EIN3-like gene expression during fruit ripening of Cavendish banana (Musa acuminata cv. Grande naine)

Salicylic acid delays pear fruit senescence by playing an antagonistic role toward ethylene, auxin, and glucose in regulating the expression of PpEIN3a

Salicylic acid (SA) and ethylene (ET) are crucial fruit senescence hormones. SA inhibited ET biosynthesis. However, the mechanism of SA delaying fruit senescence is less known. ETHYLENE INSENSITIVE 3 (EIN3), a key positive switch in ET perception, functions as a transcriptional activator and binds to the primary ET response element that is present in the promoter of the ETHYLENE RESPONSE FACTOR1 gene. In this study, a gene encoding putative EIN3 protein was cloned from sand pear and designated as PpEIN3a. The deduced PpEIN3a contains a conserved EIN3 domain. The evolutionary analysis results indicated that PpEIN3a belonged to the EIN3 superfamily. Real-time quantitative PCR analysis revealed that the accumulation of PpEIN3a transcripts were detected in all tissues of this pear. Moreover, PpEIN3a expression was regulated during fruit development. Interestingly, the expression of PpEIN3a was downregulated by SA but upregulated by ET, auxin, and glucose. Additionally, the contents of free and conjugated SA were higher than those of the control after SA treatment. While the content of ET and auxin (indole-3-acetic acid, IAA) dramatically decreased after SA treatment compared with control during fruit senescence. The content of glucose increased when fruit were treated by SA for 12 h and then there were no differences between SA treatment and control fruit during the shelf life. SA also delayed the decrease in sand pear (Pyrus pyrifolia Nakai. 'Whangkeumbae') fruit firmness. The soluble solid content remained relatively stable between the SA treated and control fruits. This study showed that SA plays an antagonistic role toward ET, auxin, and glucose in regulating the expression of PpEIN3a to delay fruit senescence.

Methionine oxidation and reduction of the ethylene signaling component MaEIL9 are involved in banana fruit ripening

The ethylene insensitive 3/ethylene insensitive 3-like (EIN3/EIL) plays an indispensable role in fruit ripening. However, the regulatory mechanism that links post-translational modification of EIN3/EIL to fruit ripening is largely unknown. Here, we studied the expression of 13 MaEIL genes during banana fruit ripening, among which MaEIL9 displayed higher enhancement particularly in the ripening stage. Consistent with its transcript pattern, abundance of MaEIL9 protein gradually increased during the ripening process, with maximal enhancement in the ripening. DNA affinity purification (DAP)-seq analysis revealed that MaEIL9 directly targets a subset of genes related to fruit ripening, such as the starch hydrolytic genes MaAMY3D and MaBAM1. Stably overexpressing MaEIL9 in tomato fruit hastened fruit ripening, whereas transiently silencing this gene in banana fruit retarded the ripening process, supporting a positive role of MaEIL9 in fruit ripening. Moreover, oxidation of methionines (Met-129, Met-130, and Met-282) in MaEIL9 resulted in the loss of its DNA-binding capacity and transcriptional activation activity. Importantly, we identified MaEIL9 as a potential substrate protein of methionine sulfoxide reductase A MaMsrA4, and oxidation of Met-129, Met-130, and Met-282 in MaEIL9 could be restored by MaMsrA4. Collectively, our findings reveal a novel regulatory network controlling banana fruit ripening, which involves MaMsrA4-mediated redox regulation of the ethylene signaling component MaEIL9.

GhEIN3, a cotton (Gossypium hirsutum) homologue of AtEIN3, is involved in regulation of plant salinity tolerance

Ethylene insensitive 3 (EIN3), a key transcription factor in ethylene signal transduction, play important roles in plant stress signaling pathways. In this study, we isolated and characterized an EIN3-like gene from cotton (Gossypium hirsutum), designated as GhEIN3. GhEIN3 is highly expressed in vegetative tissues, and its expression is induced by 1-aminocyclopropane-1-carboxylic acid (ACC) and NaCl. Ectopic expression of GhEIN3 in Arabidopsis elevated plants' response to ethylene, which exhibit smaller leaves, more root hairs, shorter roots and hypocotyls. The germination rate, survival rate and root length of GhEIN3 transgenic plants were significantly improved compared to wild type under salt stress. GhEIN3 transgenic plants accumulated less H₂O₂ and malondialdehyde (MDA), while higher superoxide dismutase (SOD) and peroxidase (POD) activities were detected under salt stress. In addition, expression of several genes related to reactive oxygen species (ROS) pathway and ABA signaling pathway was increased in the GhEIN3 transgenic plants under salt stress. In contrast, virus-induced gene silencing (VIGS) of GhEIN3 in cotton enhanced the sensitivity of transgenic plants to salt stress, accumulating higher H₂O₂ and MDA and lower SOD and POD activities compared to control plants. Collectively, our results revealed that GhEIN3 might be involved in the regulation of plant response to salt stress by regulating ABA and ROS pathway during plant growth and development.

Expression and Regulation of PpEIN3b during Fruit Ripening and Senescence via Integrating SA, Glucose, and ACC Signaling in Pear (Pyrus pyrifolia Nakai. Whangkeumbae)

The economic value of fruit is reduced by having a short shelf life. Whangkeumbae is a type of sand pear (Pyrus pyrifolia) considered a climacteric fruit. The pear is famous for its smooth surface and good flavor. However, its shelf life is very short because of senescence and disease after harvest and a burst of ethylene (ET) production prompting the onset of fruit ripening. In plants, ETHYLENE INSENSITIVE3 (EIN3) and EIN3like (EIL), located in the nucleus, are important components of the ET signaling pathway and act as transcription factors. EIN3s and EILs belong to a small family involved in regulating the expression of ethylene response factor gene (ERF), whose encoding protein is the final component in the ET signaling pathway. The mutation of these components will cause defects in the ethylene pathway. In this study, one gene encoding an EIN3 was cloned and identified from Whangkeumbae and designated PpEIN3b. The deduced PpEIN3b contained a conserved EIN3 domain, a bipartite nuclear localization signal profile (NLS_BP), and an N-6 adenine-specific DNA methylase signature (N6_MTASE). PpEIN3b belongs to the EIN3 super-family by phylogenetic analysis. Quantitative RT-PCR (qRT-PCR) analysis revealed that PpEIN3b was preferentially expressed in fruit. Additionally, its expression was developmentally regulated during fruit ripening and senescence. Furthermore, PpEIN3b transcripts were obviously repressed by salicylic acid (SA) and glucose treatment in pear fruit and in diseased fruit, while it was significantly induced by 1-aminocyclopropane-1-carboxylic acid (ACC) treatment. Taken together, our results reveal the expression and regulation profiles of PpEIN3b and suggest that PpEIN3b might integrate SA, glucose, and ACC signaling to regulate fruit ripening and senescence in pear, which would provide a candidate gene for this regulation to obtain fruit with a long shelf life and improved economic value.

