Red to Far-Red Light Ratio Modulates Hormonal and Genetic Control of Axillary bud Outgrowth in Chrysanthemum (Dendranthema grandiflorum 'Jinba')

Transcriptome analysis reveals the key network of axillary bud outgrowth modulated by topping in citrus

Topping, an important tree shaping and pruning technique, can promote the outgrowth of citrus axillary buds. However, the underlying molecular mechanism is still unclear. In this study, spring shoots of Citrus reticulata 'Huagan No.2' were topped and transcriptome was compared between axillary buds of topped and untopped shoots at 6 and 11 days after topping (DAT). 1944 and 2394 differentially expressed genes (DEGs) were found at 6 and 11 DAT, respectively. KEGG analysis revealed that many DEGs were related to starch and sucrose metabolism, signal transduction of auxin, cytokinin and abscisic acid. Specially, transcript levels of auxin synthesis, transport, and signaling-related genes (SAURs and ARF5), cytokinin signal transduction related genes (CRE1, AHP and Type-A ARRs), ABA signal responsive genes (PYL and ABF) were up-regulated by topping; while transcript levels of auxin receptor TIR1, auxin responsive genes AUX/IAAs, ABA signal transduction related gene PP2Cs and synthesis related genes NCED3 were down-regulated. On the other hand, the contents of sucrose and fructose in axillary buds of topped shoots were significantly higher than those in untopped shoots; transcript levels of 16 genes related to sucrose synthase, hexokinase, sucrose phosphate synthase, endoglucanase and glucosidase, were up-regulated in axillary buds after topping. In addition, transcript levels of genes related to trehalose 6-phosphate metabolism and glycolysis/tricarboxylic acid (TCA) cycle, as well to some transcription factors including Pkinase, Pkinase_Tyr, Kinesin, AP2/ERF, P450, MYB, NAC and Cyclin_c, significantly responded to topping. Taken together, the present results suggested that topping promoted citrus axillary bud outgrowth through comprehensively regulating plant hormone and carbohydrate metabolism, as well as signal transduction. These results deepened our understanding of citrus axillary bud outgrowth by topping and laid a foundation for further research on the molecular mechanisms of citrus axillary bud outgrowth.

Influence of Additional White, Red and Far-Red Light on Growth, Secondary Metabolites and Expression of Hormone Signaling Genes in Scots Pine under Sunlight

The influence of short-term additional white (WL), red (RL) and far-red (FRL) light and combined RL+FRL on the physiological morphological and molecular characteristics of two-year-old Scots pine plants grown in a greenhouse under sunlight was studied. Additional RL and RL+FRL increased the number of xylem cells, transpiration and the expression of a group of genes responsible for the biosynthesis and signaling of auxins (AUX/IAA, ARF3/4, and ARF16) and brassinosteroids (BR-α-RED and BRZ2), while the expression of genes related to the signaling pathway related to jasmonic acid was reduced. Additionally, WL, RL and RL+FRL increased the content of proanthocyanidins and catechins in young needles; however, an increase in the expression of the chalcone synthase gene (CHS) was found under RL, especially under RL+FRL, which possibly indicates a greater influence of light intensity than observed in the spectrum. Additional WL increased photosynthetic activity, presumably by increasing the proportion and intensity of blue light; at the same time, the highest transpiration index was found under RL. The results obtained indicate that the combined effect of additional RL+FRL can accelerate the development of pine plants by increasing the number of xylem cells and increasing the number of aboveground parts but not the photosynthetic activity or the accumulation of secondary metabolites.

Supplementary Low Far-Red Light Promotes Proliferation and Photosynthetic Capacity of Blueberry In Vitro Plantlets

Far-red light exerts an important regulatory influence on plant growth and development. However, the mechanisms underlying far-red light regulation of morphogenesis and photosynthetic characteristics in blueberry plantlets in vitro have remained elusive. Here, physiological and transcriptomic analyses were conducted on blueberry plantlets in vitro supplemented with far-red light. The results indicated that supplementation with low far-red light, such as 6 μmol m-2 s-1 and 14 μmol m-2 s-1 far-red (6FR and 14FR) light treatments, significantly increased proliferation-related indicators, including shoot length, shoot number, gibberellin A3, and trans-zeatin riboside content. It was found that 6FR and 14 FR significantly reduced chlorophyll content in blueberry plantlets but enhanced electron transport rates. Weighted correlation network analysis (WGCNA) showed the enrichment of iron ion-related genes in modules associated with photosynthesis. Genes such as NAC, ABCG11, GASA1, and Erf74 were significantly enriched within the proliferation-related module. Taken together, we conclude that low far-red light can promote the proliferative capacity of blueberry plantlets in vitro by affecting hormone pathways and the formation of secondary cell walls, concurrently regulating chlorophyll content and iron ion homeostasis to affect photosynthetic capacity.

