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Timing of ripening initiation in grape berries and its relationship to seed content and pericarp auxin levels.

Gouthu S, Deluc LG - BMC Plant Biol. (2015)

Bottom Line: In the prevéraison cluster, the expression of auxin-response factor genes was significantly higher in the pericarp of high-SB berries and remained higher until véraison compared to low-SB berries.This results in higher auxin-signaling activity that lasts longer in the pericarp of high-SB berries.In contrast, in low-SB berries, concomitant with an earlier decrease of auxin level, the features of ripening initiation, such as increases in ABA and sugar accumulation begin earlier.

View Article: PubMed Central - PubMed

ABSTRACT

Background: Individual berries in a grape (Vitis vinifera L.) cluster enter the ripening phase at different times leading to an asynchronous cluster in terms of ripening. The factors causing this variable ripening initiation among berries are not known. Because the influence via hormonal communication of the seed on fruit set and growth is well known across fruit species, differences in berry seed content and resultant quantitative or qualitative differences in the hormone signals to the pericarp likely influence the relative timing of ripening initiation among berries of the cluster.

Results: At the time of the initiation of cluster ripening (véraison), underripe green berries have higher seed content compared to the riper berries and there is a negative correlation between the seed weight-to-berry weight ratio (SB) and the sugar level in berries of a cluster. Auxin levels in seeds relative to the pericarp tissues are two to 12 times higher at pre-ripening stages. The pericarp of berries with high-SB had higher auxin and lower abscisic acid (ABA) levels compared to those with low-SB from two weeks before véraison. In the prevéraison cluster, the expression of auxin-response factor genes was significantly higher in the pericarp of high-SB berries and remained higher until véraison compared to low-SB berries. The expression level of auxin-biosynthetic genes in the pericarp was the same between both berry groups based upon similar expression activity of YUC genes that are rate-limiting factors in auxin biosynthesis. On the other hand, in low-SB berries, the expression of ABA-biosynthetic and ABA-inducible NCED and MYB genes was higher even two weeks before véraison.

Conclusions: Differences in the relative seed content among berries plays a major role in the timing of ripening initiation. Towards the end of berry maturation phase, low and high levels of auxin are observed in the pericarp of low- and high-SB berries, respectively. This results in higher auxin-signaling activity that lasts longer in the pericarp of high-SB berries. In contrast, in low-SB berries, concomitant with an earlier decrease of auxin level, the features of ripening initiation, such as increases in ABA and sugar accumulation begin earlier.

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Screening for Auxin response genes induced in grape cultured cells and in the pericarp of low and high seed containing berries. (A) Induction of auxin response genes in cultured grape cells with auxin treatment (20 μM indole-3-acetic acid). The expression level is relative to the control. (B) Expression of auxin response genes in the pericarp of berries with low and high seed weight-to-berry weight (SB) from one-week prevéraison clusters. Expression level of high-SB is shown relative to that of low-SB. (C) Expression of ARF4 gene in the pericarp of low and high-SB berry groups at two- and one-week prevéraison (PV), and at mid-véraison (MV). Expression levels of ARF4 are relative to those of low-SB berries at 2-wk PV. Gene expression was analyzed by qRT-PCR and all data represent means of five replicates and error bars indicate ± SEM. Significant differences between low- and high-SB berries at each cluster stage are indicated by asterisks (Student’s t-test, p < 0.05). Significant differences of each SB group between the ripening stages are denoted by different letters (lower and upper case letters are used for low- and high-SB berries, respectively (Tukey’s HSD, p < 0.05)).
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Fig5: Screening for Auxin response genes induced in grape cultured cells and in the pericarp of low and high seed containing berries. (A) Induction of auxin response genes in cultured grape cells with auxin treatment (20 μM indole-3-acetic acid). The expression level is relative to the control. (B) Expression of auxin response genes in the pericarp of berries with low and high seed weight-to-berry weight (SB) from one-week prevéraison clusters. Expression level of high-SB is shown relative to that of low-SB. (C) Expression of ARF4 gene in the pericarp of low and high-SB berry groups at two- and one-week prevéraison (PV), and at mid-véraison (MV). Expression levels of ARF4 are relative to those of low-SB berries at 2-wk PV. Gene expression was analyzed by qRT-PCR and all data represent means of five replicates and error bars indicate ± SEM. Significant differences between low- and high-SB berries at each cluster stage are indicated by asterisks (Student’s t-test, p < 0.05). Significant differences of each SB group between the ripening stages are denoted by different letters (lower and upper case letters are used for low- and high-SB berries, respectively (Tukey’s HSD, p < 0.05)).