The Starch Is (Not) Just Another Brick in the Wall: The Primary Metabolism of Sugars During Banana Ripening

The monocot banana fruit is one of the most important crops worldwide. As a typical climacteric fruit, the harvest of commercial bananas usually occurs when the fruit is physiologically mature but unripe. The universal treatment of green bananas with ethylene or ethylene-releasing compounds in order to accelerate and standardize the ripening of a bunch of bananas mimics natural maturation after increasing the exogenous production of ethylene. The trigger of autocatalytic ethylene production regulated by a dual positive feedback loop circuit derived from a NAC gene and three MADS genes results in metabolic processes that induce changes in the primary metabolism of bananas. These changes include pulp softening and sweetening which are sensorial attributes that determine banana postharvest quality. During fruit development, bananas accumulate large amounts of starch (between 15 and 35% w/w of their fresh weight, depending on the cultivar). Pulp softening and sweetening during banana ripening are attributed not only to changes in the activities of cell wall hydrolases but also to starch-to-sugar metabolism. Therefore, starch granule erosion and disassembling are key events that lead bananas to reach their optimal postharvest quality. The knowledge of the mechanisms that regulate sugar primary metabolism during banana ripening is fundamental to reduce postharvest losses and improve final product quality, though. Recent studies have shown that ethylene-mediated regulation of starch-degrading enzymes at transcriptional and translational levels is crucial for sugar metabolism in banana ripening. Furthermore, the crosstalk between ethylene and other hormones including indole-3-acetic acid and abscisic acid also influences primary sugar metabolism. In this review, we will describe the state-of-the-art sugar primary metabolism in bananas and discuss the recent findings that shed light on the understanding of the molecular mechanisms involved in the regulation of this metabolism during fruit ripening.

The effect of 1-methylcyclopropene (1-MCP) on expression of ethylene receptor genes in durian pulp during ripening

Rapid fruit ripening is a significant problem that limits the shelf life of durian, with ethylene having a major impact on the regulation of this event. Durian treated with ethephon ripened 3 d after treatment with increased pulp total soluble solids, ethylene production of the whole fruit and decreased pulp firmness compared to the control fruit. 1-MCP treatment delayed ripening by up to 9 d with inhibited accumulation of total soluble solids, color change, softening and ethylene production. Genes related to ethylene perception (DzETR1 and DzETR2) and the signaling pathway (DzCTR1, DzEIL1 and DzEIL2) in the pulp were investigated during this process, using qPCR to quantify changes in gene transcription. All candidate genes were significantly up-regulated in ripening durian pulp. Ethephon treatment increased the expression of DzETR1 and DzETR2 genes, while expression of DzCTR1, DzEIL1 and DzEIL2 were slightly affected. 1-MCP treatment significantly inhibited the expression of the DzETR2 and DzEIL1 genes. The promoters of DzETR2 genes were isolated and their activation by fruit transcription factors studied using transient expression in tobacco leaves. It was found that members of the kiwifruit and apple EIL1, EIL2 and EIL3 genes strongly activated the DzETR2 promoter. These results suggest that ethylene-induced ripening of durian is via the regulation of DzETR2 by EIL transcription factors.

Genome-wide identification and expression profiling of EIL gene family in woody plant representative poplar (Populus trichocarpa)

This study aimed to improve current understanding on ethylene-insensitive 3-like (EIL) members, least explored in woody plants such as poplar (Populus trichocarpa Torr. & Grey). Herein, seven putative EIL members were identified in P. trichocarpa genome and were roughly annotated either as EIN3-like sequence associated with ethylene pathway or EIL3-like sequences related with sulfur (S)-pathway. Motif-distribution pattern of proteins also corroborated this annotation. They were distributed on six chromosomes (chr1, 3, 4 and 8-10), and were revealed to encode a protein of 509-662 residues with nuclear localization. The presence of ethylene insensitive 3 (EIN3; PF04873) domain (covering first 80-280 residues from N-terminus) was confirmed by Hidden Markov Model-based search. The first half of EIL proteins (∼80-280 residues including EIN3 domain) was substantially conserved. The second half (∼300-600 residues) was considerably diverged. Additionally, first half of proteins harbored acidic, proline-rich and glutamine-rich sites, and supported the essentiality of these regions in the transcriptional-activation and protein-function. Moreover, identified six segmental and one-tandem duplications demonstrated the negative or purifying selective nature of mutations. Furthermore, expression profile analysis indicated the possibility of a crosstalk between EIN3- and EIL3-like genes, and co-expression networks implicated their interactions with very diverse panels of biological molecules.

The banana fruit Dof transcription factor MaDof23 acts as a repressor and interacts with MaERF9 in regulating ripening-related genes

The DNA binding with one finger (Dof) proteins, a family of plant-specific transcription factors, are involved in a variety of plant biological processes. However, little information is available on their involvement in fruit ripening. We have characterized 25 MaDof genes from banana fruit (Musa acuminata), designated as MaDof1-MaDof25 Gene expression analysis in fruit subjected to different ripening conditions revealed that MaDofs were differentially expressed during different stages of ripening. MaDof10, 23, 24, and 25 were ethylene-inducible and nuclear-localized, and their transcript levels increased during fruit ripening. Moreover, yeast two-hybrid and bimolecular fluorescence complementation analyses demonstrated a physical interaction between MaDof23 and MaERF9, a potential regulator of fruit ripening reported in a previous study. We determined that MaDof23 is a transcriptional repressor, whereas MaERF9 is a transcriptional activator. We suggest that they might act antagonistically in regulating 10 ripening-related genes, including MaEXP1/2/3/5, MaXET7, MaPG1, MaPME3, MaPL2, MaCAT, and MaPDC, which are associated with cell wall degradation and aroma formation. Taken together, our findings provide new insight into the transcriptional regulation network controlling banana fruit ripening.

The Banana Transcriptional Repressor MaDEAR1 Negatively Regulates Cell Wall-Modifying Genes Involved in Fruit Ripening

Ethylene plays an essential role in many biological processes including fruit ripening via modulation of ethylene signaling pathway. Ethylene Response Factors (ERFs) are key transcription factors (TFs) involved in ethylene perception and are divided into AP2, RAV, ERF, and DREB sub-families. Although a number of studies have implicated the involvement of DREB sub-family genes in stress responses, little is known about their roles in fruit ripening. In this study, we identified a DREB TF with a EAR motif, designated as MaDEAR1, which is a nucleus-localized transcriptional repressor. Expression analysis indicated that MaDEAR1 expression was repressed by ethylene, with reduced levels of histone H3 and H4 acetylation at its regulatory regions during fruit ripening. In addition, MaDEAR1 promoter activity was also suppressed in response to ethylene treatment. More importantly, MaDEAR1 directly binds to the DRE/CRT motifs in promoters of several cell wall-modifying genes including MaEXP1/3, MaPG1, MaXTH10, MaPL3, and MaPME3 associated with fruit softening during ripening and represses their activities. These data suggest that MaDEAR1 acts as a transcriptional repressor of cell wall-modifying genes, and may be negatively involved in ethylene-mediated ripening of banana fruit. Our findings provide new insights into the involvement of DREB TFs in the regulation of fruit ripening.