Interaction of Phytohormones and External Environmental Factors in the Regulation of the Bud Dormancy in Woody Plants

Bud dormancy and release are essential phenomena that greatly assist in adapting to adverse growing conditions and promoting the holistic growth and development of perennial plants. The dormancy and release process of buds in temperate perennial trees involves complex interactions between physiological and biochemical processes influenced by various environmental factors, representing a meticulously orchestrated life cycle. In this review, we summarize the role of phytohormones and their crosstalk in the establishment and release of bud dormancy. External environmental factors, such as light and temperature, play a crucial role in regulating bud germination. We also highlight the mechanisms of how light and temperature are involved in the regulation of bud dormancy by modulating phytohormones. Moreover, the role of nutrient factors, including sugar, in regulating bud dormancy is also discussed. This review provides a foundation for enhancing our understanding of plant growth and development patterns, fostering agricultural production, and exploring plant adaptive responses to adversity.

Low red/far-red ratio can induce cytokinin degradation resulting in the inhibition of tillering in wheat (Triticum aestivum L.)

Shoot branching is inhibited by a low red/far-red ratio (R/FR). Prior studies have shown that the R/FR suppressed Arabidopsis thaliana branching by promotes bud abscisic acid (ABA) accumulation directly. Given that wheat tiller buds are wrapped in leaf sheaths and may not respond rapidly to a R/FR, systemic cytokinin (CTK) may be more critical. Here, systemic hormonal signals including indole-3-acetic acid (IAA), gibberellins (GA) and CTK and bud ABA signals in wheat were tested under a low R/FR. The results showed that a low R/FR reduced the percentage of tiller occurrence of tiller IV and the tiller number per plant. The low R/FR did not rapidly induced ABA accumulation in the tiller IV because of the protection of the leaf sheath and had little effect on IAA content and signaling in the tiller nodes. The significant change in the CTK levels was observed earlier than those of other hormone (ABA, IAA and GA) and exogenous cytokinin restored the CTK levels and tiller number per plant under low R/FR conditions. Further analysis revealed that the decrease in cytokinin levels was mainly associated with upregulation of cytokinin degradation genes (TaCKX5, TaCKX11) in tiller nodes. In addition, exposure to a decreased R/FR upregulated the expression of GA biosynthesis genes (TaGA20ox1, TaGA3ox2), resulting in elevated GA levels, which might further promote CTK degradation in tiller nodes and inhibit tillering. Therefore, our results provide evidence that the enhancement of cytokinin degradation is a novel mechanism underlying the wheat tillering response to a low R/FR.

Involvement of sugar and abscisic acid in the genotype-specific response of rose to far-red light

Plant architecture determines yield (fruit or flowers) and product quality in many horticultural species. It results from growth and branching processes and is dependent on genetic and environmental factors such as light quality. Highly significant genotype and light quality effects and their interaction have been demonstrated on the architecture of rose. Far-red (FR) light is known for its favourable effect on plant growth and development. We evaluated the effect of FR on rose growth and development and its interaction with the genotype through architectural, eco-physiological (net photosynthesis rate) and biochemical (sugar and hormone concentrations) approaches. Two cultivars ('The Fairy' - TF - and Knock Out^® Radrazz - KO) with contrasting architectures were grown in a climate chamber under FR or in the absence of FR at an average photosynthetic photon flux density (400-700 nm) of 181.7 ± 12.8 μmol m^-2 s^-1 for 16 h. A significant effect of FR on the architecture of TF was demonstrated, marked by greater stem elongation, shoot branching and flowering, while KO remained insensitive to FR, supporting a genotype x FR interaction. The response of TF to FR was associated with improved photosynthetic capabilities, while KO exhibited an elevated level of abscisic acid (ABA) in its leaves. FR-dependent ABA accumulation might inhibit photosynthesis and prevent the increased plant carbon status required for growth. From a practical perspective, these findings argue in favour of a better reasoning of the choice of the cultivars grown in lighted production systems. Further investigations will be necessary to better understand these genotype-specific responses to FR and to unravel their molecular determinants.

