On the role of ethylene, auxin and a GOLVEN-like peptide hormone in the regulation of peach ripening

Abstract

Background: In melting flesh peaches, auxin is necessary for system-2 ethylene synthesis and a cross-talk between ethylene and auxin occurs during the ripening process. To elucidate this interaction at the transition from maturation to ripening and the accompanying switch from system-1 to system-2 ethylene biosynthesis, fruits of melting flesh and stony hard genotypes, the latter unable to produce system-2 ethylene because of insufficient amount of auxin at ripening, were treated with auxin, ethylene and with 1-methylcyclopropene (1-MCP), known to block ethylene receptors. The effects of the treatments on the different genotypes were monitored by hormone quantifications and transcription profiling.

Results: In melting flesh fruit, 1-MCP responses differed according to the ripening stage. Unexpectedly, 1-MCP induced genes also up-regulated by ripening, ethylene and auxin, as CTG134, similar to GOLVEN (GLV) peptides, and repressed genes also down-regulated by ripening, ethylene and auxin, as CTG85, a calcineurin B-like protein. The nature and transcriptional response of CTG134 led to discover a rise in free auxin in 1-MCP treated fruit. This increase was supported by the induced transcription of CTG475, an IAA-amino acid hydrolase. A melting flesh and a stony hard genotype, differing for their ability to synthetize auxin and ethylene amounts at ripening, were used to study the fine temporal regulation and auxin responsiveness of genes involved in the process. Transcriptional waves showed a tight interdependence between auxin and ethylene actions with the former possibly enhanced by the GLV CTG134. The expression of genes involved in the regulation of ripening, among which are several transcription factors, was similar in the two genotypes or could be rescued by auxin application in the stony hard. Only GLV CTG134 expression could not be rescued by exogenous auxin.

Conclusions: 1-MCP treatment of peach fruit is ineffective in delaying ripening because it stimulates an increase in free auxin. As a consequence, a burst in ethylene production speeding up ripening occurs. Based on a network of gene transcriptional regulations, a model in which appropriate level of CTG134 peptide hormone might be necessary to allow the correct balance between auxin and ethylene for peach ripening to occur is proposed.

Fig. 1

Flesh firmness (solid lines, filled symbols, left Y axe) and ethylene production (dashed lines, open symbols, right Y axe) during post-harvest of peaches either treated (1-MCP) or not (air) with 1-MCP (1 or 5 μL L⁻¹). The Y scale is the same in the three panels for FF (left), while it differs for ethylene production (right). I_AD was used to group S4 fruit according to their ripening stages: class 0 (pre-climacteric, panel (a)), class 1 (onset of climacteric, panel (b)), and class 2 (climacteric, panel (c)). The arrow at the bottom indicates the end of the 1-MCP treatment in 1-MCP-exposed fruit. Thereafter, fruit were kept in air at 25 °C. Data represent the mean (n = 40) ± S.D

Fig. 2

Relative expression profiles of selected genes in leaf, flower and fruit at different stages of development (S1, S2, S3I, S3II, S4I, and S4II, corresponding to 40, 65, 85, 95, 115 and 120 days after full bloom, respectively; sector A), in fruit at S3II following ethylene (ET) and NAA treatment (sector B) and in preclimacteric S4 fruits belonging to class 0 (cl0) or class 1 (cl1) treated with 1-MCP (sector C). Genes belonging to the ethylene domain (upper group), auxin domain (second group), transcription factors (third group) or with the unexpected transcriptional response following 1-MCP treatment are grouped. Genes belonging to the same family are boxed. Expression values, determined by qRT-PCR, were related to the highest expression of each gene (100 %, blue) within each experiment (a, b carried out with RH samples and c, carried out with SRG fruits; both RH and SRG produce melting flesh fruits). ppa no. indicate the peach gene identifier as described in [30], while CTG name indicate the cDNA identifiers on the microarray μPEACH1.0 as described in [28]. Hormone treatments (ET: ethylene; NAA: 1-naphthalene acetic acid, a synthetic auxin) lasted for 48 h (group B). SRG fruits were collected at commercial maturity date and sampled after 36 h of storage either in air or in 1-MCP (12 h) plus air (i.e. 24 h in air after the end of the 1-MCP treatment; group C)

Fig. 3

Structure of the CTG134 protein. Hydrophobicity plot of the protein sequence predicted from CTG134 (ppa012311m) and amino acid alignment of the C-ter with the corresponding part of some Arabidopsis RGFs/GLVs. The mature peptide hormone (dark grey) is released from the mature protein (light grey) after delivery in the cell wall (a signal sequence, SS, directs the protein to the secretion pathway)

Fig. 4

Auxin, ethylene (ET) and ABA levels during fruit ripening (panel (a)) and following 1-MCP treatment (IAA in panel (b), ABA in panel (c)). SRG peaches were sampled after 36 h of storage either in air or in 1-MCP (12 h) plus air (i.e. 24 h in air after the end of the 1-MCP treatment). Bars are the standard deviations from the means of three or more replicates. Letters above columns indicate significant differences with a Tuckey HSD test at p <0.05

Fig. 5

Absolute expression profiles of selected genes at the transition from maturation to ripening in melting flesh (Redhaven, RH) and stony-hard (194RXXIII43, RXX) genotypes. Gene groups and colors are as for Fig. 2 but for the last column (Max Val). As quantification was carried out with a standard, comparison of the relative abundance among members of the same gene family has been added (from white to red from the lowest to the highest, marked with an asterisk)

Fig. 6

Effect of auxin treatment on the ability to produce ethylene in fruit at different ripening stages in melting flesh (Redhaven, RH, from class −2 to class 2, (a)–(c), respectively) and stony-hard (194RXXIII43, RXX, class 1, (d) genotypes. c: control, fruits treated with a mock solution; NAA: fruits treated with a solution containing NAA (1-naphthalene acetic acid, a synthetic auxin). Bars are the standard deviations from the means of three or more replicates

Fig. 7

Relative expression profiles of selected genes following auxin treatment in fruit at different ripening stages in melting flesh (Redhaven, RH) and stony-hard (194RXXIII43, RXX) genotypes. Gene groups and colors are as for Fig. 2