Identification of miRNAs involved in fruit ripening in Cavendish bananas by deep sequencing

BACKGROUND

MicroRNAs (miRNAs) are a family of non-coding small RNAs that play an important regulatory role in various biological processes. Previous studies have reported that miRNAs are closely related to the ripening process in model plants. However, the miRNAs that are closely involved in the banana fruit ripening process remain unknown.

METHODS

Here, we investigated the miRNA populations from banana fruits in response to ethylene or 1-MCP treatment using a deep sequencing approach and bioinformatics analysis combined with quantitative RT-PCR validation.

RESULTS

A total of 125 known miRNAs and 26 novel miRNAs were identified from three libraries. MiRNA profiling of bananas in response to ethylene treatment compared with 1-MCP treatment showed differential expression of 82 miRNAs. Furthermore, the differentially expressed miRNAs were predicted to target a total of 815 target genes. Interestingly, some targets were annotated as transcription factors and other functional proteins closely involved in the development and the ripening process in other plant species. Analysis by qRT-PCR validated the contrasting expression patterns between several miRNAs and their target genes.

CONCLUSIONS

The miRNAome of the banana fruit in response to ethylene or 1-MCP treatment were identified by high-throughput sequencing. A total of 82 differentially expressed miRNAs were found to be closely associated with the ripening process. The miRNA target genes encode transcription factors and other functional proteins, including SPL, APETALA2, EIN3, E3 ubiquitin ligase, β-galactosidase, and β-glucosidase. These findings provide valuable information for further functional research of the miRNAs involved in banana fruit ripening.

Characterization and expression of genes involved in the ethylene biosynthesis and signal transduction during ripening of mulberry fruit

Although ethylene is well known as an essential regulator of fruit development, little work has examined the role ethylene plays in the development and maturation of mulberry (Morus L.) fruit. To study the mechanism of ethylene action during fruit development in this species, we measured the ethylene production, fruit firmness, and soluble solids content (SSC) during fruit development and harvest. By comparing the results with those from other climacteric fruit, we concluded that Morus fruit are probably climacteric. Genes associated with the ethylene signal transduction pathway of Morus were characterized from M. notabilis Genome Database, including four ethylene receptor genes, a EIN2-like gene, a CTR1-like gene, four EIN3-like genes, and a RTE1-like gene. The expression patterns of these genes were analyzed in the fruit of M. atropurpurea cv. Jialing No.40. During fruit development, transcript levels of MaETR2, MaERS, MaEIN4, MaRTE, and MaCTR1 were lower at the early stages and higher after 26 days after full bloom (DAF), while MaETR1, MaEIL1, MaEIL2, and MaEIL3 remained constant. In ripening fruit, the transcripts of MaACO1 and MaACS3 increased, while MaACS1 and MaACO2 decreased after harvest. The transcripts of MaACO1, MaACO2, and MaACS3 were inhibited by ethylene, and 1-MCP (1–methylcyclopropene) upregulated MaACS3. The transcripts of the MaETR-like genes, MaRTE, and MaCTR1 were inhibited by ethylene and 1-MCP, suggesting that ethylene may accelerate the decline of MaETRs transcripts. No significant changes in the expression of MaEIN2, MaEIL1, and MaEIL3 were observed during ripening or in response to ethylene, while the expressions of MaEIL2 and MaEIL4 increased rapidly after 24 h after harvest (HAH) and were upregulated by ethylene. The present study provides insights into ethylene biosynthesis and signal transduction in Morus plants and lays a foundation for the further understanding of the mechanisms underlying Morus fruit development and ripening.

A ripening-induced transcription factor MaBSD1 interacts with promoters of MaEXP1/2 from banana fruit

KEY MESSAGE

The ripening-induced MaBSD1 acts as a transcriptional activator, and might be involved in banana fruit ripening partly through directly activating the expression of two ripening-associated genes, MaEXP1/2. BSD (BTF2-like transcription factors, synapse-associated proteins and DOS2-like proteins) transcription factors are characterized by a typical BSD domain. However, little information is available concerning their possible roles in plant growth and development, especially in fruit ripening. In the present study, one BSD gene, designated as MaBSD1, was isolated from banana fruit. MaBSD1 has an open reading frame (ORF) of 921 bp which encodes a polypeptide of 306 amino acid residues with molecular weight of 34.80 kDa, and isoelectric point (pI) of 4.54. Subcellular localization and transcriptional activation assays showed that MaBSD1 was localized in both the nucleus and cytoplasm and possessed transcriptional activity. RT-qPCR and promoter activity analysis indicated that MaBSD1 was ethylene and ripening inducible, and the accumulation of MaBSD1 transcript was correlated well with the evolution of ethylene production and ripening process. Moreover, transient assay showed that MaBSD1 could activate the expression of two cell wall modification-related genes, MaEXP1/2, via directly interacting with their promoters. Together, these data suggest that ripening-induced MaBSD1 acts as a transcriptional activator and might be associated with banana fruit ripening, at least partially through directly activating the expression of MaEXP1/2, expanding the limited information concerning the BSD transcription factor in relation to fruit ripening.

Changes in ethylene signaling and MADS box gene expression are associated with banana finger drop

Banana finger drop was examined in ripening banana harvested at immature (iMG), early (eMG) and late mature green (lMG) stages, with contrasting ripening rates and ethylene sensitivities. Concomitantly, 11 ethylene signal transduction components (ESTC) and 6 MADS box gene expressions were comparatively studied in median (control zone, CZ) and pedicel rupture (drop zone DZ) areas in peel tissue. iMG fruit did not ripen or develop finger drop while eMG and lMG fruits displayed a similar finger drop pattern. Several ESTC and MADS box gene mRNAs were differentially induced in DZ and CZ and sequentially in eMG and lMG fruits. MaESR2, 3 and MaEIL1, MaMADS2 and MaMADS5 had a higher mRNA level in eMG and acted earlier, whereas MaERS1, MaCTR1, MaEIL3/AB266319, MaEIL4/AB266320 and MaEIL5/AB266321, MaMADS4 and to a lesser extent MaMADS2 and 5 acted later in lMG. In this fruit, MaERS1 and 3, MaCTR1, MaEIL3, 4 and MaEIL5/AB266321, and MaMADS4 were enhanced by finger drop, suggesting their specific involvement in this process. MaEIL1, MaMADS1 and 3, induced at comparable levels in DZ and CZ, are probably related to the overall fruit ripening process. These findings led us to consider that developmental cues are the predominant finger drop regulation factor.