Exposure of Brassica to Red Light Antagonizes Low Production of IAA in Leaf Through Root Signaling Under Stress Conditions

Plant leaf is highly sensitive to various growth promoting and restraining components. This sensitivity is normally caused by the alteration of different phyto-hormones (predominately by IAA), when the plants exposed to certain environmental conditions. We exposed the hydroponically grown Brassica campestris seedlings (7 days old) to red and green light in order to observe its effect on IAA secretion at leaf. The evaluated data showed that red light antagonized the low production of IAA in leaf by initiating the root signaling through flavonoids production and high redox activity. The study also explored the link between the differential phytohormonal response and biotic or abiotic stress elimination in leaf through root signaling under green or red light. The results exhibited that the biotic (P. syringae or F. alni) or abiotic stresses (100 mM AgNO₃ or 100 mM tert-butyl alcohol) inhibited flavonoids at the roots and resisted the restoration of IAA at the leaf. However, under green light where IAA was not inhibited, the stresses could not produce flavonoid at the root and further passing the signals to leaf. The results concluded that the growth and photosynthetic rates of the seedlings were improved under red light exposure through flavonoid inducing stresses.

Phytochrome interacting factors 4 and 5 regulate axillary branching via bud abscisic acid and stem auxin signalling

The ratio of red light to far-red light (R:FR) is perceived by phytochrome B (phyB) and informs plants of nearby competition. A low R:FR indicative of competition induces the shade avoidance syndrome and suppresses branching by incompletely understood mechanisms. Phytochrome interacting factors (PIFs) are transcription factors targeted by phytochromes to evoke photomorphogenic responses. PIF4 and PIF5 promote shade avoidance responses and become inactivated by direct interaction with active phyB in the nucleus. Here, genetic and physiological assays show that PIF4 and PIF5 contribute to the suppression of branching resulting from phyB loss of function and a low R:FR, although roles for other PIFs or pathways may exist. The suppression of branching is associated with PIF4/PIF5 promotion of the expression of the branching inhibitor BRANCHED 1 and abscisic acid (ABA) accumulation in axillary buds and is dependent on the function of the key ABA biosynthetic enzyme Nine-cis-epoxycarotenoid dioxygenase 3. However, PIF4/PIF5 function is not confined to a single hormonal pathway, as they also promote stem indole-3-acetic acid accumulation and stimulate systemic auxin signalling, which contribute to the suppression of bud growth when phyB is inactive.

The CmbZIP1 transcription factor of chrysanthemum negatively regulates shoot branching

The basic region/leucine zipper (bZIP) transcription factors play key roles in regulating diverse biological processes in plants. However, their participation in shoot branching has been rarely reported. Here, we isolated a CmbZIP1 transcription factor gene, a member of the bZIP family, from chrysanthemum. Subcellular localization analysis indicated that CmbZIP1 is a nuclear protein. Tissue-specific expression analysis indicated that CmbZIP1 was principally expressed in apical bud and axillary bud. Expression patterns analysis results showed that CmbZIP1 expression was suppressed in axillary buds in response to decapitation but increased in response to shade. Overexpression of CmbZIP1 in Arabidopsis inhibits its shoot branching. In addition, expression of auxin efflux protein PIN-FORMED 1 (PIN1) and auxin signaling components AUXIN RESISTANT 1/3 (AXR1, AXR3) were significantly up-regulated in overexpressing plants in comparison with wild type plants. Moreover, the transcript expression of BRANCHED 2 (AtBRC2) was also significantly up-regulated in overexpressing plants compared with the wild type. Altogether, these results suggest important and negative roles of CmbZIP1 in shoot branching. Our study extends the understanding of the function of bZIP transcription factors in plants and provides valuable gene resources for improving the architectural traits of ornamental plants.

