Musa paradisica RCI complements AtRCI and confers Na+ tolerance and K+ sensitivity in Arabidopsis

Effects of drought and rehydration on root gene expression in seedlings of Pinus massoniana Lamb

The mechanisms underlying plant response to drought involve the expression of numerous functional and regulatory genes. Transcriptome sequencing based on the second- and/or third-generation high-throughput sequencing platforms has proven to be powerful for investigating the transcriptional landscape under drought stress. However, the full-length transcriptomes related to drought responses in the important conifer genus Pinus L. remained to be delineated using the third-generation sequencing technology. With the objectives of identifying the candidate genes responsible for drought and/or rehydration and clarifying the expression profile of key genes involved in drought regulation, we combined the third- and second-generation sequencing techniques to perform transcriptome analysis on seedling roots under drought stress and rewatering in the drought-tolerant conifer Pinus massoniana Lamb. A sum of 294,114 unique full-length transcripts were produced with a mean length of 3217 bp and N50 estimate of 5075 bp, including 279,560 and 124,438 unique full-length transcripts being functionally annotated and Gene Ontology enriched, respectively. A total of 4076, 6295 and 18,093 differentially expressed genes (DEGs) were identified in three pair-wise comparisons of drought-treatment versus control transcriptomes, including 2703, 3576 and 8273 upregulated and 1373, 2719 and 9820 downregulated DEGs, respectively. Moreover, 157, 196 and 691 DEGs were identified as transcription factors in the three transcriptome comparisons and grouped into 26, 34 and 44 transcription factor families, respectively. Gene Ontology enrichment analysis revealed that a remarkable number of DEGs were enriched in soluble sugar-related and cell wall-related processes. A subset of 75, 68 and 97 DEGs were annotated to be associated with starch, sucrose and raffinose metabolism, respectively, while 32 and 70 DEGs were associated with suberin and lignin biosynthesis, respectively. Weighted gene co-expression network analysis revealed modules and hub genes closely related to drought and rehydration. This study provides novel insights into root transcriptomic changes in response to drought dynamics in Masson pine and serves as a fundamental work for further molecular investigation on drought tolerance in conifers.

Membrane Proteins in Plant Salinity Stress Perception, Sensing, and Response

Plants have several mechanisms to endure salinity stress. The degree of salt tolerance varies significantly among different terrestrial crops. Proteins at the plant's cell wall and membrane mediate different physiological roles owing to their critical positioning between two distinct environments. A specific membrane protein is responsible for a single type of activity, such as a specific group of ion transport or a similar group of small molecule binding to exert multiple cellular effects. During salinity stress in plants, membrane protein functions: ion homeostasis, signal transduction, redox homeostasis, and solute transport are essential for stress perception, signaling, and recovery. Therefore, comprehensive knowledge about plant membrane proteins is essential to modulate crop salinity tolerance. This review gives a detailed overview of the membrane proteins involved in plant salinity stress highlighting the recent findings. Also, it discusses the role of solute transporters, accessory polypeptides, and proteins in salinity tolerance. Finally, some aspects of membrane proteins are discussed with potential applications to developing salt tolerance in crops.

The root transcriptome dynamics reveals new valuable insights in the salt-resilience mechanism of wild grapevine (Vitis vinifera subsp. sylvestris)

INTRODUCTION

Most of elite cultivated grapevine varieties (Vitis vinifera L.), conventionally grafted on rootstocks, are becoming more and more affected by climate changes, such as increase of salinity. Therefore, we revisited the valuable genetic resources of wild grapevines (V. sylvestris) to elaborate strategies for a sustainable viticulture.

METHODS

Here, we compared physiological and biochemical responses of two salt-tolerant species: a wild grapevine genotype "Tebaba" from our previous studies and the conventional rootstock "1103 Paulsen". Interestingly, our physio-biochemical results showed that under 150mM NaCl, "Tebaba" maintains higher leaf osmotic potential, lower Na+/K+ ratio and a significant peaked increase of polyphenol content at the first 8h of salinity stress. This behavior allowed to hypothesis a drastic repatterning of metabolism in "Tebaba's" roots following a biphasic response. In order to deepen our understanding on the "Tebaba" salt tolerance mechanism, we investigated a time-dependent transcriptomic analysis covering three sampling times, 8h, 24h and 48h.