Expansion of banana (Musa acuminata) gene families involved in ethylene biosynthesis and signalling after lineage-specific whole-genome duplications

Whole-genome duplications (WGDs) are widespread in plants, and three lineage-specific WGDs occurred in the banana (Musa acuminata) genome. Here, we analysed the impact of WGDs on the evolution of banana gene families involved in ethylene biosynthesis and signalling, a key pathway for banana fruit ripening. Banana ethylene pathway genes were identified using comparative genomics approaches and their duplication modes and expression profiles were analysed. Seven out of 10 banana ethylene gene families evolved through WGD and four of them (1-aminocyclopropane-1-carboxylate synthase (ACS), ethylene-insensitive 3-like (EIL), ethylene-insensitive 3-binding F-box (EBF) and ethylene response factor (ERF)) were preferentially retained. Banana orthologues of AtEIN3 and AtEIL1, two major genes for ethylene signalling in Arabidopsis, were particularly expanded. This expansion was paralleled by that of EBF genes which are responsible for control of EIL protein levels. Gene expression profiles in banana fruits suggested functional redundancy for several MaEBF and MaEIL genes derived from WGD and subfunctionalization for some of them. We propose that EIL and EBF genes were co-retained after WGD in banana to maintain balanced control of EIL protein levels and thus avoid detrimental effects of constitutive ethylene signalling. In the course of evolution, subfunctionalization was favoured to promote finer control of ethylene signalling.

Identification and expression analysis of ethylene biosynthesis and signaling genes provides insights into the early and late coffee cultivars ripening pathway

The plant hormone ethylene is involved in the regulation of a multitude of plant processes, ranging from seed germination to organ senescence. Ethylene induces fruit ripening in climacteric fruits, such as coffee, being directly involved in fruit ripening time and synchronization. Coffee early cultivars usually show a more uniform ripening process although little is known about the genetic factors that promote the earliness of ripening. Thus, this work aimed to characterize the putative members of the coffee (Coffea arabica) ethylene biosynthesis and signaling pathways, as well as to analyze the expression patterns of these members during fruit ripening of early (Catucaí 785-15) and late (Acauã) coffee cultivars. Reverse Transcription-qPCR analysis of the four biosynthesis genes (CaACS1-like; CaACO1-like; CaACO4-like e CaACO5-like) analyzed in this study showed that CaACO1-like and CaACO4-like displayed an expression pattern typically observed in climacteric fruits, being up-regulated during ripening. CaACS1-like gene expression was also up-regulated during fruit ripening of both cultivars, although in a much lesser extent when compared to the changes in CaACO1-like and CaACO4-like gene expression. CaACO5-like was only induced in raisin fruit and may be related to senescence processes. On the other hand, members of the ethylene signaling pathway (CaETR1-like, CaETR4-like, CaCTR2-like, CaEIN2-like, CaEIN3-like, CaERF1) showed slightly higher expression levels during the initial stages of development (green and yellow-green fruits), except for the ethylene receptors CaETR1-like and CaETR4-like, which were constitutively expressed and induced in cherry fruits, respectively. The higher ethylene production levels in Catucaí 785-15 fruits, indicated by the expression analysis of CaACO1-like and CaACO4-like, suggest that it promotes an enhanced CaETR4-like degradation, leading to an increase in ethylene sensitivity and consequently to an earliness in the ripening process of this cultivar. Ethylene production in Acauã fruits may not be sufficient to inactivate the CaETR4-like levels and thus ripening changes occur in a slower pace. Thus, the expression analysis of the ethylene biosynthesis and signaling genes suggests that ethylene is directly involved in the determination of the ripening time of coffee fruits, and CaACO1-like, CaACO4-like and CaETR4-like may display essential roles during coffee fruit ripening.

To control and to be controlled: understanding the Arabidopsis SLIM1 function in sulfur deficiency through comprehensive investigation of the EIL protein family

Sulfur limitation 1 (SLIM1), a member of the EIN3-like (EIL) family of transcription factors in Arabidopsis, is the regulator of many sulfur deficiency responsive genes. Among the five other proteins of the family, three regulate ethylene (ET) responses and two have unassigned functions. Contrary to the well-defined ET signaling, the pathway leading from sensing sulfate status to the activation of its acquisition via SLIM1 is completely unknown. SLIM1 binds to the 20 nt-long specific UPE-box sequence; however, it also recognizes the shorter TEIL sequence, unique for the whole EIL family. SLIM1 takes part in the upregulation and downregulation of various sulfur metabolism genes, but also it controls the degradation of glucosinolates under sulfur deficient conditions. Besides facilitating the increased flux through the sulfate assimilation pathway, SLIM1 induces microRNA395, specifically targeting ATP sulfurylases and a low-affinity sulfate transporter, SULTR2;1, thus affecting sulfate translocation to the shoot. Here, we briefly review the identification, structural characteristics, and molecular function of SLIM1 from the perspective of the whole EIL protein family.

Banana ethylene response factors are involved in fruit ripening through their interactions with ethylene biosynthesis genes

The involvement of ethylene response factor (ERF) transcription factor (TF) in the transcriptional regulation of ethylene biosynthesis genes during fruit ripening remains largely unclear. In this study, 15 ERF genes, designated as MaERF1-MaERF15, were isolated and characterized from banana fruit. These MaERFs were classified into seven of the 12 known ERF families. Subcellular localization showed that MaERF proteins of five different subfamilies preferentially localized to the nucleus. The 15 MaERF genes displayed differential expression patterns and levels in peel and pulp of banana fruit, in association with four different ripening treatments caused by natural, ethylene-induced, 1-methylcyclopropene (1-MCP)-delayed, and combined 1-MCP and ethylene treatments. MaERF9 was upregulated while MaERF11 was downregulated in peel and pulp of banana fruit during ripening or after treatment with ethylene. Furthermore, yeast-one hybrid (Y1H) and transient expression assays showed that the potential repressor MaERF11 bound to MaACS1 and MaACO1 promoters to suppress their activities and that MaERF9 activated MaACO1 promoter activity. Interestingly, protein-protein interaction analysis revealed that MaERF9 and -11 physically interacted with MaACO1. Taken together, these results suggest that MaERFs are involved in banana fruit ripening via transcriptional regulation of or interaction with ethylene biosynthesis genes.

Expression patterns of ethylene biosynthesis genes from bananas during fruit ripening and in relationship with finger drop

BACKGROUND AND AIMS

Banana finger drop is defined as dislodgement of individual fruits from the hand at the pedicel rupture area. For some banana varieties, this is a major feature of the ripening process, in addition to ethylene production and sugar metabolism. The few studies devoted to assessing the physiological and molecular basis of this process revealed (i) the similarity between this process and softening, (ii) the early onset of related molecular events, between the first and fourth day after ripening induction, and (iii) the putative involvement of ethylene as a regulatory factor. This study was conducted with the aim of identifying, through a candidate gene approach, a quality-related marker that could be used as a tool in breeding programmes. Here we examined the relationship between ripening ethylene biosynthesis (EB) and finger drop in order to gain further insight into the upstream regulatory steps of the banana finger drop process and to identify putative related candidate genes.

