Flower Morphogenesis in Rosa hybrida 'Mercedes' as Studied by Cryo-scanning Electron and Light Microscopy. Effects on Light and Shoot Position on a Branch

Flower bud development in Rosa hybrida cv. 'Mercedes' was studied in shoots grown at different irradiances and sprouting from axillary buds at different branch positions. Cryo-scanning electron microscopy and light microscopy were used to visualize, characterize and determine flower morphogenesis during early shoot development. Up to the moment of visible flower bud appearance on the plant, flower morphogenesis was divided into nine stages. This classification was based on external and internal characteristics of the primordium. All shoots of the rose 'Mercedes' whether positioned uppermost or second on a branch and whether grown at 300 or 150 μmol m^-2 s^-1 PAR (12 h d^-1) developed equally up to flower stage 7, i.e. the stage just before visible initiation of stamen and pistils. Signs of flower bud abortion were the compactness of the flower bud at developmental stage 7 (height/width < 1·5) and the sprouting of axillary buds positioned just below the flower bud primordium. It was concluded that once a flower bud has reached a height to width ratio larger than 1·5, and once stamen and pistil developmental has started, it has passed the critical developmental stage in which abortion may occur. Flower developmental stage was closely related to shoot length. This relationship was not affected by irradiance level nor by shoot position on a branch. Therefore, cultivation treatments aimed to improve flower production by reducing flower abortion, such as supplementary lighting, will be most effective when applied during the first 2 weeks of shoot growth in which the flower develops up to stage 7.     

The ontogenetic basis for floral diversity in <Emphasis Type="Italic">Agonis</Emphasis>, <Emphasis Type="Italic">Leptospermum</Emphasis> and <Emphasis Type="Italic">Kunzea</Emphasis> (Myrtaceae)

Floral development in three species each of Leptospermum and Kunzea, and one species of Agonis, is described and compared. Differences in the number of stamens and their arrangement in the flower at anthesis are determined by the growth dynamics of the bud. In Leptospermum, early expansion of the bud is predominantly in the axial direction and causes the stamen primordia to be initiated in antepetalous chevrons. In Kunzea, early expansion occurs predominantly in the lateral direction and successive iterations of stamen primordia are inserted alternately at more or less the same level. In both genera, further expansion in the lateral plane spreads the stamens into a ring around the hypanthium. Agonis flexuosa is similar to Leptospermum. Other variable factors include the timing at which stamen initiation commences (earlier in Leptospermum than Kunzea), the duration of stamen initiation (hence the total number of stamens produced – varies within genera), and very late differential expansion that forces stamens into secondary antesepalous groups in A. flexuosa and L. myrsinoides.The authors thank Dr H. Toelken for kindly providing some material and the impetus for this project. This research was supported by Australian Research Council grant AS19131815.     

Woodland strawberry axillary bud fate is dictated by a crosstalk of environmental and endogenous factors

Plant architecture is defined by fates and positions of meristematic tissues and has direct consequences on yield potential and environmental adaptation of the plant. In strawberries (Fragaria vesca L. and F. × ananassa Duch.), shoot apical meristems can remain vegetative or differentiate into a terminal inflorescence meristem. Strawberry axillary buds (AXBs) are located in leaf axils and can either remain dormant or follow one of the two possible developmental fates. AXBs can either develop into stolons needed for clonal reproduction or into branch crowns (BCs) that can bear their own terminal inflorescences under favorable conditions. Although AXB fate has direct consequences on yield potential and vegetative propagation of strawberries, the regulation of AXB fate has so far remained obscure. We subjected a number of woodland strawberry (F. vesca L.) natural accessions and transgenic genotypes to different environmental conditions and growth regulator treatments to demonstrate that strawberry AXB fate is regulated either by environmental or endogenous factors, depending on the AXB position on the plant. We confirm that the F. vesca GIBBERELLIN20-oxidase4 (FvGA20ox4) gene is indispensable for stolon development and under tight environmental regulation. Moreover, our data show that apical dominance inhibits the outgrowth of the youngest AXB as BCs, although the effect of apical dominance can be overrun by the activity of FvGA20ox4. Finally, we demonstrate that the FvGA20ox4 is photoperiodically regulated via FvSOC1 (F. vesca SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1) at 18°C, but at higher temperature of 22°C an unidentified FvSOC1-independent pathway promotes stolon development.

Environmental conditions and apical dominance dictate woodland strawberry plant architecture by regulating axillary bud fate.     

