Banana Ripening: Implications of Changes in Internal Ethylene and CO(2) Concentrations, Pulp Fructose 2,6-Bisphosphate Concentration, and Activity of Some Glycolytic Enzymes

Atypical Climacteric and Functional Ethylene Metabolism and Signaling During Fruit Ripening in Blueberry (Vaccinium sp.)

Climacteric fruits display an increase in respiration and ethylene production during the onset of ripening, while such changes are minimal in non-climacteric fruits. Ethylene is a primary regulator of ripening in climacteric fruits. The ripening behavior and role of ethylene in blueberry (Vaccinium sp.) ripening is controversial. This work aimed to clarify the fruit ripening behavior and the associated role of ethylene in blueberry. Southern highbush (Vaccinium corymbosum hybrids) and rabbiteye (Vaccinium ashei) blueberry displayed an increase in the rate of respiration and ethylene evolution, both reaching a maxima around the Pink and Ripe stages of fruit development, consistent with climacteric fruit ripening behavior. Increase in ethylene evolution was associated with increases in transcript abundance of its biosynthesis genes, AMINOCYCLOPROPANE CARBOXYLATE (ACC) SYNTHASE1 (ACS1) and ACC OXIDASE2 (ACO2), implicating them in developmental ethylene production during ripening. Blueberry fruit did not display autocatalytic system 2 ethylene during ripening as ACS transcript abundance and ACC concentration were not enhanced upon treatment with an ethylene-releasing compound (ethephon). However, ACO transcript abundance was enhanced in response to ethephon, suggesting that ACO was not rate-limiting. Transcript abundance of multiple genes associated with ethylene signal transduction was upregulated concomitant with developmental increase in ethylene evolution, and in response to exogenous ethylene. As these changes require ethylene signal transduction, fruit ripening in blueberry appears to involve functional ethylene signaling. Together, these data indicate that blueberry fruit display atypical climacteric ripening, characterized by a respiratory climacteric, developmentally regulated but non-autocatalytic increase in ethylene evolution, and functional ethylene signaling.

Global Transcriptomic Analysis of Targeted Silencing of Two Paralogous ACC Oxidase Genes in Banana

Among 18 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase homologous genes existing in the banana genome there are two genes, Mh-ACO1 and Mh-ACO2, that participate in banana fruit ripening. To better understand the physiological functions of Mh-ACO1 and Mh-ACO2, two hairpin-type siRNA expression vectors targeting both the Mh-ACO1 and Mh-ACO2 were constructed and incorporated into the banana genome by Agrobacterium-mediated transformation. The generation of Mh-ACO1 and Mh-ACO2 RNAi transgenic banana plants was confirmed by Southern blot analysis. To gain insights into the functional diversity and complexity between Mh-ACO1 and Mh-ACO2, transcriptome sequencing of banana fruits using the Illumina next-generation sequencer was performed. A total of 32,093,976 reads, assembled into 88,031 unigenes for 123,617 transcripts were obtained. Significantly enriched Gene Oncology (GO) terms and the number of differentially expressed genes (DEGs) with GO annotation were ‘catalytic activity’ (1327, 56.4%), ‘heme binding’ (65, 2.76%), ‘tetrapyrrole binding’ (66, 2.81%), and ‘oxidoreductase activity’ (287, 12.21%). Real-time RT-PCR was further performed with mRNAs from both peel and pulp of banana fruits in Mh-ACO1 and Mh-ACO2 RNAi transgenic plants. The results showed that expression levels of genes related to ethylene signaling in ripening banana fruits were strongly influenced by the expression of genes associated with ethylene biosynthesis.

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.

Characterization of differential ripening pattern in association with ethylene biosynthesis in the fruits of five naturally occurring banana cultivars and detection of a GCC-box-specific DNA-binding protein

MA-ACS1 and MA-ACO1 are the two major ripening genes in banana and play crucial role in the regulation of ethylene production during ripening. Here, we report a comparative ripening pattern in five different naturally occurring banana cultivars namely Cavendish (AAA), Rasthali (AAB), Kanthali (AB), Poovan (AAB) and Monthan (ABB), which have distinct genome composition. We found a distinct variation in the climacteric ethylene production and in-vivo ACC oxidase activity level during the ripening stages in the five cultivars. We identified the cDNAs for MA-ACS1 and MA-ACO1 from the five cultivars and studied the transcript accumulation patterns of the two genes, which correlated well with the differential timing in the expression of these two genes during ripening. The GCC-box is one of the ethylene-responsive elements (EREs) found in the promoters of many ethylene-inducible genes. We have identified a GCC-box motif (putative ERE) in the promoters of MA-ACS1 and MA-ACO1 in banana cultivars. DNA-protein interaction studies revealed the presence of a GCC-box-specific DNA-binding activity in the fruit nuclear extract and such DNA-binding activity was enhanced following ethylene treatment. South-Western blotting revealed a 25-kDa nuclear protein that binds specifically to GCC-box DNA in the climacteric banana fruit. Together, these results indicate the probable involvement of the GCC-box motif as the cis-acting ERE in the regulation of MA-ACS1 and MA-ACO1 during ripening in banana fruits via binding of specific ERE-binding protein.