METHODS

Postharvest ripening of green banana fruit was induced by acetylene treatment and fruit taken at 1-4 days after ripening induction, and total RNA extracted from the median area [control zone (CZ)] and the pedicel rupture area [drop zone (DZ)] of peel tissue. Then the expression patterns of EB genes (MaACO1, MaACO2, MaACS1, MaACS2, MaACS3 and MaACS4) were comparatively examined in CZ and DZ via real-time quantitative polymerase chain reaction.

PRINCIPAL RESULTS

Differential expression of EB gene was observed in CZ and DZ during the postharvest period examined in this study. MaACO1, MaACS2 and MaACS1 were more highly induced in DZ than in the control, while a slight induction of the MaACS4 gene was observed. No marked differences between the two zones were observed for the MaACO2 gene.

CONCLUSIONS

The finger drop process enhanced EB gene expression including developmental- and ripening-induced genes (MaACO1), specific ripening-induced genes (MaACS1) and wound-induced genes (MaACS2). Thus, this process might be associated with a specific ethylene production in DZ of the pedicel area and the result of crosstalk between developmental, ripening and wound regulatory pathways. MaACO1, MaACS1, MaACS2, and to a lesser extent MaACS4 genes, which are more highly induced in DZ than in CZ, could be considered as putative candidates of the finger drop process.

Molecular characterization of banana NAC transcription factors and their interactions with ethylene signalling component EIL during fruit ripening

The plant-specific NAC (NAM, ATAF1/2, and CUC2) transcription factors (TFs) play important roles in plant growth, development, and stress responses. However, the precise role of NAC TFs in relation to fruit ripening is poorly understood. In this study, six NAC genes, designated MaNAC1-MaNAC6, were isolated and characterized from banana fruit. Subcellular localization showed that MaNAC1-MaNAC5 proteins localized preferentially to the nucleus, while MaNAC6 was distributed throughout the entire cell. A transactivation assay in yeast demonstrated that MaNAC4 and MaNAC6, as well as their C-terminal regions, possessed trans-activation activity. Gene expression profiles in fruit with four different ripening characteristics, including natural, ethylene-induced, 1-methylcyclopropene (1-MCP)-delayed, and a combination of 1-MCP with ethylene treatment, revealed that the MaNAC genes were differentially expressed in peel and pulp during post-harvest ripening. MaNAC1 and MaNAC2 were apparently upregulated by ethylene in peel and pulp, consistent with the increase in ethylene production. In contrast, MaNAC3 in peel and pulp and MaNAC5 in peel were constitutively expressed, and transcripts of MaNAC4 in peel and pulp and MaNAC6 in peel decreased, while MaNAC5 or MaNAC6 in pulp increased slightly during fruit ripening. Furthermore, the MaNAC2 promoter was activated after ethylene application, further enhancing the involvement of MaNAC2 in fruit ripening. More importantly, yeast two-hybrid and bimolecular fluorescence complementation analyses confirmed that MaNAC1/2 physically interacted with a downstream component of ethylene signalling, ethylene insensitive 3 (EIN3)-like protein, termed MaEIL5, which was downregulated during ripening. Taken together, these results suggest that MaNACs such as MaNAC1/MaNAC2, may be involved in banana fruit ripening via interaction with ethylene signalling components.

Expression profiles of a MhCTR1 gene in relation to banana fruit ripening

The banana (Musa spp.) is a typical climacteric fruit of high economic importance. The development of bananas from maturing to ripening is characterized by increased ethylene production accompanied by a respiration burst. To elucidate the signal transduction pathway involved in the ethylene regulation of banana ripening, a gene homologous to Arabidopsis CTR1 (constitutive triple response 1) was isolated from Musa spp. (Hsien Jin Chiao, AAA group) and designated as MhCTR1. MhCTR1 spans 11.5 kilobases and consists of 15 exons and 14 introns with consensus GT-AG nucleotides situated at their boundaries. MhCTR1 encodes a polypeptide of 805 amino acid residues with a calculated molecular weight of 88.6 kDa. The deduced amino acid sequence of MhCTR1 demonstrates 55%, 56% and 55% homology to AtCTR1, RhCTR1, and LeCTR1, respectively. MhCTR1 is expressed mostly in the mature green pulp and root organs. During fruit development MhCTR1 expression increases just before ethylene production rises. Moreover, MhCTR1 expression was detected mainly in the pulps at ripening stage 3, and correlated with the onset of peel yellowing, while MhCTR1 was constitutively expressed in the peels. MhCTR1 expression could be induced by ethylene treatment (0.01 μL L(-1)), and MhCTR1 expression decreased in both peel and pulp 24 h after treatment. Overall, changes observed in MhCTR1 expression in the pulp closely related to the regulation of the banana ripening process.

Identification and expression analysis of four 14-3-3 genes during fruit ripening in banana (Musa acuminata L. AAA group, cv. Brazilian)

To investigate the regulation of 14-3-3 proteins in banana (Musa acuminata L. AAA group, cv. Brazilian) fruit postharvest ripening, four cDNAs encoding 14-3-3 proteins were isolated from banana and designated as Ma-14-3-3a, Ma-14-3-3c, Ma-14-3-3e, and Ma-14-3-3i, respectively. Amino acid sequence alignment showed that the four 14-3-3 proteins shared a highly conserved core structure and variable C-terminal as well as N-terminal regions with 14-3-3 proteins from other plant species. Phylogenetic analysis revealed that the four 14-3-3 genes belong to the non-ε groups. They were differentially and specifically expressed in various tissues. Real-time RT-PCR analysis indicated that these four genes function differentially during banana fruit postharvest ripening. Three genes, Ma-14-3-3a, Ma-14-3-3c, and Ma-14-3-3e, were significantly induced by exogenous ethylene treatment. However, gene function differed in naturally ripened fruits. Ethylene could induce Ma-14-3-3c expression during postharvest ripening, but expression patterns of Ma-14-3-3a and Ma-14-3-3e suggest that these two genes appear to be involved in regulating ethylene biosynthesis during fruit ripening. No obvious relationship emerged between Ma-14-3-3i expression in naturally ripened and 1-MCP (1-methylcyclopropene)-treated fruit groups during fruit ripening. These results indicate that the 14-3-3 proteins might be involved in various regulatory processes of banana fruit ripening. Further studies will mainly focus on revealing the detailed biological mechanisms of these four 14-3-3 genes in regulating banana fruit postharvest ripening.

cDNA cloning and functional characterization of ETHYLENE INSENSITIVE 3 orthologs from Oncidium Gower Ramsey involved in flower cutting and pollinia cap dislodgement