Mentions: Auxin signaling regulates cell responses to different levels of auxin. The main components of signaling are auxin response factors (ARFs) that activate or repress the expression of auxin-dependent genes [54,55]. Recent evidence in other models implicates specific ARF proteins that mediate auxin responses at different stages of fruit development. Genetic studies in tomato and Arabidopsis have shown that ARF6, ARF7, ARF8, and AUX/IAA9 [56-59] function at the fruit initiation stage, while ARF4 is important at the ripening transition stage [60]. To examine if any of these auxin-responsive genes function in mediating the observed auxin level changes during the ripening transition in grape, expression of these ARFs was examined in the pericarp of low- and high-SB berries from 1-wk PV clusters, which had lower and higher auxin levels, respectively (Figure 5B). Further, we assessed the auxin-responsiveness of these genes in grape cultured cells and found that ARF4 and ARF6 genes were significantly induced in cells treated with 20 μM IAA for 2 h (Figure 5A). While both ARF4 and ARF6 had higher expression in the pericarp of high-SB berries, the expression of ARF4 was more than sixfold higher. In Arabidopsis the expression of ARF4 and ARF19 is induced by auxin [61], and the expression of ARF genes that mediate auxin responses is cell- and development-context specific [62] (reviewed by [63,64]). The expression of ARF7, a negative regulator of auxin response that inhibits fruit set in tomato, is highest in the ovary and decreases with increasing auxin levels in this tissue [57]. Similarly in tomato, ARF4 is preferentially expressed in fruit around the breaker stage and its expression levels follow the ripening-related auxin changes in the tissue [60]. Under-expressing ARF4, formerly designated DR12, in tomato results in dark green, immature fruits and up-regulates the expression of sugar metabolism-related genes [60,65]. Based upon this evidence, our results indicate that ARF4 mediates the response to auxin changes during the grape berry ripening initiation, and is likely a negative regulator of the ripening-related changes in the pericarp during véraison.Figure 5


Timing of ripening initiation in grape berries and its relationship to seed content and pericarp auxin levels.

Gouthu S, Deluc LG - BMC Plant Biol. (2015)

Screening for Auxin response genes induced in grape cultured cells and in the pericarp of low and high seed containing berries. (A) Induction of auxin response genes in cultured grape cells with auxin treatment (20 μM indole-3-acetic acid). The expression level is relative to the control. (B) Expression of auxin response genes in the pericarp of berries with low and high seed weight-to-berry weight (SB) from one-week prevéraison clusters. Expression level of high-SB is shown relative to that of low-SB. (C) Expression of ARF4 gene in the pericarp of low and high-SB berry groups at two- and one-week prevéraison (PV), and at mid-véraison (MV). Expression levels of ARF4 are relative to those of low-SB berries at 2-wk PV. Gene expression was analyzed by qRT-PCR and all data represent means of five replicates and error bars indicate ± SEM. Significant differences between low- and high-SB berries at each cluster stage are indicated by asterisks (Student’s t-test, p < 0.05). Significant differences of each SB group between the ripening stages are denoted by different letters (lower and upper case letters are used for low- and high-SB berries, respectively (Tukey’s HSD, p < 0.05)).
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Related In: Results  -  Collection