Partial shoot reiteration in Wollemia nobilis (Araucariaceae) does not arise from 'axillary meristems'

Background and Aims

Conifers are characterized by the paucity of axillary buds which in dicotyledonous trees usually occur at every node. To compensate, conifers also produce ‘axillary meristems’, which may be stimulated to late development. In juvenile material of Wollemia nobilis (Araucariaceae: Massart''s model) first-order (plagiotropic) branches lack both axillary buds and, seemingly, axillary meristems. This contrasts with orthotropic (trunk) axes, which produce branches, either within the terminal bud or as reiterated orthotropic axes originating from axillary meristems. However, plagiotropic axes do produce branches if they are decapitated. This study investigated how this can occur if axillary meristems are not the source.

Methods

The terminal buds of a series of plagiotropic branches on juvenile trees were decapitated in order to generate axillary shoots. Shoots were culled at about weekly intervals to obtain stages in lateral shoot development. Serial sections were cut with a sliding microtome from the distal end of each sample and scanned sequentially for evidence of axillary meristems and early bud development.

Key Results

Anatomical search produced no clear evidence of pre-existing axillary meristems but did reveal stages of bud initiation. Buds were initiated in a group of small starch-rich cortical cells. Further development involved de-differentiation of these small cells and the development of contrasting outer and inner regions. The outer part becomes meristematic and organizes the apex of the new branch. The inner part develops a callus-like tissue of vacuolated cells within which vascular cambia are developed. This kind of insertion of a branch on the parent axis seems not to have been described before.

Conclusions

Axillary meristems in Wollemia characterize the leaf axils of trunk axes so that the origin of reiterated shoots is clear. Plagiotropic axes seemingly lack axillary meristems but still produce axillary branches by distinctive developmental processes. These observations demonstrate limited understanding of branch initiation in trees generally.     

Floral primordia-targeted ACS (1-aminocyclopropane-1-carboxylate synthase) expression in transgenic Cucumis melo implicates fine tuning of ethylene production mediating unisexual flower development

Main conclusion

Floral primordia-targeted expression of the ethylene biosynthetic gene, ACS , in melon suggests that differential timing and ethylene response thresholds combine to promote carpels, inhibit stamens, and prevent asexual bud formation. Typical angiosperm flowers produce both male and female reproductive organs. However, numerous species have evolved unisexuality. Melons (Cucumis melo L.) can produce varying combinations of male, female or bisexual flowers. Regardless of final sex, floral development begins with sequential initiation of all four floral whorls; unisexuality results from carpel or stamen primordia arrest regulated by the G and A loci, respectively. Ethylene, which promotes femaleness, is a key factor regulating sex expression. We sought to further understand the location, timing, level, and relationship to sex gene expression required for ethylene to promote carpel development or inhibit stamen development. Andromonoecious melons (GGaa) were transformed with the ethylene biosynthetic enzyme gene, ACS (1-aminocyclopropane-1-carboxylate synthase), targeted for expression in stamen and petal, or carpel and nectary, primordia using Arabidopsis APETALA3 (AP3) or CRABSCLAW (CRC) promoters, respectively. CRC::ACS plants did not exhibit altered sex phenotype. AP3::ACS melons showed increased femaleness manifested by gain of a bisexual-only phase not seen in wild type, decreased male buds and flowers, and loss of the initial male-only phase. In extreme cases, plants became phenotypically hermaphrodite, rather than andromonoecious. A reduced portion of buds progressed beyond initial whorl formation. Both the ACS transgene and exogenous ethylene reduced the expression of the native carpel-suppressing gene, G, while elevating expression of the stamen-suppressing gene, A. These results show ethylene-mediated regulation of key sex expression genes and suggest a mechanism by which temporally regulated ethylene production and differential ethylene response thresholds can promote carpels, inhibit stamens, and prevent the formation of asexual buds.     

INITIATION AND DEVELOPMENT OF INFLORESCENCE AND FLOWER IN ANEMOPSIS CALIFORNICA (SAURURACEAE)

All flowers of Anemopsis californica, the most specialized taxon of the family Saururaceae, are initiated as individual primordia subtended by previously initiated bracts, in contrast to the common-primordium initiation of all flowers of Saururus cernuus and of most flowers of Houttuynia cordata. Floral symmetry is bilateral and zygomorphic, and the sequence of initiation among floral parts is paired or whorled. In A. californica, the six stamens arise as three common primordia, each of which later bifurcates to form a pair. The three common primordia occupy sites corresponding to the positions of the three stamens in H. cordata flowers. In Anemopsis, the filaments of each pair are connate. Each stamen pair is vascularized by a single bifurcating vascular bundle. The three carpels per flower are usually initiated simultaneously although there may be some variation. Adnation between stamens and carpels results from zonal growth. Downward extension of the locule, and proliferation and expansion of receptacular tissue and inflorescence cortical tissue around the locule below the bases of the carpels produce the inferior ovary. The inflorescence terminates its activity as a flattened apical residuum, surrounded by bracts subtending reduced flowers most of which have stamens only.     

