Sprouting of paradormant and endodormant grapevine buds under conditions of forced growth: similarities and differences

Comparative transcriptome analysis of genes involved in paradormant bud release response in 'Summer Black' grape

Grapevines possess a hierarchy of buds, and the fruitful winter bud forms the foundation of the two-crop-a-year cultivation system, yielding biannual harvests. Throughout its developmental stages, the winter bud sequentially undergoes paradormancy, endodormancy, and ecodormancy to ensure survival in challenging environmental conditions. Releasing the endodormancy of winter bud results in the first crop yield, while breaking the paradormancy of winter bud allows for the second crop harvest. Hydrogen cyanamide serves as an agent to break endodormancy, which counteracting the inhibitory effects of ABA, while H2O2 and ethylene function as signaling molecules in the process of endodormancy release. In the context of breaking paradormancy, common agronomic practices include short pruning and hydrogen cyanamide treatment. However, the mechanism of hydrogen cyanamide contributes to this process remains unknown. This study confirms that hydrogen cyanamide treatment significantly improved both the speed and uniformity of bud sprouting, while short pruning proved to be an effective method for releasing paradormancy until August. This observation highlights the role of apical dominance as a primary inhibitory factor in suppressing the sprouting of paradormant winter bud. Comparative transcriptome analysis revealed that the sixth node winter bud convert to apical tissue following short pruning and established a polar auxin transport canal through the upregulated expression of VvPIN3 and VvTIR1. Moreover, short pruning induced the generation of reactive oxygen species, and wounding, ethylene, and H2O2 collectively acted as stimulating signals and amplified effects through the MAPK cascade. In contrast, hydrogen cyanamide treatment directly disrupted mitochondrial function, resulting in ROS production and an extended efficacy of the growth hormone signaling pathway induction.

Dual functions of PsmiR172b-PsTOE3 module in dormancy release and flowering in tree peony (Paeonia suffruticosa)

MicroRNAs (miRNAs) are non-coding RNAs that interact with target genes and are involved in many physiological processes in plants. miR172-AP2 mainly plays a role in the regulation of flowering time and floral organ differentiation. Bud dormancy release is necessary for forcing culture of tree peony in winter, but the mechanism of dormancy regulation is unclear. In this study, we found that a miR172 family member, PsmiR172b, was downregulated during chilling-induced bud dormancy release in tree peony, exhibiting a trend opposite to that of PsTOE3. RNA ligase-mediated (RLM) 5'-RACE (rapid amplification of cDNA ends) confirmed that miR172b targeted PsTOE3, and the cleavage site was between bases 12 (T) and 13 (C) within the complementary site to miR172b. The functions of miR172b and PsTOE3 were detected by virus-induced gene silencing (VIGS) and their overexpression in tree peony buds. PsmiR172b negatively regulated bud dormancy release, but PsTOE3 promoted bud dormancy release, and the genes associated with bud dormancy release, including PsEBB1, PsEBB3, PsCYCD, and PsBG6, were upregulated. Further analysis indicated that PsTOE3 directly regulated PsEBB1 by binding to its promoter, and the specific binding site was a C-repeat (ACCGAC). Ectopic expression in Arabidopsis revealed that the PsmiR172b-PsTOE3 module displayed conservative function in regulating flowering. In conclusion, our results provided a novel insight into the functions of PsmiR172-PsTOE3 and possible molecular mechanism underlying bud dormancy release in tree peony.

Sucrose accumulation and endodormancy are synchronized events induced by the short-day photoperiod in grapevine buds

At the end of the summer season, grapevine buds (Vitis vinifera L) grown in temperate climates enter a state of winter recess or endodormancy (ED), which is induced by the shortening of the photoperiod, and during this period, the buds accumulate sucrose. In this study, we investigated whether the shortening of the photoperiod regulates the accumulation of sucrose in the buds in the same way as it regulates its entry into the ED. Because sucrose accumulation is regulated by genes that control its transport and degradation, the effect of the SD photoperiod and the transition of buds from paradormancy (PD) to ED on the expression of sucrose transporter (VvSUTs) and invertase genes (VvINVs) was studied. To analyze the possible role of sucrose during ED development, its effect on bud swelling and sprouting was studied on dormant and nondormant buds under forced growth conditions. The results showed that the SD photoperiod upregulates the expression of the VvSUT genes and downregulates that of the VvINV genes in grapevine buds. Additionally, during the transition of buds from PD to ED, the sucrose content increased, the expression of the VvINV genes decreased, and the expression of the VvSUT genes did not change significantly. Sucrose delayed bud swelling and sprouting when applied to dormant buds but had no effect when applied to nondormant buds. Therefore, we concluded that ED development and sucrose accumulation were synchronized events induced by the SD photoperiod and that a sucrose peak marks the end of ED development in grapevine buds.