Morpho-physiological integrators, transcriptome and coexpression network analyses signify the novel molecular signatures associated with axillary bud in chrysanthemum

BACKGROUND

Axillary bud is an important agronomic and economic trait in cut chrysanthemum. Bud outgrowth is an intricate process controlled by complex molecular regulatory networks, physio-chemical integrators and environmental stimuli. Temperature is one of the key regulators of bud's fate. However, little is known about the temperature-mediated control of axillary bud at molecular levels in chrysanthemum. A comprehensive study was designed to study the bud outgrowth at normal and elevated temperature in cut chrysanthemum. Leaf morphology, histology, physiological parameters were studied to correlate the leaf activity with bud morphology, sucrose and hormonal regulation and the molecular controllers.

RESULTS

Temperature caused differential bud outgrowth along bud positions. Photosynthetic leaf area, physiological indicators and sucrose utilization were changed considerable due to high temperature. Comparative transcriptome analysis identified a significant proportion of bud position-specific genes.Weighted Gene Co-expression Network Analysis (WGCNA) showed that axillary bud control can be delineated by modules of coexpressed genes; especially, MEtan3, MEgreen2 and MEantiquewhite presented group of genes specific to bud length. A comparative analysis between different bud positions in two temperatures revealed the morpho-physiological traits associated with specific modules. Moreover, the transcriptional regulatory networks were configured to identify key determinants of bud outgrowth. Cell division, organogenesis, accumulation of storage compounds and metabolic changes were prominent during the bud emergence.

CONCLUSIONS

RNA-seq data coupled with morpho-physiological integrators from three bud positions at two temperature regimes brings a robust source to understand bud outgrowth status influenced by high temperature in cut chrysanthemum. Our results provide helpful information for elucidating the regulatory mechanism of temperature on axillary bud growth in chrysanthemum.

Sugar Transporter, CmSWEET17, Promotes Bud Outgrowth in Chrysanthemum Morifolium

We previously demonstrated that 20 mM sucrose promotes the upper axillary bud outgrowth in two-node stems of Chrysanthemum morifolium. In this study, we aimed to screen for potential genes involved in this process. Quantitative reverse transcription (qRT)-PCR analysis of sugar-related genes in the upper axillary bud of plants treated with 20 mM sucrose revealed the specific expression of the gene CmSWEET17. Expression of this gene was increased in the bud, as well as the leaves of C. morifolium, following exogenous sucrose treatment. CmSWEET17 was isolated from C. morifolium and a subcellular localization assay confirmed that the protein product was localized in the cell membrane. Overexpression of CmSWEET17 promoted upper axillary bud growth in the two-node stems treatment as compared with the wild-type. In addition, the expression of auxin transporter genes CmAUX1, CmLAX2, CmPIN1, CmPIN2, and CmPIN4 was upregulated in the upper axillary bud of CmSWEET17 overexpression lines, while indole-3-acetic acid content decreased. The results suggest that CmSWEET17 could be involved in the process of sucrose-induced axillary bud outgrowth in C. morifolium, possibly via the auxin transport pathway.

Light Regulation of Axillary Bud Outgrowth Along Plant Axes: An Overview of the Roles of Sugars and Hormones

Apical dominance, the process by which the growing apical zone of the shoot inhibits bud outgrowth, involves an intricate network of several signals in the shoot. Auxin originating from plant apical region inhibits bud outgrowth indirectly. This inhibition is in particular mediated by cytokinins and strigolactones, which move from the stem to the bud and that respectively stimulate and repress bud outgrowth. The action of this hormonal network is itself modulated by sugar levels as competition for sugars, caused by the growing apical sugar sink, may deprive buds from sugars and prevents bud outgrowth partly by their signaling role. In this review, we analyze recent findings on the interaction between light, in terms of quantity and quality, and apical dominance regulation. Depending on growth conditions, light may trigger different pathways of the apical dominance regulatory network. Studies pinpoint to the key role of shoot-located cytokinin synthesis for light intensity and abscisic acid synthesis in the bud for R:FR in the regulation of bud outgrowth by light. Our analysis provides three major research lines to get a more comprehensive understanding of light effects on bud outgrowth. This would undoubtedly benefit from the use of computer modeling associated with experimental observations to deal with a regulatory system that involves several interacting signals, feedbacks, and quantitative effects.