Pseudodiplostemony, and its implications for the evolution of the androecium in the Caryophyllaceae

The androecium of the Caryophyllaceae is varied, ranging from a two-whorled condition to a single stamen. A number of species belonging to the three subfamilies, Caryophyl-loideae, Alsinoideae and Paronychioideae have been studied ontogenetically with the SEM to understand their peculiar androecial development in the broader context of the Caryophyllales alliance. Although patterns of initiation are highly variable among species, there are three ontogenetic modes of stamen initiation: all stamens simultaneous within a whorl, the antepetalous stamens simultaneous and the antesepalous sequentially with a reversed direction, or both whorls sequentially with or without a reversed direction. The most common floral (ontogenetic) sequence of the Caryophyllaceae runs as follows: five sepals (in a 2/5 sequence), the stamens in front of the three inner sepals successively, stamens opposite the two outermost sepals, five antepetalous stamens (simultaneously or in a reversed spiral superimposed on the spiral of the antesepalous stamens), five outer sterile (petaloid) organs arising before, simultaneously or after the antesepalous stamens, often by the division of common primordia. A comparison with the floral configurations of the Phytolaccaceae and Molluginaceae indicates that the outer petaline whorl of the Caryophyllaceae corresponds positionally to the alternisepalous stamens of somePhytolacca, such asP. dodecandra. The difference withP. dodecandra lies in the fact that an extra inner or outer whorl is formed in the Caryophyl-laceae, in alternation with the sepals. A comparable arrangement exists in the Molluginaceae, though the initiation of stamens is centrifugal. A comparison of floral ontogenies and the presence of reduction series in the Caryophyllaceae support the idea that the pentamerous arrangement is derived from a trimerous prototype. Petals correspond to sterillized stamens and are comparable to two stamen pairs opposite the outer sepals and a single stamen alternating with the third and fifth sepals. Petals are often in a state of reduction; they may be confused with staminodes and they often arise from common stamenpetal primordia. The antesepalous stamen whorl represents an amalgamation of two whorls: initiation is reversed with the stamens opposite the fourth and fifth formed sepals arising before the other, while the stamens opposite the first and second formed sepals are frequently reduced or lost. Reductive trends are correlated with the mode of initiation of the androecium, as well as changes in the number of carpels, and affect the antesepalous and antepetalous whorls in different proportions. It is concluded that the androecium of the Caryophyllaceae is pseudodiplos-temonous and is not comparable to diplostemonous forms in the Dilleniidae and Rosidae. The basic floral formula of Caryophyllaceae is as follows: sepals 5—petals 5 (sterile stamens)—antesepalous stamens 3+2—antepetalous stamens 5 gynoecium 5.     

Expression of ASK1-like genes in arrested stamens of female Silene latifolia plants

Silene latifolia is a dioecious plant in which sex is determined by heteromorphic sex chromosomes. In female plants, stamen development is arrested before microspore mother cells are formed. In this study, we isolated four cDNAs (SlSKP1-1 to 4) encoding ASK1-like protein as expression markers to reveal when expression levels are reduced in arrested stamens of female flowers. Expression patterns of the SlSKP1 genes were analyzed by in-situ hybridization. We use the flower development classification of Grant et al. (in Plant J 6:471–480, 1994). SlSKP1 genes were highly expressed in primary parietal cells and primary sporogenous cells that develop into microspore mother cells in male flowers. Expression levels started to be reduced in the external stamens of the female flowers when stamen development was arrested at stage 7. Although microspore mother cells could not be developed in female flowers and SlSKP1 expression may be unnecessary in arrested stamens, SlSKP1 genes were still expressed in sporogenous cells of degenerated stamens at stage 8. Parietal cells stopped differentiating earlier than sporogenous cells in arrested stamens. These results suggest that not all types of cell are arrested simultaneously at a particular stage of stamen development during stamen suppression of S. latifolia.     