RESULTS

The dynamic analysis indicated that "Tebaba" root cells detect and respond on a large scale within 8h to an accumulation of ROS by enhancing a translational reprogramming process and inducing the transcripts of glycolytic metabolism and flavonoids biosynthesis as a predominate non-enzymatic scavenging process. Afterwards, there is a transition to a largely gluconeogenic stage followed by a combined response mechanism based on cell wall remodeling and lignin biosynthesis with an efficient osmoregulation between 24 and 48 h.

DISCUSSION

This investigation explored for the first time in depth the established cross-talk between the physiological, biochemical and transcriptional regulators contributing to propose a hypothetical model of the dynamic salt mechanism tolerance of wild grapevines. In summary, these findings allowed further understanding of the genetic regulation mechanism of salt-tolerance in V. sylvestris and identified specific candidate genes valuable for appropriate breeding strategies.

Overexpression of MsRCI2D and MsRCI2E Enhances Salt Tolerance in Alfalfa (Medicago sativa L.) by Stabilizing Antioxidant Activity and Regulating Ion Homeostasis

Rare cold-inducible 2 (RCI2) genes from alfalfa (Medicago sativa L.) are part of a multigene family whose members respond to a variety of abiotic stresses by regulating ion homeostasis and stabilizing membranes. In this study, salt, alkali, and ABA treatments were used to induce MsRCI2D and MsRCI2E expression in alfalfa, but the response time and the expression intensity of the MsRCI2D,-E genes were different under specific treatments. The expression intensity of the MsRCI2D gene was the highest in salt- and alkali-stressed leaves, while the MsRCI2E gene more rapidly responded to salt and ABA treatment. In addition to differences in gene expression, MsRCI2D and MsRCI2E differ in their subcellular localization. Akin to MtRCI2D from Medicago truncatula, MsRCI2D is also localized in the cell membrane, while MsRCI2E is different from MtRCI2E, localized in the cell membrane and the inner membrane. This difference might be related to an extra 20 amino acids in the C-terminal tail of MsRCI2E. We investigated the function of MsRCI2D and MsRCI2E proteins in alfalfa by generating transgenic alfalfa chimeras. Compared with the MsRCI2E-overexpressing chimera, under high-salinity stress (200 mmol·L⁻¹ NaCl), the MsRCI2D-overexpressing chimera exhibited a better phenotype, manifested as a higher chlorophyll content and a lower MDA content. After salt treatment, the enzyme activities of SOD, POD, CAT, and GR in MsRCI2D- and -E-overexpressing roots were significantly higher than those in the control. In addition, after salt stress, the Na⁺ content in MsRCI2D- and -E-transformed roots was lower than that in the control; K⁺ was higher than that in the control; and the Na⁺/K⁺ ratio was lower than that in the control. Correspondingly, H⁺-ATPase, SOS1, and NHX1 genes were significantly up-regulated, and the HKT gene was significantly down-regulated after 6 h of salt treatment. MsRCI2D was also found to regulate the expression of the MsRCI2B and MsRCI2E genes, and the MsRCI2E gene could alter the expression of the MsRCI2A, MsRCI2B, and MsRCI2D genes. MsRCI2D- and -E-overexpressing alfalfa was found to have higher salt tolerance, manifested as improved activity of antioxidant enzymes, reduced content of reactive oxygen species, and sustained Na⁺ and K⁺ ion balance by regulating the expression of the H⁺-ATPase, SOS1, NHX1, HKT, and MsRCI2 genes.

Overexpression of MsRCI2A, MsRCI2B, and MsRCI2C in Alfalfa (Medicago sativa L.) Provides Different Extents of Enhanced Alkali and Salt Tolerance Due to Functional Specialization of MsRCI2s

Rare cold-inducible 2/plasma membrane protein 3 (RCI2/PMP3) genes are ubiquitous in plants and belong to a multigene family whose members respond to a variety of abiotic stresses by regulating ion homeostasis and stabilizing membranes, thus preventing damage. In this study, the expression of MsRCI2A, MsRCI2B, and MsRCI2C under high-salinity, alkali and ABA treatments was analyzed. The results showed that the expression of MsRCI2A, MsRCI2B, and MsRCI2C in alfalfa (Medicago sativa L.) was induced by salt, alkali and ABA treatments, but there were differences between MsRCI2 gene expression under different treatments. We investigated the functional differences in the MsRCI2A, MsRCI2B, and MsRCI2C proteins in alfalfa (Medicago sativa L.) by generating transgenic alfalfa plants that ectopically expressed these MsRCI2s under the control of the CaMV35S promoter. The MsRCI2A/B/C-overexpressing plants exhibited different degrees of improved phenotypes under high-salinity stress (200 mmol.L^-1 NaCl) and weak alkali stress (100 mmol.L^-1 NaHCO₃, pH 8.5). Salinity stress had a more significant impact on alfalfa than alkali stress. Overexpression of MsRCI2s in alfalfa caused the same physiological response to salt stress. However, in response to alkali stress, the three proteins encoded by MsRCI2s exhibited functional differences, which were determined not only by their different expression regulation but also by the differences in their regulatory relationship with MsRCI2s or H⁺-ATPase.