Insight into Carbohydrate Metabolism and Signaling in Grapevine Buds during Dormancy Progression

Perennial fruit crops enter dormancy to ensure bud tissue survival during winter. However, a faster phenological advancement caused by global warming exposes bud tissue to a higher risk of spring frost damage. Tissue dehydration and soluble sugars accumulation are connected to freezing tolerance, but non-structural carbohydrates also act as metabolic substrates and signaling molecules. A deepened understanding of sugar metabolism in the context of winter freezing resistance is required to gain insight into adaptive possibilities to cope with climate changes. In this study, the soluble sugar content was measured in a cold-tolerant grapevine hybrid throughout the winter season. Moreover, the expression of drought-responsive hexose transporters VvHT1 and VvHT5, raffinose synthase VvRS and grapevine ABA-, Stress- and Ripening protein VvMSA was analyzed. The general increase in sugars in December and January suggests that they can participate in protecting bud tissues against low temperatures. The modulation of VvHT5, VvINV and VvRS appeared consistent with the availability of the different sugar species; challenging results were obtained for VvHT1 and VvMSA, suggesting interesting hypotheses about their role in the sugar-hormone crosstalk. The multifaceted role of sugars on the intricate phenomenon, which is the response of dormant buds to changing temperature, is discussed.

The bud dormancy disconnect: latent buds of grapevine are dormant during summer despite a high metabolic rate

Grapevine (Vitis vinifera L.) displays wide plasticity to climate; however, the physiology of dormancy along a seasonal continuum is poorly understood. Here we investigated the apparent disconnect between dormancy and the underlying respiratory physiology and transcriptome of grapevine buds, from bud set in summer to bud burst in spring. The establishment of dormancy in summer was pronounced and reproducible; however, this was coupled with little or no change in physiology, indicated by respiration, hydration, and tissue oxygen tension. The release of dormancy was biphasic; the depth of dormancy declined substantially by mid-autumn, while the subsequent decline towards spring was moderate. Observed changes in physiology failed to explain the first phase of dormancy decline, in particular. Transcriptome data contrasting development from summer through to spring also indicated that dormancy was poorly reflected by metabolic quiescence during summer and autumn. Gene Ontology and enrichment data revealed the prevailing influence of abscisic acid (ABA)-related gene expression during the transition from summer to autumn, and promoter motif analysis suggested that photoperiod may play an important role in regulating ABA functions during the establishment of dormancy. Transcriptomic data from later transitions reinforced the importance of oxidation and hypoxia as physiological cues to regulate the maintenance of quiescence and resumption of growth. Collectively these data reveal a novel disconnect between growth and metabolic quiescence in grapevine following bud set, which requires further experimentation to explain the phenology and dormancy relationships.

Ascorbate-glutathione pathways mediated by cytokinin regulate H2O2 levels in light-controlled rose bud burst

Rosebush (Rosa "Radrazz") plants are an excellent model to study light control of bud outgrowth since bud outgrowth only arises in the presence of light and never occurs in darkness. Recently, we demonstrated high levels of hydrogen peroxide (H2O2) present in the quiescent axillary buds strongly repress the outgrowth process. In light, the outgrowing process occurred after H2O2 scavenging through the promotion of Ascorbic acid-Glutathione (AsA-GSH)-dependent pathways and the continuous decrease in H2O2 production. Here we showed Respiratory Burst Oxidase Homologs expression decreased in buds during the outgrowth process in light. In continuous darkness, the same decrease was observed although H2O2 remained at high levels in axillary buds, as a consequence of the strong inhibition of AsA-GSH cycle and GSH synthesis preventing the outgrowth process. Cytokinin (CK) application can evoke bud outgrowth in light as well as in continuous darkness. Furthermore, CKs are the initial targets of light in the photocontrol process. We showed CK application to cultured buds in darkness decreases bud H2O2 to a level that is similar to that observed in light. Furthermore, this treatment restores GSH levels and engages bud burst. We treated plants with buthionine sulfoximine, an inhibitor of GSH synthesis, to solve the sequence of events involving H2O2/GSH metabolisms in the photocontrol process. This treatment prevented bud burst, even in the presence of CK, suggesting the sequence of actions starts with the positive CK effect on GSH that in turn stimulates H2O2 scavenging, resulting in initiation of bud outgrowth.

Hydrogen Peroxide Increases during Endodormancy and Decreases during Budbreak in Grapevine (Vitis vinifera L.) Buds

Changes in the level of hydrogen peroxide (H₂O₂) is a good indicator to monitor fluctuations in cellular metabolism and in the stress responses. In this study, the changes in H₂O₂ content during bud endodormancy (ED) and budbreak were analysed in grapevine (Vitis vinifera L.). The results showed a gradual increase in the H₂O₂ content during the development of bud ED, which was mainly due to an increase in the activity of peroxidases (PODs). The maximum H₂O₂ content reached in the grapevine buds coincided with the maximum depth of bud ED. In contrast, during budbreak, the H₂O₂ content decreased. As the plant hormones cytokinin (CK) and auxin play an important role in budbreak and growth resumption in grapevine, the effect of exogenous applications of H₂O₂ on the expression of genes involved in CK and auxin metabolism was analysed. The results showed that H₂O₂ represses the expression of the CK biosynthesis genes VvIPT3a and VvLOG1 and induces the expression of the CK-inactivating gene VvCKX3, thus reducing potentially the CK content in the grapevine bud. On the other hand, H₂O₂ induced the expression of the auxin biosynthesis genes VvAMI1 and VvYUC3 and of the auxin transporter gene VvPIN3, thus increasing potentially the auxin content and auxin transport in grapevine buds. In general, the results suggest that H₂O₂ in grapevine buds is associated with the depth of ED and negatively regulates its budbreak.

