Identification of Chilling-Responsive Genes in Litchi chinensis by Transcriptomic Analysis Underlying Phytohormones and Antioxidant Systems

Litchi (Litchi chinensis Sonn.) is an important subtropical and tropical evergreen fruit tree that is seriously affected by chilling stress. In order to identify genes that may be involved in the response to chilling in litchi, we investigate the physiological and biochemical changes under chilling stress and construct 12 RNA-Seq libraries of leaf samples at 0, 4, 8, and 12 days of chilling. The results show that antioxidant enzymes are activated by chilling treatments. Comparing the transcriptome data of the four time points, we screen 2496 chilling-responsive genes (CRGs), from which we identify 63 genes related to the antioxidant system (AO-CRGs) and 54 ABA, 40 IAA, 37 CTK, 27 ETH, 21 BR, 13 GA, 35 JA, 29 SA, and 4 SL signal transduction-related genes. Expression pattern analysis shows that the expression trends of the 28 candidate genes detected by qRT-PCR are similar to those detected by RNA-Seq, indicating the reliability of our RNA-Seq data. Partial Least Squares Structural Equation Modeling (PLS-SEM) analysis of the RNA-Seq data suggests a model for the litchi plants in response to chilling stress that alters the expression of the plant hormone signaling-related genes, the transcription factor-encoding genes LcICE1, LcCBFs, and LcbZIPs, and the antioxidant system-related genes. This study provides candidate genes for the future breeding of litchi cultivars with high chilling resistance, and elucidates possible pathways for litchi in response to chilling using transcriptomic data.

Comparative physiological and proteomic analyses of mangrove plant Kandelia obovata under cold stress

Cold events had broadly affected the survival and geographic distribution of mangrove plants. Kandelia obovata, has an excellent cold tolerance as a true halophyte and widespread mangrove species. In this study, physiological characters and comparative proteomics of leaves of K. obovata were performed under cold treatment. The physiological analysis showed that K. obovata could alleviate its cold-stress injuries through increasing the levels of antioxidants, the activities of related enzymes, as well as osmotic regulation substances (proline). It was detected 184 differentially expressed protein spots, and of 129 (70.11%) spots were identified. These proteins have been involved in several pathways such as the stress and defense, photosynthesis and photorespiration, signal transduction, transcription factors, protein biosynthesis and degradation, molecular chaperones, ATP synthesis, the tricarboxylic acid (TCA) cycle and primary metabolisms. The protein post-translational modification may be a common phenomenon and plays a key role in cold-response process in K. obovata. According to our precious work, a schematic diagram was drawn for the resistance or adaptation strategy of mangrove plants under cold stress. This study provided valuable information to understand the mechanism of cold tolerance of K. obovata.

Isolation and expression analysis of a CBF transcriptional factor gene from the mangrove Bruguiera gymnorrhiza

The present work isolated a CBF/DREB1 gene from mangrove Bruguiera gymnorrhiza (BgCBF1) and compared its expression levels in various tissues under normal condition and cold stress, and in leaves exposed to various environmental stimuli. Results showed that the BgCBF1 deduced protein showed almost 100% similarities to that of AcCBF1 from Aegiceras corniculatum and AmCBF1 from Avicennia marina. Real-time quantitative PCR analysis showed that BgCBF1 gene displayed constitute expression in leaf, stem and root samples of plantlets under normal condition, but with different expression levels and tissue preference. When exposed to cold, BgCBF1 could be rapidly, slightly and transiently induced in all tissues. Furthermore, the BgCBF1 gene in leaves displayed a transient and small induction after salt and drought (PEG) exposure, while exhibited relatively high up-regulated expression after the phytohormone abscisic acid (ABA) treatment. These results suggest that the BgCBF1 gene may participate in the ABA mediated development and protection of plant against cold and drought. Further studies on its promoters and downstream genes will be needed to better understand its functions.