Genomic organization of a diverse ACC synthase gene family in banana and expression characteristics of the gene member involved in ripening of banana fruits

The banana is one of the typical climacteric fruits with great economic importance in agriculture. To understand the basic mechanism underlying banana ripening, gene clones for banana ACC synthase (EC 4.4.1.14), a key regulatory enzyme in the ethylene biosynthetic pathway, were characterized. Genomic clones were analyzed by restriction mapping, and the data in conjunction with sequence comparisons with the previously isolated PCR fragments indicated that at least nine ACC synthase genes (MACS1-9) exist in the banana genome. Southern blot analysis showed they are located in different regions of the banana genome. Three lambda genomic clones (GMACS-1, -9, and -12) were completely sequenced, and gene structures of MACS1 (corresponding to alleles of GMACS-9 and -12) and MACS2 (corresponding to GMACS-1) were elucidated. The coding regions of these three genes were all interrupted by three introns. The size and location of introns are similar to the ACC synthase genes from tomato and Arabidopsis. Northern analysis showed that only MACS1 expressed during fruit ripening and was inducible by exogenous ethylene treatment, which indicates MACS1 is a significant member of the ACC synthase gene family related to ripening in banana fruit. The transcription initiation site of GMACS-12 containing MACS1 was defined. There is a TATTAAT sequence located at position -31 to -25 that qualifies as a TATA box. The delineation of transcription unit in MACS1 will facilitate the promoter studies for this gene and allow its specific functions involved in fruit ripening to be determined.

Molecular cloning and characterization of a novel 1-aminocyclopropane-1-carboxylate oxidase gene involved in ripening of banana fruits

One novel banana fruit ripening related 1-aminocyclopropane-1-carboxylate (ACC) oxidase gene quite different from ACC oxidase genes from other species was cloned. In contrast to other studies, the polypeptide encoded by this gene, named Mh-ACO1, lacks the putative leucine zipper motif which is conserved in all known ACC oxidases including the other previously reported banana ACC oxidase, Mh-ACO2. The locus consists of two nearly identical paralogous ACC oxidase genes arranged in opposite orientation and separated by a 3.1-kb intergenic region. The has only two introns, at positions identical to , which comprises a coding region interrupted by three introns. The predicted amino acid sequence of Mh-ACO1 shares less than 50% identity to those of ACC oxidase from other climacteric fruits, while that of Mh-ACO2 shows more than 65% homology. When expressed in Saccharomyces cerevisiae -encoded protein possessed the enzyme activity for ethylene conversion. The levels of mRNA corresponding to both and increased during fruit ripening and were induced by exogenous ethylene. We conclude that both and contribute to increased ethylene production in fruits and these two genes are differentially expressed in fruits and other organs in banana.

Regulatory phosphorylation of banana fruit phosphoenolpyruvate carboxylase by a copurifying phosphoenolpyruvate carboxylase-kinase

Phosphoenolpyruvate (P-pyruvate) carboxylase from ripened banana fruit was purified to near homogeneity and a final specific activity of 20-23 U/mg protein; P-pyruvate carboxylase-kinase copurified with P-pyruvate carboxylase throughout the purification. Gel filtration FPLC indicated that the two proteins form a tightly associated 425-kDa complex. Both the 103-kDa and 100-kDa subunits of the P-pyruvate carboxylase alpha2beta2 hetetrotetramer were phosphorylated and subsequently dephosphorylated in vitro in a time-dependent manner when the final preparation was incubated with 0.1 mM [gamma-32P]ATP followed by rabbit muscle protein phosphatase type 2A1. Phosphoamino acid and phosphopeptide map analyses indicated that in vitro phosphorylation of both subunits likely occurs at an identical Ser residue. Maximal stoichiometry of 32P incorporation was 0.2 and 0.4 mol/mol 103-kDa and 100-kDa subunit, respectively. The level of 32P incorporation was correlated with the enzyme's activation state when assayed under suboptimal assay conditions (pH 7.3, 75 microM P-pyruvate, 0.2 mM L-malate). The main kinetic effect of phosphorylation was to decrease the enzyme's Km(P-pyruvate), as well as its sensitivity to inhibition by L-malate and L-glutamate. Banana P-pyruvate carboxylase-kinase: (a) also phosphorylated maize leaf P-pyruvate carboxylase, histone III-S, and dephosphorylated casein; (b) demonstrated Mg2+ dependence and Ca2+ independence, (c) exhibited a broad pH activity optimum of pH 8.0-8.5, and (d) was inhibited by L-malate and activated by Ba2+ and Co2+. Time-course kinetic studies suggested that P-pyruvate carboxylase exists mainly in the dephosphorylated form in preclimacteric, climacteric and postclimacteric fruit, but that its kinase is expressed throughout ripening. In situ 32P-labeling indicated that, while both subunits of ripe banana P-pyruvate carboxylase are phosphorylated in vivo, it is primarily the 100-kDa subunit that is radiolabeled. The results suggest that similar to the enzyme from leaves, root nodules and seeds, a fruit P-pyruvate carboxylase may be subject to regulatory seryl phosphorylation by an endogenous P-pyruvate carboxylase-kinase.