The cDNAs encoding ETHYLENE INSENSITIVE3 (EIN3) transcription factor, OgEIL1 and OgEIL2 of Oncidium were cloned, sequenced and characterized. The deduced amino acid sequences of OgEIL1 and OgEIL2 of identified cDNA clones contain all structural features found in the Arabidopsis EIN3, such as an amino terminal acidic domain, a proline-rich region, and five basic conserved domains. Complementation test for OgEIL1 in Arabidopsis ein3 mutant indicate that function of OgEIL1 is the same as Arabidopsis EIN3. RNA gel blot analysis indicated that OgEIL1 and OgEIL2 expressed differentially in the roots, stem, leaves and flower buds of Oncidium. OgEIL1 and OgEIL2 mRNA levels in fully opened flowers increased as time progressed after cutting and reached a maximum in the fifth day and decreased on seventh day, which is consistent with the hypothesis that flowers initiated to wilt when ethylene raised abruptly. In de-capped flowers, OgEIL2 mRNA showed a decrease, while OgEIL1 mRNA exhibited an increase. Exogenous application of ethylene increased the mRNA levels of OgEIL1 and OgEIL2 in flower buds and flowers after cutting compared prior to ethylene treatment, however, in pollinia de-capped flowers, both OgEIL1 and OgEIL2 mRNA levels responded to a decline to exogenous ethylene immediately after treatment. Collectively, it is suggested that the main functions of OgEIL1 and OgEIL2 are to modulate the senescence of Oncidium flowers.

Polyamine-induced modulation of genes involved in ethylene biosynthesis and signalling pathways and nitric oxide production during olive mature fruit abscission

After fruit ripening, many fruit-tree species undergo massive natural fruit abscission. Olive (Olea europaea L.) is a stone-fruit with cultivars such as Picual (PIC) and Arbequina (ARB) which differ in mature fruit abscission potential. Ethylene (ET) is associated with abscission, but its role during mature fruit abscission remains largely uncharacterized. The present study investigates the possible roles of ET and polyamine (PA) during mature fruit abscission by modulating genes involved in the ET signalling and biosynthesis pathways in the abscission zone (AZ) of both cultivars. Five ET-related genes (OeACS2, OeACO2, OeCTR1, OeERS1, and OeEIL2) were isolated in the AZ and adjacent cells (AZ-AC), and their expression in various olive organs and during mature fruit abscission, in relation to interactions between ET and PA and the expression induction of these genes, was determined. OeACS2, OeACO2, and OeEIL2 were found to be the only genes that were up-regulated in association with mature fruit abscission. Using the inhibition of ET and PA biosynthesis, it is demonstrated that OeACS2 and OeEIL2 expression are under the negative control of PA while ET induces their expression in AZ-AC. Furthermore, mature fruit abscission depressed nitric oxide (NO) production present mainly in the epidermal cells and xylem of the AZ. Also, NO production was differentially responsive to ET, PA, and different inhibitors. Taken together, the results indicate that PA-dependent ET signalling and biosynthesis pathways participate, at least partially, during mature fruit abscission, and that endogenous NO and 1-aminocyclopropane-1-carboxylic acid maintain an inverse correlation, suggesting an antagonistic action of NO and ET in abscission signalling.

Validation of reference genes for RT-qPCR studies of gene expression in banana fruit under different experimental conditions

Reverse transcription quantitative real-time PCR (RT-qPCR) is a sensitive technique for quantifying gene expression, but its success depends on the stability of the reference gene(s) used for data normalization. Only a few studies on validation of reference genes have been conducted in fruit trees and none in banana yet. In the present work, 20 candidate reference genes were selected, and their expression stability in 144 banana samples were evaluated and analyzed using two algorithms, geNorm and NormFinder. The samples consisted of eight sample sets collected under different experimental conditions, including various tissues, developmental stages, postharvest ripening, stresses (chilling, high temperature, and pathogen), and hormone treatments. Our results showed that different suitable reference gene(s) or combination of reference genes for normalization should be selected depending on the experimental conditions. The RPS2 and UBQ2 genes were validated as the most suitable reference genes across all tested samples. More importantly, our data further showed that the widely used reference genes, ACT and GAPDH, were not the most suitable reference genes in many banana sample sets. In addition, the expression of MaEBF1, a gene of interest that plays an important role in regulating fruit ripening, under different experimental conditions was used to further confirm the validated reference genes. Taken together, our results provide guidelines for reference gene(s) selection under different experimental conditions and a foundation for more accurate and widespread use of RT-qPCR in banana.

Expression of genes associated with ethylene-signalling pathway in harvested banana fruit in response to temperature and 1-MCP treatment

BACKGROUND

Little attention has been paid to characterising the ethylene-signalling pathway genes in relation to abnormal ripening of harvested banana fruit during storage at high temperature. The aim of the present study was to investigate banana fruit abnormal ripening and the expression of ten genes associated with the ethylene-signalling pathway, namely MaACS1, MaACO1, MaERS1-4 and MaEIL1-4, at high temperature. Changes in these parameters of banana fruit at high temperature in response to 1-MCP pretreatment were also investigated.

RESULTS

High temperature accelerated the decline in fruit firmness, increased ethylene production and inhibited degreening in banana fruit, resulting in fruit abnormal ripening. In addition, the expression of MaACS1, MaACO1, MaERS2, MaERS3, MaERS4, MaEIL1, MaEIL3 and MaEIL4 was enhanced in banana fruit stored at high temperature. However, application of 1-MCP prior to high temperature storage delayed fruit abnormal ripening and simultaneously suppressed the expression of MaACS1, MaERS2, MaERS3, MaEIL1, MaEIL3 and MaEIL4.

CONCLUSION

The findings of this study suggested that the expression of genes associated with the ethylene-signalling pathway might be involved in banana fruit abnormal ripening at high temperature. Application of 1-MCP suppressed the expression of genes associated with the ethylene-signalling pathway, which may be attributed at least partially to 1-MCP delaying fruit abnormal ripening at high temperature.

Mechanisms of haplotype divergence at the RGA08 nucleotide-binding leucine-rich repeat gene locus in wild banana (Musa balbisiana)

BACKGROUND

Comparative sequence analysis of complex loci such as resistance gene analog clusters allows estimating the degree of sequence conservation and mechanisms of divergence at the intraspecies level. In banana (Musa sp.), two diploid wild species Musa acuminata (A genome) and Musa balbisiana (B genome) contribute to the polyploid genome of many cultivars. The M. balbisiana species is associated with vigour and tolerance to pests and disease and little is known on the genome structure and haplotype diversity within this species. Here, we compare two genomic sequences of 253 and 223 kb corresponding to two haplotypes of the RGA08 resistance gene analog locus in M. balbisiana "Pisang Klutuk Wulung" (PKW).