Isolation and expression analysis of two novel C-repeat binding factor (CBF) genes involved in plant growth and abiotic stress response in mangrove Kandelia obovata

Kandelia obovata is one of the cold tolerant mangrove plants along the China coast. To reveal the cold tolerant mechanism of K. obovata, the present work isolated two CBF/DREB1 genes (designated KoCBF1 and KoCBF3) from cold-stressed K. obovata and characterized their expression profiles in various organs and in response to multiple abiotic stresses. The deduced proteins of KoCBF1 and 3 all contain specific features of CBFs, and show high similarity to AmCBF1 and 3 from Avicennia marina, respectively. Different expression patterns of the two CBF orthologous under various abiotic stresses and exogenous hormone suggested that they may have different regulators and be involved in different regulatory pathway. The high basal and cold induced expression of the two genes indicated that they may all play important roles in growth and cold resistance of plants. The significant induction of KoCBF3 after salt and lead (Pb²⁺) treatments suggested that this CBF gene may also participate in response to salinity and heavy metal stresses. This study will provide a better understanding of CBF-regulated stress-resistant mechanism, which may be benefit in mangrove biotechnological breeding, high-latitude transplanting, and bioremediation of heavy metal pollutions.

Mechanism of Stomatal Closure in Plants Exposed to Drought and Cold Stress

Drought is one of the abiotic stresses which impairs the plant growth/development and restricts the yield of many crops throughout the world. Stomatal closure is a common adaptation response of plants to the onset of drought condition. Stomata are microscopic pores on the leaf epidermis, which regulate the transpiration/CO₂ uptake by leaves. Stomatal guard cells can sense various abiotic and biotic stress stimuli from the internal and external environment and respond quickly to initiate closure under unfavorable conditions. Stomata also limit the entry of pathogens into leaves, restricting their invasion. Drought is accompanied by the production and/or mobilization of the phytohormone, abscisic acid (ABA), which is well-known for its ability to induce stomatal closure. Apart from the ABA, various other factors that accumulate during drought and affect the stomatal function are plant hormones (auxins, MJ, ethylene, brassinosteroids, and cytokinins), microbial elicitors (salicylic acid, harpin, Flg 22, and chitosan), and polyamines . The role of various signaling components/secondary messengers during stomatal opening or closure has been a matter of intense investigation. Reactive oxygen species (ROS) , nitric oxide (NO) , cytosolic pH, and calcium are some of the well-documented signaling components during stomatal closure. The interrelationship and interactions of these signaling components such as ROS, NO, cytosolic pH, and free Ca²⁺ are quite complex and need further detailed examination.Low temperatures can have deleterious effects on plants. However, plants evolved protection mechanisms to overcome the impact of this stress. Cold temperature inhibits stomatal opening and causes stomatal closure. Cold-acclimated plants often exhibit marked changes in their lipid composition, particularly of the membranes. Cold stress often leads to the accumulation of ABA, besides osmolytes such as glycine betaine and proline. The role of signaling components such as ROS, NO, and Ca²⁺ during cold acclimation is yet to be established, though the effects of cold stress on plant growth and development are studied extensively. The information on the mitigation processes is quite limited. We have attempted to describe consequences of drought and cold stress in plants, emphasizing stomatal closure. Several of these factors trigger signaling components in roots, shoots, and atmosphere, all leading to stomatal closure. A scheme is presented to show the possible signaling events and their convergence and divergence of action during stomatal closure. The possible directions for future research are discussed.

Characterization and expression analysis of a gene encoding CBF/DREB1 transcription factor from mangrove Aegiceras corniculatum

Several transcription factors play important roles in survival of plants under cold, drought and salt stresses by serving as master regulator of sets of downstream stress-responsive genes. A gene encoding CBF/DREB1 transcription factor (C-repeat binding factor/dehydration responsive element-binding factor 1) was isolated from mangrove Aegiceras corniculatum and designated AcCBF1. The full-length cDNA of AcCBF1 was 896 bp containing 618 bp ORF encoding a protein of 205 amino acids. Multiple sequence analysis showed that the corresponding protein had 100 % identity to AmCBF1 (KC776908) from mangrove Avicennia marina, and contains an AP2/ERE DNA-binding domain and two CBF signature sequences. Expression analyses based on quantitative real-time PCR revealed that the AcCBF1 gene was expressed in all tissues of A. corniculatum under normal condition with the highest expression level detected in leaves. When exposed to abiotic stresses, AcCBF1 gene showed different expression patterns in different tissues. Generally, AcCBF1 gene could be rapidly and strongly induced by cold and drought, while slightly induced by abscisic acid and salinity. Furthermore, light could positively regulate the cold-induction level of AcCBF1. These results suggest that the AcCBF1 may be playing important role in the signaling pathway of cold stress and also involved in the cross-talk among abiotic stresses. Further studies focusing on the promotors and downstream stress-responsive genes of AcCBF1 will help to better understand the regulatory mechanisms of mangrove A. corniculatum under abiotic stresses.