Subcellular Journey of Rare Cold Inducible 2 Protein in Plant Under Stressful Condition

Rare cold inducible 2 (RCI2) proteins are small hydrophobic membrane proteins in plants, and it has been widely reported that RCI2 expressions are dramatically induced by salt, cold, and drought stresses in many species. The RCI2 proteins have been shown to regulate plasma membrane (PM) potential and enhance abiotic stress tolerance when over-expressed in plants. RCI2 protein structures contain two transmembrane domains that are thought to be PM intrinsic proteins and have been observed at the PM and endomembranes. However, cellular trafficking of RCI2s are not fully understood. In this review, we discussed (i) general properties of RCI2s characterized in many species, (ii) the uses of RCI2s as a tracer in live cell imaging analyses and when they are fused to fluorescence proteins during investigations into vesicle trafficking, and (iii) RCI2 functionalities such as their involvement in rapid diffusion, endocytosis, and protein interactions. Consequently, the connection between physiological characteristics of RCI2s and traffic of RCI2s interacting membrane proteins might be helpful to understand role of RCI2s contributing abiotic stresses tolerance.

Characterization of the Esi3/RCI2/PMP3 gene family in the Triticeae

Background

Members of the Early Salt Induced 3 (Esi3/RCI2/PMP3) gene family in plants have been shown to be induced in response to both biotic and abiotic stresses and to enhance stress tolerance in both transgenic plants and Saccharomyces cerevisiae. Esi3 was first identified as a salt stress induced gene in the salt tolerant wild wheat grass, Lophopyrum elongatum, and subsequently homologous genes in many other species were found to be members of the gene family. These include Arabidopsis thaliana and Oryza sativa where they are referred to as Rare Cold Inducible 2 (RCI2), and Zea mays where they are referred to as Plasma Membrane Protein 3 (PMP3). This study characterizes the Esi3 family members in Triticum aestivum and explores the tissue specific expression patterns of the gene family members as well as their response to a variety of environmental stresses.

Results

The Esi3 gene family was found to have a total of 29 family members comprised of ten paralogous groups in the hexaploid T. aestivum. Each paralogous group contains three homeologous copies, one in each of the A, B and D genomes with the exception of Esi3–2 which is missing the B copy. The genes of the Esi3 gene family were also identified in four other monocot species, Aegilops tauschii, Hordeum vulgare, Secale cereale and Sorghum bicolor, and were confirmed or corrected for Brachypodium distachyon, Oryza sativa and Zea mays, as well as the dicot Arabidopsis thaliana. Gene expression of the Esi3s was analyzed using tissue-specific, abiotic and biotic stress RNA-Seq 454 sequence libraries and Affymetrix microarray data for T. aestivum.

Conclusions

Members of nearly all paralogous groups of the Esi3 genes in T. aestivum have altered gene expression in response to abiotic or biotic stress conditions. In addition, there are modest differences in gene expression among homeologous members of the gene family. This suggests that the Esi3 gene family plays an important role in the plants response to the stresses presented in this study. The Esi3–9 in T. aestivum has a unique N terminal extension placing it into Group III, a new group for the Esi3/RCI2/PMP3 gene family.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5311-8) contains supplementary material, which is available to authorized users.