Influence of Elevated Fructose-2,6-Bisphosphate Levels on Starch Mobilization in Transgenic Tobacco Leaves in the Dark

The aim of this work was to study the effect of elevated fructose-2,6-bisphosphate (Fru-2,6-bisP) levels on carbohydrate metabolism in leaves in the dark. In transgenic tobacco (Nicotiana tabacum L.) lines containing mammalian 6-phosphofructo-2-kinase activity there is an inverse relationship between the level of Fru-2,6-bisP in leaves and the rate of starch breakdown in the dark. Estimates of the flux response coefficient for the rate of net starch degradation with respect to changes in Fru-2,6-bisP level are -0.57 for whole leaves and -0.69 to -0.89 for excised leaf discs. We suggest that this decrease in the net rate of starch breakdown is caused, at least in part, by stimulation of unidirectional starch synthesis. Measurements of the levels of metabolic intermediates and the metabolism of [U-14C]glucose indicate that the stimulation of starch synthesis in the dark is a result of high Fru-2,6-bisP levels, increasing the 3-phosphoglycerate:inorganic phosphate ratio in leaves. We argue that the observed response to changes in the level of Fru-2,6-bisP are effected through activation of pyrophosphate:fructose-6-phosphate 1-phosphotransferase. However, the extent to which changes in Fru-2,6-bisP influence starch metabolism in wild-type plants is not known.

Glycolysis at the climacteric of bananas

This work was carried out to investigate the relative roles of phosphofructokinase and pyrophosphate-fructose-6-phosphate 1-phosphotransferase during the increased glycolysis at the climacteric in ripening bananas (Musa cavendishii Lamb ex Paxton). Fruit were ripened in the dark in a continuous stream of air in the absence of ethylene. CO2 production, the contents of glucose 6-phosphate, fructose 6-phosphate, fructose 1,6-bisphosphate, phosphoenolpyruvate and PPi; and the maximum catalytic activities of pyrophosphate-fructose-6-phosphate 1-phosphotransferase, 6-phosphofructokinase, pyruvate kinase and phosphoenolpyruvate carboxylase were measured over a 12-day period that included the climacteric. Cytosolic fructose-1,6- bisphosphatase could not be detected in extracts of climacteric fruit. The peak of CO2 production was preceded by a threefold rise in phosphofructokinase, and accompanied by falls in fructose 6-phosphate and glucose 6-phosphate, and a rise in fructose 1,6-bisphosphate. No change in pyrophosphate-fructose-6-phosphate 1-phosphotransferase or pyrophosphate was found. It is argued that phosphofructokinase is primarily responsible for the increased entry of fructose 6-phosphate into glycolysis at the climacteric.

Banana ripening: implications of changes in glycolytic intermediate concentrations, glycolytic and gluconeogenic carbon flux, and fructose 2,6-bisphosphate concentration

In ripening banana (Musa sp. [AAA group, Cavendish subgroup] cv Valery) fruit, the concentration of glycolytic intermediates increased in response to the rapid conversion of starch to sugars and CO(2). Glucose 6-phosphate (G-6-P), fructose 6-phosphate (Fru 6-P), and pyruvate (Pyr) levels changed in synchrony, increasing to a maximum one day past the peak in ethylene synthesis and declining rapidly thereafter. Fructose 1,6-bisphosphate (Fru 1,6-P(2)) and phosphoenolpyruvate (PEP) levels underwent changes dissimilar to those of G 6-P, Fru 6-P, and Pyr, indicating that carbon was regulated at the PEP/Pyr and Fru 6-P/Fru 1,6-P(2) interconversion sites. During the climacteric respiratory rise, gluconeogenic carbon flux increased 50- to 100-fold while glycolytic carbon flux increased only 4- to 5-fold. After the climacteric peak in CO(2) production, gluconeogenic carbon flux dropped dramatically while glycolytic carbon flux remained elevated. The steady-state fructose 2,6-bisphosphate (Fru 2,6-P(2)) concentration decreased to (1/2) that of preclimacteric fruit during the period coinciding with the rapid increase in gluconeogenesis. Fru 2,6-P(2) concentration increased thereafter as glycolytic carbon flux increased relative to gluconeogenic carbon flux. It appears likely that the initial increase in respiration in ripening banana fruit is due to the rapid influx of carbon into the cytosol as starch is degraded. As starch reserves are depleted and the levels of intermediates decline, the continued enhancement of respiration may, in part, be maintained by an increased steady-state Fru 2,6-P(2) concentration acting to promote glycolytic carbon flux at the step responsible for the interconversion of Fru 6-P and Fru 1,6-P(2).