Ecophysiological differences between three mangrove seedlings (Kandelia obovata, Aegiceras corniculatum, and Avicennia marina) exposed to chilling stress

Although the cold-resistant ability of mangroves varies greatly with species, the physiological mechanism remains unclear. The chilling stress effects on morphological changes, photosynthetic pigments, reactive oxygen species (ROS), malondialdehyde (MDA) and several antioxidants, were studied in leaves of three mangrove seedlings (Kandelia obovata, Aegiceras corniculatum and Avicennia marina). Results showed that both K. obovata and A. corniculatum exhibited lighter chilling damage, lower chilling injury rates and higher survival rates compared to A. marina. Reductions of chlorophylls (Chls) were observed in all the three mangroves, and the highest was detected in A. marina. Significant increases in content of ROS (hydrogen peroxide, H2O2; hydroxyl radicals, OH⋅) and MDA were observed in both A. marina and A. corniculatum, whereas chilling stressed K. obovata showed a decrease in H2O2 content, constant OH⋅ level and instantaneous increase of MDA. The contents of proline and water-soluble protein exhibited similar stress-time dependent increases in all mangroves, while A. corniculatum showed the highest increase of proline and relatively higher increase of water-soluble protein. The catalase activities significantly decreased with stress time in all mangroves, while K. obovata showed the least reduction. An increase in ascorbic acid (AsA) content and activities of superoxide dismutase, peroxidase (POD), and ascorbate peroxidase (APX) were also detected in all the three mangroves, while K. obovata showed the highest increases. These results indicate that chilling-tolerance of mangroves is associated with the efficiency of antioxidants, as confirmed by principal component analysis. The AsA, APX and POD in K. obovata may play more important role in control of oxidative stresses than those in the other two species. Furthermore, the higher cold-resistance of A. corniculatum compared to A. marina may be partly associated with its higher proline accumulation. The results indicate that enzymatic and non-enzymatic antioxidants (POD, APX, AsA, proline and Car) play key roles in scavenging of excess ROS in mangroves. Further studies focusing on these stress-responsive genes will enable better understanding of the cold-resistance mechanism from molecular level.

Characterization and expression analysis of three CBF/DREB1 transcriptional factor genes from mangrove Avicennia marina

Three CBF/DREB1 (C-repeat binding factor/dehydration responsive element-binding factor 1) homologues were isolated from mangrove Avicennia marina and designated AmCBF1, 2 and 3. Multiple sequence analysis showed that the three deduced proteins all contain an AP2 DNA-binding domain and two CBF signature sequences. According to the phylogenetic analysis, these proteins belong to the A-1 subgroup of the DREB subfamily. Expression analyses based on quantitative real-time PCR revealed that the AmCBF2 displayed relatively high expression under normal conditions, with the highest level in stems, while both AmCBF1 and 3 were weakly expressed without stress. The three genes also showed different responses to various environmental stimuli. The AmCBF2 was inducible by cold, drought, high salinity, heavy metals, as well as abscisic acid (ABA), and exhibited much stronger induction by cold, drought, Pb(2+) or Zn(2+) than by NaCl, ABA or Cd(2+). In contrast, both AmCBF1 and AmCBF3 displayed insignificant changes under these stimuli. These results indicate that the three AmCBF genes play different roles in A. marina and the AmCBF2 might be involved in the signaling pathway of cold, drought and heavy metal stress response.