Overexpression of AlTMP2 gene from the halophyte grass Aeluropus littoralis in transgenic tobacco enhances tolerance to different abiotic stresses by improving membrane stability and deregulating some stress-related genes

Herein, we report isolation of the AlTMP2 gene from the halophytic C4 grass Aeluropus littoralis. The subcellular localization suggested that AlTMP2 is a plasma membrane protein. In A. littoralis exposed to salt and osmotic stresses, the AlTMP2 gene was induced early and at a high rate, but was upregulated relatively later in response to abscisic acid and cold treatments. Expression of AlTMP2 in tobacco conferred improved tolerance against salinity, osmotic, H2O2, heat, and freezing stresses at the germination and seedling stages. Under control conditions, no growth or yield penalty were mentioned in transgenic plants due to the constitutive expression of AlTMP2. Interestingly, under greenhouse conditions, the seed yield of transgenic plants was significantly higher than that of non-transgenic (NT) plants grown under salt or drought stress. Furthermore, AlTMP2 plants had less electrolyte leakage, higher membrane stability, and lower Na+ and higher K+ accumulation than NT plants. Finally, six stress-related genes were shown to be deregulated in AlTMP2 plants relative to NT plants under both control and stress conditions. Collectively, these results indicate that AlTMP2 confers abiotic stress tolerance by improving ion homeostasis and membrane integrity, and by deregulating certain stress-related genes.

T-homoeolog specific plasma membrane protein 3 [Nt(t)PMP3-2] in polyploid Nicotiana tabacum shows conserved alternative splicing, derived from extant Nicotiana tomentosiformis parent

Abiotic stress induced plasma membrane protein 3 (PMP3) genes occur as multigene families in plants, coding for hydrophobic proteins. Group I PMP3s code for shorter ORFs while Group II PMP3s code for proteins with C-terminal extensions. Allotetraploid Nicotiana tabacum (SSTT; 2n = 48) derives its parentage from extant ancestors related to Nicotiana sylvestris (SS) and Nicotiana tomentosiformis (TT). Polyploidization triggers complex genetic and epigenetic changes, often leading to homoeolog-specific retention or loss of function, sub-functionalization or neo-functionalization. Genomic sequences of Nt(t)PMP3-1/Nt(t)PMP3-2 cloned from N. tabacum show near identity with N. tomentosiformis NtoPMP3-1/NtoPMP3-2 genomic sequences respectively (distinct from N. sylvestris NsPMP3-1/NsPMP3-2 genomic regions). RT-PCR with exon 1,2 primer pairs amplified only single fragments for Nt(t)PMP3-1 and Nt(t)PMP3-2. In contrast, for Nt(t)PMP3-2, three variants were detected using exon 2,3 primers by RT-PCR. Cloning revealed (i) a transcript coding for a Group I PMP3 [Nt(t)PMP3-2CS], (ii) a transcript with complete retention of the second intron [Nt(t)PMP3-2IR] and (iii) a transcript with an alternative (exon 2) 5' splice site [Nt(t)PMP3-2AS], coding for a longer protein, similar to ORFs of Group II PMP3 genes. All three Nt(t)PMP3-2 variants have conserved counterparts in the N. tomentosiformis transcriptome, suggesting the transcriptional machinery governing alternative splicing of Nt(t)PMP3-2 in N. tabacum has conserved origins, derived from a N. tomenosiformis lineage. The above data shows alternative splicing of PMP3 genes contributes to transcript and ORF diversity in plants. All three Nt(t)PMP3-2 splice variants show increased root-specific expression. Implications of Nt(t)PMP3-2 alternative splicing on transcript stability and ORF features are discussed.

Ectopic Expression of Aeluropus littoralis Plasma Membrane Protein Gene AlTMP1 Confers Abiotic Stress Tolerance in Transgenic Tobacco by Improving Water Status and Cation Homeostasis

We report here the isolation and functional analysis of AlTMP1 gene encoding a member of the PMP3 protein family. In Aeluropus littoralis, AlTMP1 is highly induced by abscisic acid (ABA), cold, salt, and osmotic stresses. Transgenic tobacco expressing AlTMP1 exhibited enhanced tolerance to salt, osmotic, H₂O₂, heat and freezing stresses at the seedling stage. Under greenhouse conditions, the transgenic plants showed a higher level of tolerance to drought than to salinity. Noteworthy, AlTMP1 plants yielded two- and five-fold more seeds than non-transgenic plants (NT) under salt and drought stresses, respectively. The leaves of AlTMP1 plants accumulated lower Na⁺ but higher K⁺ and Ca2+ than those of NT plants. Tolerance to osmotic and salt stresses was associated with higher membrane stability, low electrolyte leakage, and improved water status. Finally, accumulation of AlTMP1 in tobacco altered the regulation of some stress-related genes in either a positive (NHX1, CAT1, APX1, and DREB1A) or negative (HKT1 and KT1) manner that could be related to the observed tolerance. These results suggest that AlTMP1 confers stress tolerance in tobacco through maintenance of ion homeostasis, increased membrane integrity, and water status. The observed tolerance may be due to a direct or indirect effect of AlTMP1 on the expression of stress-related genes which could stimulate an adaptive potential not present in NT plants.