RESULTS

Sequence comparison revealed two regions of contrasting features. The first is a highly colinear gene-rich region where the two haplotypes diverge only by single nucleotide polymorphisms and two repetitive element insertions. The second corresponds to a large cluster of RGA08 genes, with 13 and 18 predicted RGA genes and pseudogenes spread over 131 and 152 kb respectively on each haplotype. The RGA08 cluster is enriched in repetitive element insertions, in duplicated non-coding intergenic sequences including low complexity regions and shows structural variations between haplotypes. Although some allelic relationships are retained, a large diversity of RGA08 genes occurs in this single M. balbisiana genotype, with several RGA08 paralogs specific to each haplotype. The RGA08 gene family has evolved by mechanisms of unequal recombination, intragenic sequence exchange and diversifying selection. An unequal recombination event taking place between duplicated non-coding intergenic sequences resulted in a different RGA08 gene content between haplotypes pointing out the role of such duplicated regions in the evolution of RGA clusters. Based on the synonymous substitution rate in coding sequences, we estimated a 1 million year divergence time for these M. balbisiana haplotypes.

CONCLUSIONS

A large RGA08 gene cluster identified in wild banana corresponds to a highly variable genomic region between haplotypes surrounded by conserved flanking regions. High level of sequence identity (70 to 99%) of the genic and intergenic regions suggests a recent and rapid evolution of this cluster in M. balbisiana.

Kiwifruit EIL and ERF genes involved in regulating fruit ripening

Kiwifruit (Actinidia deliciosa) is a climacteric fruit sensitive to low concentrations of ethylene. To investigate the transcriptional mechanisms underlying kiwifruit ethylene response, transcription factors encoding four EIN3-Like (EILs) and 14 Ethylene Response Factors (ERFs) were cloned from kiwifruit. Expression of these transcription factors was examined during fruit development. The expression of transcripts of most AdERFs was higher during early fruit development, with the exception of AdERF3, which increased with maturity. Several AdERFs were apparently down-regulated by ethylene, as they were affected by the ethylene inhibitor 1-methylcyclopropene and by antisense suppression of ACO (for 1-aminocyclopropane-1-carboxylic acid oxidase) in the fruit. In contrast, AdEILs were constitutively expressed during fruit development and ripening. The transcription factors AdEIL2 and AdEIL3 activated transcription of the ripening-related genes AdACO1 and AdXET5 (xyloglucan endotransglycosylase gene) and, when overexpressed in Arabidopsis (Arabidopsis thaliana), stimulated ethylene production. The potential repressor AdERF9 suppressed this promoter activity. These results support a role for kiwifruit EILs and ERFs in transcriptional regulation of ripening-related genes and in the regulation of kiwifruit fruit-ripening processes.

The role of ethylene and cold temperature in the regulation of the apple POLYGALACTURONASE1 gene and fruit softening

Fruit softening in apple (Malus x domestica) is associated with an increase in the ripening hormone ethylene. Here, we show that in cv Royal Gala apples that have the ethylene biosynthetic gene ACC OXIDASE1 suppressed, a cold treatment preconditions the apples to soften independently of added ethylene. When a cold treatment is followed by an ethylene treatment, a more rapid softening occurs than in apples that have not had a cold treatment. Apple fruit softening has been associated with the increase in the expression of cell wall hydrolase genes. One such gene, POLYGALACTURONASE1 (PG1), increases in expression both with ethylene and following a cold treatment. Transcriptional regulation of PG1 through the ethylene pathway is likely to be through an ETHYLENE-INSENSITIVE3-like transcription factor, which increases in expression during apple fruit development and transactivates the PG1 promoter in transient assays in the presence of ethylene. A cold-related gene that resembles a COLD BINDING FACTOR (CBF) class of gene also transactivates the PG1 promoter. The transactivation by the CBF-like gene is greatly enhanced by the addition of exogenous ethylene. These observations give a possible molecular mechanism for the cold- and ethylene-regulated control of fruit softening and suggest that either these two pathways act independently and synergistically with each other or cold enhances the ethylene response such that background levels of ethylene in the ethylene-suppressed apples is sufficient to induce fruit softening in apples.

The regulation of MADS-box gene expression during ripening of banana and their regulatory interaction with ethylene

Six MaMADS-box genes have been cloned from the banana fruit cultivar Grand Nain. The similarity of these genes to tomato LeRIN is low and neither MaMADS2 nor MaMADS1 complement the tomato rin mutation. Nevertheless, the expression patterns, specifically in fruit and the induction during ripening and in response to ethylene and 1-MCP, suggest that some of these genes may participate in ripening. MaMADS1, 2, and 3, are highly expressed in fruit only, while the others are expressed in fruit as well as in other organs. Moreover, the suites of MaMADS-box genes and their temporal expression differ in peel and pulp during ripening. In the pulp, the increase in MaMADS2, 3, 4, and 5 expression preceded an increase in ethylene production, but coincides with the CO(2) peak. However, MaMADS1 expression in pulp coincided with ethylene production, but a massive increase in its expression occurred late during ripening, together with a second wave in the expression of MaMADS2, 3, and 4. In the peel, on the other hand, an increase in expression of MaMADS1, 3, and to a lesser degree also of MaMADS4 and 2 coincided with an increase in ethylene production. Except MaMADS3, which was induced by ethylene in pulp and peel, only MaMADS4, and 5 in pulp and MaMADS1 in peel were induced by ethylene. 1-MCP applied at the onset of the increase in ethylene production, increased the levels of MaMADS4 and MaMADS1 in pulp, while it decreased MaMADS1, 3, 4, and 5 in peel, suggesting that MaMADS4 and MaMADS1 are negatively controlled by ethylene at the onset of ethylene production only in pulp. Only MaMADS2 is neither induced by ethylene nor by 1-MCP, and it is expressed mainly in pulp. Our results suggest that two independent ripening programs are employed in pulp and peel which involve the activation of mainly MaMADS2, 4, and 5 and later on also MaMADS1 in pulp, and mainly MaMADS1, and 3 in peel. Hence, our results are consistent with MaMADS2, a SEP3 homologue, acting in the pulp upstream of the increase in ethylene production similarly to LeMADS-RIN.

Nicotiana tabacum EIL2 directly regulates expression of at least one tobacco gene induced by sulphur starvation

Sulphur deficiency severely affects plant growth and their agricultural productivity leading to diverse changes in development and metabolisms. Molecular mechanisms regulating gene expression under low sulphur conditions remain largely unknown. AtSLIM1, a member of the EIN3-like (EIL) family was reported to be a central transcriptional regulator of the plant sulphur response, however, no direct interaction of this protein with any sulphur-responsive promoters was demonstrated. The focus of this study was on the analysis of a promoter region of UP9C, a tobacco gene strongly induced by sulphur limitation. Cloning and subsequent examination of this promoter resulted in the identification of a 20-nt sequence (UPE-box), also present in the promoters of several Arabidopsis genes, including three out of four homologues of UP9C. The UPE-box, consisting of two parallel tebs sequences (TEIL binding site), proved to be necessary to bind the transcription factors belonging to the EIL family and of a 5-nt conserved sequence at the 3'-end. The yeast one-hybrid analysis resulted in the identification of one transcription factor (NtEIL2) capable of binding to the UPE-box. The interactions of NtEIL2, and its homologue from Arabidopsis, AtSLIM1, with DNA were affected by mutations within the UPE-box. Transient expression assays in Nicotiana benthamiana have further shown that both factors, NtEIL2 and AtSLIM1, activate the UP9C promoter. Interestingly, activation by NtEIL2, but not by AtSLIM1, was dependent on the sulphur-deficiency of the plants.

