary origin. PTC52 also harbors conserved motifs with bacterial oxygenases such as the terminal oxygenase component of 3-ketosteroid 9-alpha-hydroxylase (KshA) from Rhodococcus rhodochrous. 3D-modeling of PTC52 structure permitted the prediction of amino acid residues that contribute to the substrate specificity of this enzyme. In vitro-mutagenesis was used to test the predicted PTC52 model and provide insights into the reaction mechanism of this Rieske non-heme oxygenase.

Photochemical activity changes accompanying the embryogenesis of pea (Pisum sativum) with yellow and green cotyledons

The pea seeds are photosynthetically active until the end of the maturation phase, when the embryonic chlorophylls degrade. However, in some cultivars, the underlying mechanisms are compromised, and the mature seeds preserve green colour. The residual chlorophylls can enhance oxidative degradation of reserve biomolecules, and affect thereby the quality, shelf life and nutritive value of seeds. Despite this, the formation, degradation, and physical properties of the seed chlorophylls are still not completely characterised. So here we address the dynamics of seed photochemical activity in the yellow- and green-seeded pea cultivars by the pulse amplitude modulation (PAM) fluorometric analysis. The experiments revealed the maximal photochemical activity at the early- and mid-cotyledon stages. Thereby, the active centres of PSII were saturated at the light intensity of 15-20µmol photons m-2 s-1. Despite of their shielding from the light by the pod wall and seed coat, photochemical reactions can be registered in the seeds with green embryo. Importantly, even at the low light intensities, the photochemical activity in the coats and cotyledons could be detected. The fast transients of the chlorophyll a fluorescence revealed a higher photochemical activity in the coat of yellow-seeded cultivars in comparison to those with the green-seeded ones. However, it declined rapidly in all seeds at the late cotyledon stage, and was accompanied with the decrease of the seed water content. Thus, the termination of photosynthetic activity in seeds is triggered by their dehydration.

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.

LEC1 sequentially regulates the transcription of genes involved in diverse developmental processes during seed development

LEAFY COTYLEDON1 (LEC1), an atypical subunit of the nuclear transcription factor Y (NF-Y) CCAAT-binding transcription factor, is a central regulator that controls many aspects of seed development including the maturation phase during which seeds accumulate storage macromolecules and embryos acquire the ability to withstand desiccation. To define the gene networks and developmental processes controlled by LEC1, genes regulated directly by and downstream of LEC1 were identified. We compared the mRNA profiles of wild-type and lec1-null mutant seeds at several stages of development to define genes that are down-regulated or up-regulated by the lec1 mutation. We used ChIP and differential gene-expression analyses in Arabidopsis seedlings overexpressing LEC1 and in developing Arabidopsis and soybean seeds to identify globally the target genes that are transcriptionally regulated by LEC1 in planta Collectively, our results show that LEC1 controls distinct gene sets at different developmental stages, including those that mediate the temporal transition between photosynthesis and chloroplast biogenesis early in seed development and seed maturation late in development. Analyses of enriched DNA sequence motifs that may act as cis-regulatory elements in the promoters of LEC1 target genes suggest that LEC1 may interact with other transcription factors to regulate distinct gene sets at different stages of seed development. Moreover, our results demonstrate strong conservation in the developmental processes and gene networks regulated by LEC1 in two dicotyledonous plants that diverged ∼92 Mya.

Light Deprivation-Induced Inhibition of Chloroplast Biogenesis Does Not Arrest Embryo Morphogenesis But Strongly Reduces the Accumulation of Storage Reserves during Embryo Maturation in Arabidopsis

The chloroplast is one of the most important organelles found exclusively in plant and algal cells. Previous reports indicated that the chloroplast is involved in plant embryogenesis, but the role of the organelle during embryo morphogenesis and maturation is still a controversial question demanding further research. In the present study, siliques of Arabidopsis at the early globular stage were enwrapped using tinfoil, and light deprivation-induced inhibition of the chloroplast biogenesis were validated by stereomicroscope, laser scanning confocal microscope and transmission electron microscope. Besides, the effects of inhibited chloroplast differentiation on embryogenesis, especially on the reserve deposition were analyzed using periodic acid-Schiff reaction, Nile red labeling, and Coomassie brilliant blue staining. Our results indicated that tinfoil enwrapping strongly inhibited the formation of chloroplasts, which did not arrest embryo morphogenesis, but markedly influenced embryo maturation, mainly through reducing the accumulation of storage reserves, especially starch grains and oil. Our data provide a new insight into the roles of the chloroplast during embryogenesis.