Plant abiotic stress-related RCI2/PMP3s: multigenes for multiple roles

RCI2 / PMP3 s participate in abiotic stress responses and impact the expression of other genes. Their multifunctionality is determined by differential expression and by distinct activities of their structurally different proteins. In plants, RCI2/PMP3 genes, which encode small membrane proteins of the PMP3 family, are closely associated with abiotic stress responses. Their involvement in mediating stress tolerance is supported by genetic evidence and overexpression studies. RCI2/PMP3s occur as multigenes in plant genomes and their encoded proteins belong to distinct and conserved structural groups. In addition, different isoforms appear to be targeted to the plasma membrane or to distinct endomembrane compartments in cells. Several studies have revealed that RCI2/PMP3 proteins participate in cell ion homeostasis, and in regulation of membrane stability and polarization. They also appear to potentiate plant transcriptional responses to abiotic stresses. However, their mechanisms of action remain unknown. This paper reviews the current knowledge of the multiple roles of plant RCI2/PMP3 genes resulting from their differential expression under normal and stress conditions. The structural diversity of RCI2/PMP3 proteins is analyzed and evidence supporting their functional specialization and possible activity mechanisms is examined. Finally, strategies are discussed for exploiting new and established technologies to overcome the difficulties posed by the multigene status of RCI2s and the integral membrane character of their proteins, enabling the probing of their individual functions and collective significance.

Isolation and functional characterization of salt-stress induced RCI2-like genes from Medicago sativa and Medicago truncatula

Salt stress is one of the most significant adverse abiotic factors, causing crop failure worldwide. So far, a number of salt stress-induced genes, and genes improving salt tolerance have been characterized in a range of plants. Here, we report the isolation and characterization of a salt stress-induced Medicago sativa (alfalfa) gene (MsRCI2A), which showed a high similarity to the yeast plasma membrane protein 3 gene (PMP3) and Arabidopsis RCI2A. The sequence comparisons revealed that five genes of MtRCI2(A-E) showed a high similarity to MsRCI2A in the Medicago truncatula genome. MsRCI2A and MtRCI2(A-E) encode small, highly hydrophobic proteins containing two putative transmembrane domains, predominantly localized in the plasma membrane. The transcript analysis results suggest that MsRCI2A and MtRCI2(A-D) genes are highly induced by salt stress. The expression of MsRCI2A and MtRCI2(A-C) in yeast mutants lacking the PMP3 gene can functionally complement the salt sensitivity phenotype resulting from PMP3 deletion. Overexpression of MsRCI2A in Arabidopsis plants showed improved salt tolerance suggesting the important role of MsRCI2A in salt stress tolerance in alfalfa.

Analysis of transcriptional regulation and tissue-specific expression of Avicennia marina Plasma Membrane Protein 3 suggests it contributes to Na(+) transport and homoeostasis in A. marina

Plasma membrane proteins (PMP3) play a role in cation homoeostasis. The 5' flanking sequence of stress inducible, Avicennia marina PMP3 (AmPMP3prom) was transcriptionally fused to (a) GUS or (b) GFP-AmPMP3 and analyzed in transgenic tobacco. Tissue-histochemical GUS and GFP:AmPMP3 localization are co-incident under basal and stress conditions. AmPMP3prom directed GUS activity is highest in roots. Basal transcription is conferred by a 388bp segment upstream of the translation start site. A 463bp distal enhancer in the AmPMP3prom confers enhanced expression under salinity in all tissues and also responds to increases in salinity. The effect of a central, stem-specific negative regulatory region is suppressed by the distal enhancer. The A. marina rhizosphere encounters dynamic changes in salinity at the inter-tidal interface. The complex, tissue-specific transcriptional responsiveness of AmPMP3 to salinity appears to have evolved in response to these changes. Under salinity, guard cell and phloem-specific expression of GFP:AmPMP3 is highly enhanced. Mesophyll, trichomes, bundle sheath, parenchymatous cortex and xylem parenchyma also show GFP:AmPMP3 expression. Cis-elements conferring stress, root and vascular-specific expression are enriched in the AmPMP3 promoter. Pronounced vascular-specific AmPMP3 expression suggests a role in salinity induced Na(+) transport, storage, and secretion in A. marina.