Flower development in Abrus precatorius (Leguminosae: Papilionoideae: Abreae) and a review of androecial characters in Papilionoideae

The jequirity bean (Abrus precatorius) is well known because of its shiny black and red coloured seeds and because of the poison (abrin) it contains. The genus Abrus is placed in a monogeneric tribe Abreae which is placed in a relatively isolated systematic position at the base of Millettieae. To contribute to a better understanding of this taxon, a detailed ontogenetic and morphologic analysis of its flowers is presented. Floral primordia are subtended by an abaxial bract and preceded by two lateral bracteoles which are formed in short succession. Sepal formation is unidirectional starting abaxially. All petals are formed simultaneously. The carpel is formed concomitantly with the outer (antesepalous) stamen whorl, which arises unidirectionally, starting in an abaxial position. In the inner, antepetalous stamen whorl two abaxial stamens are formed first, followed by two lateral stamen primordia. The adaxial, antepetalous position remains organ free (i.e. this stamen is lost). Later in development the nine stamen filaments fuse to form an adaxially open sheath. The filament bases of the two adaxial outer-whorl stamens grow inwards, possibly to provide stability and to compensate for the lost stamen. In the mature flower a basal outgrowth can be found in the position of the lost stamen. However this is more likely to be an outgrowth of the filament sheath rather than a remnant of the lost stamen. These ontogenetic patterns match in parts those found in other Millettieae (unidirectional formation of sepals and stamens, simultaneous petal formation). In contrast, the complete loss of a stamen is rather unusual and supports the isolated position of Abreae and probably justifies (among other characters) its tribal status. A review of androecial characters shows that androecial merosity is on the one hand extremely variable among Leguminosae, varying from a single stamen per flower to more than 500. On the other hand it is noteworthy that the number of stamens becomes stabilised in more derived Papilionoideae such as the large non-protein-amino-acid-accumulating clade (NPAAA clade). This indicates that the androecium has played an important role in the success of a major part of Leguminosae.     

FLOWER DEVELOPMENT IN NORMAL TOMATO AND A GIBBERELLIN-DEFICIENT (ga-2) MUTANT

Flower buds of a tomato (Lycopersicon esculentum Mill.) gibberellin-deficient mutant (ga-2/ga-2) were initiated, but did not develop to maturity and eventually aborted. If GA, was applied to a developing inflorescence or stem tip, completion of flower bud development and fruit set occurred. In development of the ga-2 flowers, the corolla and stamens did not elongate and the style was misshapen or extended past the tip of the anthers. Light microscope observations indicated that meiosis of both microsporocytes and megasporocytes did not occur. Cells of the sporogenous layer were initiated, but growth was arrested and they eventually degenerated. The ovary was normal in appearance. However, the megasporocytes degenerated, giving rise to a cavity in the ovule. Thus, although GA is not required for flower initiation in tomato, it is essential for meiosis of the microsporocytes and megasporocytes and elongation of the corolla and stamens.     

海韭菜的花器官发生

运用扫描电镜（SEM）观察了海韭菜（Triglochin maritimum）的花器官发生发育过程。结果表明：海韭菜花发育是典型的单子叶植物发生模式,即两轮花被片、两轮雄蕊和两轮心皮以三基数轮状交替发生,花器官是以向心向顶的方式发生的,未发现“花被片—雄蕊复合原基”。 发育后期雄蕊和与之对生的花被片之间的共同基部可能是相继向上居间生长的结果。花被片轮和雄蕊轮二者之间在发育位置、时间和速率上存在差异,内轮花被片原基和外轮雄蕊原基的不同发育时间和发育速度使得在成熟花中内轮花被片位于外轮雄蕊的内方。观察结果不支持水麦冬属植物的花是退化（或压缩）的花序侧分枝等假花的观点。     

Phyllotaxis and the Initiation of Primordia During Flower Development in Silene

The measured divergence angles between successive primordiain the developing flower were compared with angles expectedon several hypotheses concerning primordial initiation. Theresults lead to the conclusion that the position and sequenceof initiation of the younger sepals is determined by the olderones but that the influence of an older primordium lasts foronly two plastochrons. The petals and carpels are apparentlypositioned by the sepals. The positions of the stamens are consistentwith their king determined by the sepals (antesepalous stamens)or petals (antepctalous stamens), but their sequence of initiationis consistent with its being determined, like the sepals, bythe two youngest primordia. The data indicate that there aretwo sets of factors governing the initiation of the primordiasubsequent to the sepals: one governing the positioning of theprimordia and resembling the factors governing the positionsof axillary buds, and the other governing the sequence of primordiaand resembling the factors which determine the initiation ofleaves. Measurements of the plastochron ratios were used tocalculate the sizes of the sepal, petal and stamen primordiaat initiation. At the moment of initiation the sepal primordiawere about one third, and the petal and stamen primordia aboutone sixth, of the size of the leaf primordia. In its early developmentthe Silene flower therefore resembles a condensed leafy shootwith precocious axillary buds but with primordia which are smallcompared to leaf primordia. Silene coeli-rosa, flower development, primordia, phyllotaxis     

A morphological study of the development of the second inflorescences in strawberry (Fragaria×ananassa Duch.)

