Spatio-temporal changes in endogenous abscisic acid contents during etiolated growth and photomorphogenesis in tomato seedlings

Gene Expression Analysis of Potato (Solanum tuberosum L.) Lipoxygenase Cascade and Oxylipin Signature under Abiotic Stress

The metabolism of polyunsaturated fatty acids through the lipoxygenase-catalyzed step and subsequent reactions is referred to as the lipoxygenase (LOX) pathway. The components of this system, such as jasmonates, are involved in growth, development and defense reactions of plants. In this report, we focus on dynamics of expression of different LOX pathway genes and activities of target enzymes with three abiotic stress factors: darkness, salinity and herbicide toxicity. To obtain a more complete picture, the expression profiles of marker genes for salicylic acid, abscisic acid, ethylene, auxin and gibberellin-dependent signaling systems under the same stresses were also analyzed. The gene expression in Solanum tuberosum plants was analyzed using qRT-PCR, and we found that the LOX-cascade-related genes responded to darkness, salinity and herbicide toxicity in different ways. We detected activation of a number of 9-LOX pathway genes; however, in contrast to studies associated with biotic stress (infection), the 9-divinyl ether synthase branch of the LOX cascade was inhibited under all three stresses. GC-MS analysis of the oxylipin profiles also showed the main activity of the 9-LOX-cascade-related enzymes after treatment with herbicide and darkness.

Hormonal impact on photosynthesis and photoprotection in plants

Photosynthesis is not only essential for plants, but it also sustains life on Earth. Phytohormones play crucial roles in developmental processes, from organ initiation to senescence, due to their role as growth and developmental regulators, as well as their central role in the regulation of photosynthesis. Furthermore, phytohormones play a major role in photoprotection of the photosynthetic apparatus under stress conditions. Here, in addition to discussing our current knowledge on the role of the phytohormones auxin, cytokinins, gibberellins, and strigolactones in promoting photosynthesis, we will also highlight the role of abscisic acid beyond stomatal closure in modulating photosynthesis and photoprotection under various stress conditions through crosstalk with ethylene, salicylates, jasmonates, and brassinosteroids. Furthermore, the role of phytohormones in controlling the production and scavenging of photosynthesis-derived reactive oxygen species, the duration and extent of photo-oxidative stress and redox signaling under stress conditions will be discussed in detail. Hormones have a significant impact on the regulation of photosynthetic processes in plants under both optimal and stress conditions, with hormonal interactions, complementation, and crosstalk being important in the spatiotemporal and integrative regulation of photosynthetic processes during organ development at the whole-plant level.

Light and abscisic acid interplay in early seedling development

Abscisic acid (ABA) plays a crucial role in plant development, regulating germination, seedling development and stomatal movements, especially under adverse conditions. Light interacts with the ABA signalling pathway to fine tune these processes. Here, we provide an overview of the recent investigations on ABA-light interplay during early plant development after germination. We discuss the multilayered and reciprocal interactions between ABA signalling components and several light signalling modulators, including photoreceptors, transcription factors and posttranslational modifiers. ABSCISIC ACID INSENSITIVE5 acts as a central convergence point for these interactions during postgermination seedling development. ABA also regulates the adaptation of seedlings to challenging light environments. Furthermore, we enlist the role of ABA-light cross-talk in regulating seedling establishment in crops and highlight open questions for future investigations.

Physiological and transcriptome analyses of photosynthesis and chlorophyll metabolism in variegated Citrus (Shiranuhi and Huangguogan) seedlings

Citrus species are among the most economically important fruit crops. Physiological characteristics and molecular mechanisms associated with de-etiolation have been partially revealed. However, little is known about the mechanisms controlling the expression and function of genes associated with photosynthesis and chlorophyll biosynthesis in variegated citrus seedlings. The lower biomass, chlorophyll contents, and photosynthetic parameter values recorded for the variegated seedlings suggested that chlorophyll biosynthesis was partially inhibited. Additionally, roots of the variegated seedlings were longer than the roots of green seedlings. We obtained 567.07 million clean reads and 85.05 Gb of RNA-sequencing data, with more than 94.19% of the reads having a quality score of Q30 (sequencing error rate = 0.1%). Furthermore, we detected 4,786 and 7,007 differentially expressed genes (DEGs) between variegated and green Shiranuhi and Huangguogan seedlings. Thirty common pathways were differentially regulated, including pathways related to photosynthesis (GO: 0015979) and the chloroplast (GO: 0009507). Photosynthesis (44 and 63 DEGs), photosynthesis-antenna proteins (14 and 29 DEGs), and flavonoid biosynthesis (16 and 29 DEGs) pathways were the most common KEGG pathways detected in two analyzed libraries. Differences in the expression patterns of PsbQ, PetF, PetB, PsaA, PsaN, PsbP, PsaF, Cluster-2274.8338 (ZIP1), Cluster-2274.38688 (PTC52), and Cluster-2274.78784 might be responsible for the variegation in citrus seedlings. We completed a physiological- and transcriptome-level comparison of the Shiranuhi and Huangguogan cultivars that differ in terms of seedling variegation. We performed mRNA-seq analyses of variegated and green Shiranuhi and Huangguogan seedlings to explore the genes and regulatory pathways involved in the inhibition of chlorophyll biosynthesis and decreases in Chl a and Chl b contents. The candidate genes described herein should be investigated in greater detail to further characterize variegated citrus seedlings.