Ascorbate glutathione-dependent H2O2 scavenging is an important process in axillary bud outgrowth in rosebush

BACKGROUND AND AIMS

Branching is an important mechanism of plant shape establishment and the direct consequence of axillary bud outgrowth. Recently, hydrogen peroxide (H2O2) metabolism, known to be involved in plant growth and development, has been proposed to contribute to axillary bud outgrowth. However, the involvement of H2O2 in this process remains unclear.

METHODS

We analysed the content of H2O2 during bud outgrowth and characterized its catabolism, both at the transcriptional level and in terms of its enzymatic activities, using RT-qPCR and spectrophotometric methods, respectively. In addition, we used in vitro culture to characterize the effects of H2O2 application and the reduced glutathione (GSH) synthesis inhibitor l-buthionine sulfoximine (BSO) on bud outgrowth in relation to known molecular markers involved in this process.

KEY RESULTS

Quiescent buds displayed a high content of H2O2 that declined when bud outgrowth was initiated, as the consequence of an increase in the scavenging activity that is associated with glutathione pathways (ascorbate-glutathione cycle and glutathione biosynthesis); catalase did not appear to be implicated. Modification of bud redox state after the application of H2O2 or BSO prevented axillary bud outgrowth by repressing organogenesis and newly formed axis elongation. Hydrogen peroxide also repressed bud outgrowth-associated marker gene expression.

CONCLUSIONS

These results show that high levels of H2O2 in buds that are in a quiescent state prevents bud outgrowth. Induction of ascorbate-glutathione pathway scavenging activities results in a strong decrease in H2O2 content in buds, which finally allows bud outgrowth.

Integrated analysis of miRNAs and their targets reveals that miR319c/TCP2 regulates apical bud burst in tea plant (Camellia sinensis)

MAIN CONCLUSION

The roles of microRNA-mediated epigenetic regulation were highlighted in the bud dormancy-activity cycle, implying that certain differentially expressed miRNAs play crucial roles in apical bud burst, such as csn-miR319c/TCP2. microRNAs (miRNAs) are a class of small non-coding RNAs that regulate gene expression by targeting mRNA transcripts for cleavage or directing translational inhibition. To investigate whether miRNAs regulate bud dormancy-activation transition in tea plant, which largely affects the yield and price of tea products and adaptability of tea trees, we constructed small RNA libraries from three different periods of bud dormancy-burst transition. Through sequencing analysis, 262 conserved and 83 novel miRNAs were identified, including 118 differentially expressed miRNAs. Quantitative RT-PCR results for randomly selected miRNAs exhibited that our comprehensive analysis is highly reliable and accurate. The content of caffeine increased continuously from the endodormancy bud to flushing bud, and differentially expressed miRNAs coupling with their targets associated with bud burst were identified. Remarkably, csn-miR319c was downregulated significantly from the quiescent bud to burst bud, while its target gene CsnTCP2 (TEOSINTE BRANCHED/CYCLOIDEA/PROLIFERATING CELL FACTOR 2) displayed opposite expression patterns. Co-transformation experiment in tobacco demonstrated that csn-miR319c can significantly suppress the functions of CsnTCP2. This study on miRNAs and the recognition of target genes could provide new insights into the molecular mechanism of the bud dormancy-activation transition in tea plant.

ABA promotes starch synthesis and storage metabolism in dormant grapevine buds

In grapevine (Vitis vinifera L.) buds, the short day (SD)-photoperiod induces endodormancy and increases the level of ABA and the expression of ABA key biosynthesis genes, which suggests that ABA could be the mediator of the photoperiodic induction of endodormancy. In the present study, it was established that during the development of the endodormancy, the content of ABA and the accumulation of starch increased in parallel in the buds; however, these increases occurred after the buds were already in the state of endodormancy. Despite this finding the exogenous applications of ABA to single-bud cuttings increased the starch content and up-regulated the expression of starch synthesis genes (VvSS1 and VvSS3) and down-regulated the expression of sucrose metabolism genes, invertase (VvINV) and sucrose phosphate synthase (VvSUPS). In addition, the manipulation of the endogenous content of ABA in the grapevine buds by applications of hydrogen cyanamide and uniconazole-P, revealed that the depth of the endodormancy depends on the ABA levels. Taken together, the results indicate that the development of the endodormancy in grapevine buds is associated with the accumulation of starch and a shift in metabolism towards a storage metabolism; as ABA stimulates both processes, it must play an important role in the maintenance and release but not the induction of endodormancy in grapevine buds.