To clarify the timing of the differentiation of the first and second inflorescences in strawberry (Fragaria × ananassa Duch.), morphological changes on shoot apices during short day and low night temperature treatments were observed by scanning electron microscopy (SEM) and optical microscopy. Axillary buds just below the first inflorescence (axillary bud 1) became visible when sepal primordia of the primary flower were differentiated. By this time, other axillary buds had already developed. Axillary bud 1 developed four leaf primordia, and then a differentiated inflorescence at its summit. The phase transition of shoot apices from the vegetative to the reproductive phase may therefore trigger the differentiation of axillary bud 1 which is destined to develop into extension crowns.     

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.

Light-induced bud outgrowth in rosebush results from cytokinin-mediated peroxide scavenging and glutathione metabolism stimulation.     

Floral ontogeny and anatomy inKoelreuteria with special emphasis on monosymmetry and septal cavities

The floral ontogeny and anatomy ofKoelreuteria paniculata have been investigated to understand the developmental basis for the occurring monosymmetry and the origin of the septal cavities. Petals arise sequentially and one petal is missing between sepals 3 and 5, or rarely between sepals 2 and 5. The eight stamens arise sequentially before petal initiation is completed. The last formed petal and one stamen arise on a common primordium. Two stamen positions are empty (opposite the petal between the sepals 2 and 5, and the petal between sepal 1 and 3); consequently two antesepalous stamens have become displaced. The derivation of octandry from a diplostemonous ancestry, and reduction of the petal are discussed. The triangular gynoecium has a strong impact in obliquely reorganizing the symmetry of the flower, loss of organs, and shifts of stamens. The so-called septal slits occurring within the style are a deepreaching non-nectariferous extension of the stigma. Alternating locular furrows are present which could play a role as pollen transmitting tissue and in the loculicid dehiscence of the capsule.     

Inflorescence and floral development of Dahlstedtia species (Leguminosae: Papilionoideae: Millettieae)

Inflorescence and floral development of two tropical legume trees, Dahlstedtia pinnata and Dahlstedtia pentaphylla, occurring in the Atlantic Forest of south-eastern and southern Brazil, were investigated and compared with other papilionoids. Few studies have been made of floral development in tribe Millettieae, and this paper is intended to fill that gap in our knowledge. Dahlstedtia species have an unusual inflorescence type among legumes, the pseudoraceme, which comprises axillary units of three or more flowers, each with a subtending bract. Each flower exhibits a pair of opposite bracteoles. The order of flower initiation is acropetal; inception of the floral organs is as follows: sepals (5), petals (5), carpel (1) plus outer stamens (5) and finally inner stamens (5). Organ initiation in sepal, petal and inner stamen whorls is unidirectional; the carpel cleft is adaxial. The vexillum originates from a tubular-shaped primordium in mid-development and is larger than other petals at maturity, covering the keels. The filament tube develops later after initiation of inner-stamen primordia. Floral development in Dahlstedtia is almost always similar to other papilionoids, especially species of Phaseoleae and Sophoreae. But one important difference is the precocious ovule initiation (open carpel with ovules) in Dahlstedtia, the third citation of this phenomenon for papilionoids. No suppression, organ loss or anomalies occur in the order of primordia initiation or structure. Infra-generic differences in the first stages of ontogeny are rare; however, different species of Dahlstedtia are distinguished by the differing distribution pattern of secretory cavities in the flower.     