Mitochondrial electron transport protects floating leaves of long leaf pondweed (Potamogeton nodosus Poir) against photoinhibition: comparison with submerged leaves

Investigations were carried to unravel mechanism(s) for higher tolerance of floating over submerged leaves of long leaf pondweed (Potamogeton nodosus Poir) against photoinhibition. Chloroplasts from floating leaves showed ~5- and ~6.4-fold higher Photosystem (PS) I (reduced dichlorophenol-indophenol → methyl viologen → O2) and PS II (H2O → parabenzoquine) activities over those from submerged leaves. The saturating rate (V max) of PS II activity of chloroplasts from floating and submerged leaves reached at ~600 and ~230 µmol photons m(-2) s(-1), respectively. Photosynthetic electron transport rate in floating leaves was over 5-fold higher than in submerged leaves. Further, floating leaves, as compared to submerged leaves, showed higher F v/F m (variable to maximum chlorophyll fluorescence, a reflection of PS II efficiency), as well as a higher potential to withstand photoinhibitory damage by high light (1,200 µmol photons m(-2) s(-1)). Cells of floating leaves had not only higher mitochondria to chloroplast ratio, but also showed many mitochondria in close vicinity of chloroplasts. Electron transport (NADH → O2; succinate → O2) in isolated mitochondria of floating leaves was sensitive to both cyanide (CN(-)) and salicylhydroxamic acid (SHAM), whereas those in submerged leaves were sensitive to CN(-), but virtually insensitive to SHAM, revealing the presence of alternative oxidase in mitochondria of floating, but not of submerged, leaves. Further, the potential of floating leaves to withstand photoinhibitory damage was significantly reduced in the presence of CN(-) and SHAM, individually and in combination. Our experimental results establish that floating leaves possess better photosynthetic efficiency and capacity to withstand photoinhibition compared to submerged leaves; and mitochondria play a pivotal role in protecting photosynthetic machinery of floating leaves against photoinhibition, most likely by oxidation of NAD(P)H and reduction of O2.

Dynamic Metabolic Profiles and Tissue-Specific Source Effects on the Metabolome of Developing Seeds of Brassica napus

Canola (Brassica napus) is one of several important oil-producing crops, and the physiological processes, enzymes, and genes involved in oil synthesis in canola seeds have been well characterized. However, relatively little is known about the dynamic metabolic changes that occur during oil accumulation in seeds, as well as the mechanistic origins of metabolic changes. To explore the metabolic changes that occur during oil accumulation, we isolated metabolites from both seed and silique wall and identified and characterized them by using gas chromatography coupled with mass spectrometry (GC-MS). The results showed that a total of 443 metabolites were identified from four developmental stages. Dozens of these metabolites were differentially expressed during seed ripening, including 20 known to be involved in seed development. To investigate the contribution of tissue-specific carbon sources to the biosynthesis of these metabolites, we examined the metabolic changes of silique walls and seeds under three treatments: leaf-detachment (Ld), phloem-peeling (Pe), and selective silique darkening (Sd). Our study demonstrated that the oil content was independent of leaf photosynthesis and phloem transport during oil accumulation, but required the metabolic influx from the silique wall. Notably, Sd treatment resulted in seed senescence, which eventually led to a severe reduction of the oil content. Sd treatment also caused a significant accumulation of fatty acids (FA), organic acids and amino acids. Furthermore, an unexpected accumulation of sugar derivatives and organic acid was observed in the Pe- and Sd-treated seeds. Consistent with this, the expression of a subset of genes involved in FA metabolism, sugar and oil storage was significantly altered in Pe and Sd treated seeds. Taken together, our studies suggest the metabolite profiles of canola seeds dynamically varied during the course of oil accumulation, which may provide a new insight into the mechanisms of the oil accumulation at the metabolite level.