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Fig5: Screening for Auxin response genes induced in grape cultured cells and in the pericarp of low and high seed containing berries. (A) Induction of auxin response genes in cultured grape cells with auxin treatment (20 μM indole-3-acetic acid). The expression level is relative to the control. (B) Expression of auxin response genes in the pericarp of berries with low and high seed weight-to-berry weight (SB) from one-week prevéraison clusters. Expression level of high-SB is shown relative to that of low-SB. (C) Expression of ARF4 gene in the pericarp of low and high-SB berry groups at two- and one-week prevéraison (PV), and at mid-véraison (MV). Expression levels of ARF4 are relative to those of low-SB berries at 2-wk PV. Gene expression was analyzed by qRT-PCR and all data represent means of five replicates and error bars indicate ± SEM. Significant differences between low- and high-SB berries at each cluster stage are indicated by asterisks (Student’s t-test, p < 0.05). Significant differences of each SB group between the ripening stages are denoted by different letters (lower and upper case letters are used for low- and high-SB berries, respectively (Tukey’s HSD, p < 0.05)).
Mentions: Auxin signaling regulates cell responses to different levels of auxin. The main components of signaling are auxin response factors (ARFs) that activate or repress the expression of auxin-dependent genes [54,55]. Recent evidence in other models implicates specific ARF proteins that mediate auxin responses at different stages of fruit development. Genetic studies in tomato and Arabidopsis have shown that ARF6, ARF7, ARF8, and AUX/IAA9 [56-59] function at the fruit initiation stage, while ARF4 is important at the ripening transition stage [60]. To examine if any of these auxin-responsive genes function in mediating the observed auxin level changes during the ripening transition in grape, expression of these ARFs was examined in the pericarp of low- and high-SB berries from 1-wk PV clusters, which had lower and higher auxin levels, respectively (Figure 5B). Further, we assessed the auxin-responsiveness of these genes in grape cultured cells and found that ARF4 and ARF6 genes were significantly induced in cells treated with 20 μM IAA for 2 h (Figure 5A). While both ARF4 and ARF6 had higher expression in the pericarp of high-SB berries, the expression of ARF4 was more than sixfold higher. In Arabidopsis the expression of ARF4 and ARF19 is induced by auxin [61], and the expression of ARF genes that mediate auxin responses is cell- and development-context specific [62] (reviewed by [63,64]). The expression of ARF7, a negative regulator of auxin response that inhibits fruit set in tomato, is highest in the ovary and decreases with increasing auxin levels in this tissue [57]. Similarly in tomato, ARF4 is preferentially expressed in fruit around the breaker stage and its expression levels follow the ripening-related auxin changes in the tissue [60]. Under-expressing ARF4, formerly designated DR12, in tomato results in dark green, immature fruits and up-regulates the expression of sugar metabolism-related genes [60,65]. Based upon this evidence, our results indicate that ARF4 mediates the response to auxin changes during the grape berry ripening initiation, and is likely a negative regulator of the ripening-related changes in the pericarp during véraison.Figure 5

Bottom Line: In the prevéraison cluster, the expression of auxin-response factor genes was significantly higher in the pericarp of high-SB berries and remained higher until véraison compared to low-SB berries.This results in higher auxin-signaling activity that lasts longer in the pericarp of high-SB berries.In contrast, in low-SB berries, concomitant with an earlier decrease of auxin level, the features of ripening initiation, such as increases in ABA and sugar accumulation begin earlier.

View Article: PubMed Central - PubMed

ABSTRACT

Background: Individual berries in a grape (Vitis vinifera L.) cluster enter the ripening phase at different times leading to an asynchronous cluster in terms of ripening. The factors causing this variable ripening initiation among berries are not known. Because the influence via hormonal communication of the seed on fruit set and growth is well known across fruit species, differences in berry seed content and resultant quantitative or qualitative differences in the hormone signals to the pericarp likely influence the relative timing of ripening initiation among berries of the cluster.

Results: At the time of the initiation of cluster ripening (véraison), underripe green berries have higher seed content compared to the riper berries and there is a negative correlation between the seed weight-to-berry weight ratio (SB) and the sugar level in berries of a cluster. Auxin levels in seeds relative to the pericarp tissues are two to 12 times higher at pre-ripening stages. The pericarp of berries with high-SB had higher auxin and lower abscisic acid (ABA) levels compared to those with low-SB from two weeks before véraison. In the prevéraison cluster, the expression of auxin-response factor genes was significantly higher in the pericarp of high-SB berries and remained higher until véraison compared to low-SB berries. The expression level of auxin-biosynthetic genes in the pericarp was the same between both berry groups based upon similar expression activity of YUC genes that are rate-limiting factors in auxin biosynthesis. On the other hand, in low-SB berries, the expression of ABA-biosynthetic and ABA-inducible NCED and MYB genes was higher even two weeks before véraison.

Conclusions: Differences in the relative seed content among berries plays a major role in the timing of ripening initiation. Towards the end of berry maturation phase, low and high levels of auxin are observed in the pericarp of low- and high-SB berries, respectively. This results in higher auxin-signaling activity that lasts longer in the pericarp of high-SB berries. In contrast, in low-SB berries, concomitant with an earlier decrease of auxin level, the features of ripening initiation, such as increases in ABA and sugar accumulation begin earlier.

Show MeSH