Product inhibition of potato tuber pyrophosphate:fructose-6-phosphate phosphotransferase by phosphate and pyrophosphate

The product inhibition of potato (Solanum tuberosum) tuber pyrophosphate:fructose-6-phosphate phosphotransferase by inorganic pyrophosphate and inorganic phosphate has been studied. The binding of substrates for the forward (glycolytic) and the reverse (gluconeogenic) reaction is random order, and occurs with only weak competition between the substrate pair fructose-6-phosphate and pyrophosphate, and between the substrate pair fructose-1,6-bisphosphate and phosphate. Pyrophosphate is a powerful inhibitor of the reverse reaction, acting competitively to fructose-1,6-biphosphate and noncompetitively to phosphate. At the concentrations needed for catalysis of the reverse reaction, phosphate inhibits the forward reaction in a largely noncompetitive mode with respect to both fructose-6-phosphate and pyrophosphate. At higher concentrations, phosphate inhibits both the forward and the reverse reaction by decreasing the affinity for fructose-2,6-bisphosphate and thus, for the other three substrates. These results allow a model to be proposed, which describes the interactions between the substrates at the catalytic site. They also suggest the enzyme may be regulated in vivo by changes of the relation between metabolites and phosphate and could act as a means of controlling the cytosolic pyrophosphate concentration.

Fructose 2,6-bisphosphate and the climacteric in bananas

This work was done to test the view that there is a marked rise in the content of fructose 2,6-bisphosphate during the climacteric of the fruit of banana (Musa cavendishii Lamb ex. Paxton). Bananas were ripened in the dark in a continuous stream of air in the absence of exogenous ethylene. CO2 production and the contents of fructose 2,6-bisphosphate and sucrose were monitored over a 15-day period. A range of extraction procedures for fructose 2,6-bisphosphate were compared. Recovery of fructose 2,6-bisphosphate added to samples of unripe fruit varied from poor to unmeasurable. Recoveries from samples of ripe fruit were high. It is argued that this differential recovery of fructose 2,6-bisphosphate undermines claims that the amount of this compound increases at the climacteric. When recoveries are taken into account, our data suggest that there is no major change in fructose 2,6-bisphosphate content during the onset of the climacteric in bananas.

Carbohydrate Metabolism and Activity of Pyrophosphate: Fructose-6-Phosphate Phosphotransferase in Photosynthetic Soybean (Glycine max, Merr.) Suspension Cells

Activity of pyrophosphate:fructose-6-phosphate phosphotransferase (PFP) was investigated in relation to carbohydrate metabolism and physiological growth stage in mixotrophic soybean (Glycine max Merr.) suspension cells. In the presence of exogenous sugars, log phase growth occurred and the cells displayed mixotrophic metabolism. During this stage, photosynthetic oxygen evolution was depressed and sugars were assimilated from the medium. Upon depletion of medium sugar, oxygen evolution and chlorophyll content increased, and cells entered stationary phase. Activities of various enzymes of glycolysis and sucrose metabolism, including PFP, sucrose synthase, fructokinase, glucokinase, UDP-glucose pyrophosphorylase, and fructose-1,6-bisphosphatase, changed as the cells went from log to stationary phases of growth. The largest change occurred in the activity of PFP, which was three-fold higher in log phase cells. PFP activity increased in cells grown on media initially containing sucrose, glucose, or fructose and began to decline when sugar in the medium was depleted. Western blots probed with antibody specific to the -subunit of potato PFP revealed a single 56 kilodalton immunoreactive band that changed in intensity during the growth cycle in association with changes in total PFP activity. The level of fructose-2,6-bisphosphate, an activator of the soybean PFP, increased during the first 24 hours after cell transfer and returned to the stationary phase level prior to the increase in PFP activity. Throughout the growth cycle, the calculated in vivo cytosolic concentration of fructose-2,6-bisphosphate exceeded by more than two orders of magnitude the previously reported activation coefficient (K(a)) for soybean PFP. These results indicate that metabolism of exogenously supplied sugars by these cells involves a PFP-dependent step that is not coupled directly to sucrose utilization. Activity of this pathway appears to be controlled by changes in the level of PFP, rather than changes in the total cytosolic level of fructose-2,6-bisphosphate.