Carotenoid Metabolism in Plants: The Role of Plastids

Carotenoids are indispensable to plants and critical in human diets. Plastids are the organelles for carotenoid biosynthesis and storage in plant cells. They exist in various types, which include proplastids, etioplasts, chloroplasts, amyloplasts, and chromoplasts. These plastids have dramatic differences in their capacity to synthesize and sequester carotenoids. Clearly, plastids play a central role in governing carotenogenic activity, carotenoid stability, and pigment diversity. Understanding of carotenoid metabolism and accumulation in various plastids expands our view on the multifaceted regulation of carotenogenesis and facilitates our efforts toward developing nutrient-enriched food crops. In this review, we provide a comprehensive overview of the impact of various types of plastids on carotenoid biosynthesis and accumulation, and discuss recent advances in our understanding of the regulatory control of carotenogenesis and metabolic engineering of carotenoids in light of plastid types in plants.

To Stimulate or Inhibit? That Is the Question for the Function of Abscisic Acid

Physiologically, abscisic acid (ABA) is believed to be a general inhibitor of plant growth, including during the crucial early development of seedlings. However, this view contradicts many reports of stimulatory effects of ABA that, so far, have not been considered in the debate concerning ABA's function in plant development. To address this apparent contradiction, we propose a hypothetical mechanism to explain how ABA might contribute to the promotion of cell expansion. We wish to overturn conventional views on ABA's role during juvenile plant development and put forward the idea that, as for other phytohormones, the role of ABA is determined by dose and sensitivity and ranges from stimulatory to inhibitory effects.

Variability in CitXET expression and XET activity in Citrus cultivar Huangguogan seedlings with differed degrees of etiolation

Considering the known effects of xyloglucan endotransglycosylase (XET) on plant growth and development, we aimed to determine whether XETs help to regulate the growth and elongation of Huangguogan shoots and roots. We confirmed a possible role for XET during seedling etiolation. Our results revealed that the roots of etiolated seedlings (H-E) were longer than those of green seedlings (H-G). However, shoot length exhibited the opposite pattern. We also observed positive and negative effects on the xyloglucan-degrading activity of XET in the root sub-apical region and shoots of etiolated Huangguogan seedling, respectively. There was a significant down-regulation in CitXET expression in the etiolated shoots at 15 days after seed germination. On the contrary, it was significantly increased in the root sub-apical region of etiolated and multicolored seedlings at 15 days after seed germination. The XET coding sequence (i.e., CitXET) was cloned from Huangguogan seedlings using gene-specific primers. The encoded amino acid sequence was predicted by using bioinformatics-based methods. The 990-bp CitXET gene was highly homologous to other XET genes. The CitXET protein was predicted to contain 319 amino acids, with a molecular mass of 37.45 kDa and an isoelectric point of 9.05. The predicted molecular formula was C1724H2548N448O466S14, and the resulting protein included only one transmembrane structure. The CitXET secondary structure consisted of four main structures (i.e., 21% α-helix, 30.72% extended strand, 9.09% β-turn, and 39.18% random coil). Analyses involving the NCBI Conserved Domains Database (NCBI-CDD), InterPro, and ScanProsite revealed that CitXET was a member of the glycosyl hydrolase family 16 (GH16), and included the DEIDFEFLG motif. Our results indicate that the differed degrees of etiolation influenced the CitXET expression pattern and XET activity in Huangguogan seedlings. The differential changes in XET activity and CitXET expression levels in Huangguogan seedlings may influence the regulation of root and shoot development, and may be important for seedling etiolation.

Transcriptome Analyses of Two Citrus Cultivars (Shiranuhi and Huangguogan) in Seedling Etiolation

Citrus species are among the most important fruit crops. However, gene regulation and signaling pathways related to etiolation in this crop remain unknown. Using Illumina sequencing technology, modification of global gene expression in two hybrid citrus cultivars—Huangguogan and Shiranuhi, respectively—were investigated. More than 834.16 million clean reads and 125.12 Gb of RNA-seq data were obtained, more than 91.37% reads had a quality score of Q30. 124,952 unigenes were finally generated with a mean length of 1,189 bp. 79.15%, 84.35%, 33.62%, 63.12%, 57.67%, 57.99% and 37.06% of these unigenes had been annotated in NR, NT, KO, SwissProt, PFAM, GO and KOG databases, respectively. Further, we identified 604 differentially expressed genes in multicoloured and etiolated seedlings of Shiranuhi, including 180 up-regulated genes and 424 down-regulated genes. While in Huangguogan, we found 1,035 DEGs, 271 of which were increasing and the others were decreasing. 7 DEGs were commonly up-regulated, and 59 DEGs down-regulated in multicoloured and etiolated seedlings of these two cultivars, suggesting that some genes play fundamental roles in two hybrid citrus seedlings during etiolation. Our study is the first to provide the transcriptome sequence resource for seedlings etiolation of Shiranuhi and Huangguogan.