Ethylene signal transduction elements involved in chilling injury in non-climacteric loquat fruit

Loquat (Eriobotrya japonica Lindl.) is a subtropical fruit, with some cultivars such as 'Luoyangqing' (LYQ) susceptible to chilling injury (CI), while others such as 'Baisha' (BS) are resistant. Although loquats are non-climacteric, modulation of ethylene has an effect on ripening-related post-harvest CI. Therefore the role of ethylene signalling in the development of CI was investigated in fruit of both the LYQ and BS cultivars. Three ethylene receptor genes, one CTR1-like gene, and one EIN3-like gene were isolated and characterized in ripening fruit. All of these genes were expressed differentially within and between fruit of the two cultivars. Transcripts either declined over fruit development (EjERS1a in both cultivars and EjEIL1 in LYQ) or showed an increase in the middle stages of fruit development before declining (EjETR1, EjERS1b, and EjCTR1 in both cultivars and EjEIL1 in BS). The main cultivar differences were in levels rather than in patterns of expression during post-harvest storage. EjETR1, EjCTR1, and EjEIL1 genes showed increased expression in response to low temperature and this was particularly notable for EjETR1, and EjEIL1 during CI development in LYQ fruit. The genes were also differentially responsive to ethylene treatment, 1-methycyclopropene (1-MCP) and low temperature conditioning, confirming a role for ethylene in regulation of CI in loquat fruit.

Involvement of two rice ETHYLENE INSENSITIVE3-LIKE genes in wound signaling

Ethylene and jasmonic acid (JA) have been proposed as key compounds for wound signaling in plants. In Arabidopsis, ETHYLENE INSENSITIVE3 (EIN3), which is an essential transcription factor for ethylene signaling, is regulated at the post-transcriptional level, while transcriptional regulation of EIN3 or EIN3-LIKE (EIL) genes has not been well documented. The expression of 6 rice EIL genes (OsEIL1-6) was analyzed and only OsEIL1 and 2 were found to be wound-inducible EIL. OsEIL2 was also induced by JA. Electrophoretic mobility shift assays showed that recombinant OsEIL1 and 2 proteins bound to specific DNA sequences that are recognized by a wound-inducible tobacco EIL. Accumulation of OsEIL1 and 2 transcripts reached a maximum at 1 and 0.5 h after wounding, respectively, and the corresponding DNA-binding activity in nuclear extracts of rice leaves was increased at 1 h after wounding. Candidates for OsEIL-target genes were selected by microarray analysis of wounded rice and by promoter sequence analyses of wound-inducible genes identified by microarray analysis. In OsEIL1- and/or 2-suppressed rice plants, the expression of at least four of 18 candidate genes analyzed was down-regulated. These results indicate the importance of inducible OsEILs in wound signaling in rice.

Expression patterns of cell wall-modifying genes from banana during fruit ripening and in relationship with finger drop

Few molecular studies have been devoted to the finger drop process that occurs during banana fruit ripening. Recent studies revealed the involvement of changes in the properties of cell wall polysaccharides in the pedicel rupture area. In this study, the expression of cell-wall modifying genes was monitored in peel tissue during post-harvest ripening of Cavendish banana fruit, at median area (control zone) and compared with that in the pedicel rupture area (drop zone). To this end, three pectin methylesterase (PME) and seven xyloglucan endotransglycosylase/hydrolase (XTH) genes were isolated. The accumulation of their mRNAs and those of polygalaturonase, expansin, and pectate lyase genes already isolated from banana were examined. During post-harvest ripening, transcripts of all genes were detected in both zones, but accumulated differentially. MaPME1, MaPG1, and MaXTH4 mRNA levels did not change in either zone. Levels of MaPME3 and MaPG3 mRNAs increased greatly only in the control zone and at the late ripening stages. For other genes, the main molecular changes occurred 1-4 d after ripening induction. MaPME2, MaPEL1, MaPEL2, MaPG4, MaXTH6, MaXTH8, MaXTH9, MaEXP1, MaEXP4, and MaEXP5 accumulated highly in the drop zone, contrary to MaXTH3 and MaXTH5, and MaEXP2 throughout ripening. For MaPG2, MaXET1, and MaXET2 genes, high accumulation in the drop zone was transient. The transcriptional data obtained from all genes examined suggested that finger drop and peel softening involved similar mechanisms. These findings also led to the proposal of a sequence of molecular events leading to finger drop and to suggest some candidates.

Differential transcriptional regulation of banana sucrose phosphate synthase gene in response to ethylene, auxin, wounding, low temperature and different photoperiods during fruit ripening and functional analysis of banana SPS gene promoter

Sucrose phosphate synthase (SPS) (EC 2.3.1.14) is the key regulatory component in sucrose formation in banana (Musa acuminata subgroup Cavendish, cv Giant governor) fruit during ripening. This report illustrates differential transcriptional responses of banana SPS gene following ethylene, auxin, wounding, low temperature and different photoperiods during ripening in banana fruit. Whereas ethylene strongly stimulated SPS transcript accumulation, auxin and cold treatment only marginally increased the abundance of SPS mRNA level, while wounding negatively regulated SPS gene expression. Conversely, SPS transcript level was distinctly increased by constant exposure to white light. Protein level, enzymatic activity of SPS and sucrose synthesis were substantially increased by ethylene and increased exposure to white light conditions as compared to other treatments. To further study the transcriptional regulation of SPS in banana fruit, the promoter region of SPS gene was cloned and some cis-acting regulatory elements such as a reverse GCC-box ERE, two ARE motifs (TGTCTC), one LTRE (CCGAA), a GAGA-box (GAGA...) and a GATA-box LRE (GATAAG) were identified along with the TATA and CAAT-box. DNA-protein interaction studies using these cis-elements indicated a highly specific cis-trans interaction in the banana nuclear extract. Furthermore, we specifically studied the light responsive characteristics of GATA-box containing synthetic as well as native banana SPS promoter. Transient expression assays using banana SPS promoter have also indicated the functional importance of the SPS promoter in regulating gene expression. Together, these results provide insights into the transcriptional regulation of banana SPS gene in response to phytohormones and other environmental factors during fruit ripening.