Roles for Auxin, Cytokinin, and Strigolactone in Regulating Shoot Branching

Many processes have been described in the control of shoot branching. Apical dominance is defined as the control exerted by the shoot tip on the outgrowth of axillary buds, whereas correlative inhibition includes the suppression of growth by other growing buds or shoots. The level, signaling, and/or flow of the plant hormone auxin in stems and buds is thought to be involved in these processes. In addition, RAMOSUS (RMS) branching genes in pea (Pisum sativum) control the synthesis and perception of a long-distance inhibitory branching signal produced in the stem and roots, a strigolactone or product. Auxin treatment affects the expression of RMS genes, but it is unclear whether the RMS network can regulate branching independently of auxin. Here, we explore whether apical dominance and correlative inhibition show independent or additive effects in rms mutant plants. Bud outgrowth and branch lengths are enhanced in decapitated and stem-girdled rms mutants compared with intact control plants. This may relate to an RMS-independent induction of axillary bud outgrowth by these treatments. Correlative inhibition was also apparent in rms mutant plants, again indicating an RMS-independent component. Treatments giving reductions in RMS1 and RMS5 gene expression, auxin transport, and auxin level in the main stem were not always sufficient to promote bud outgrowth. We suggest that this may relate to a failure to induce the expression of cytokinin biosynthesis genes, which always correlated with bud outgrowth in our treatments. We present a new model that accounts for apical dominance, correlative inhibition, RMS gene action, and auxin and cytokinin and their interactions in controlling the progression of buds through different control points from dormancy to sustained growth.     

Gibberellin and auxin signaling genes RGA1 and ARF8 repress accessory fruit initiation in diploid strawberry

Unlike ovary-derived botanical fruits, strawberry (Fragaria x ananassa) is an accessory fruit derived from the receptacle, the stem tip subtending floral organs. Although both botanical and accessory fruits initiate development in response to auxin and gibberellic acid (GA) released from seeds, the downstream auxin and GA signaling mechanisms underlying accessory fruit development are presently unknown. We characterized GA and auxin signaling mutants in wild strawberry (Fragaria vesca) during early stage fruit development. While mutations in FveRGA1 and FveARF8 both led to the development of larger fruit, only mutations in FveRGA1 caused parthenocarpic fruit formation, suggesting FveRGA1 is a key regulator of fruit set. FveRGA1 mediated fertilization-induced GA signaling during accessory fruit initiation by repressing the expression of cell division and expansion genes and showed direct protein–protein interaction with FveARF8. Further, fvearf8 mutant fruits exhibited an enhanced response to auxin or GA application, and the increased response to GA was due to increased expression of FveGID1c coding for a putative GA receptor. The work reveals a crosstalk mechanism between FveARF8 in auxin signaling and FveGID1c in GA signaling. Together, our work provides functional insights into hormone signaling in an accessory fruit, broadens our understanding of fruit initiation in different fruit types, and lays the groundwork for future improvement of strawberry fruit productivity and quality.

An investigation of the mechanism of accessory fruit initiation in diploid strawberry, identifying the function of two hormone signaling genes in fruit initiation.     

Vegetative axillary bud dormancy induced by shade and defoliation signals in the grasses

Vegetative axillary bud dormancy and outgrowth is regulated by several hormonal and environmental signals. In perennials, the dormancy induced by hormonal and environmental signals has been categorized as eco-, endo- or para-dormancy. Over the past several decades para-dormancy has primarily been investigated in eudicot annuals. Recently, we initiated a study using the monoculm phyB mutant (phyB-1) and the freely branching near isogenic wild type (WT) sorghum (Sorghum bicolor) to identify molecular mechanisms and signaling pathways regulating dormancy and outgrowth of axillary buds in the grasses. In a paper published in the January 2010 issue of Plant Cell and Environment, we reported the role of branching genes in the inhibition of bud outgrowth by phyB, shade and defoliation signals. Here we present a model that depicts the molecular mechanisms and pathways regulating axillary bud dormancy induced by shade and defoliation signals in the grasses.Key words: axillary bud, dormancy, shade, phytochrome, defoliation, shoot branching, teosinte branched1, MAX2, cell cycle, sorghumThe dormancy and outgrowth of axillary buds is regulated by several plant hormones such as auxin, cytokinins, abscisic acid and strigolactones, and by environmental factors such as light quality, quantity and duration as well as water, temperature and nutrient status.^1–3 Since the fate of an axillary bud is regulated by such diverse hormonal and environmental signals and their interactions, the type of dormancy induced varies. In perennials, three types of bud dormancy have been identified.^4,5 Dormancy mediated by factors within the bud is known as endo-dormancy; while dormancy induced by factors within the plant but outside the bud is called paradormancy or correlative inhibition; the best known example being apical dominance. Dormancy induced due to unfavorable environmental conditions is known as eco-dormancy. Although there is an indepth knowledge about para-dormancy in annuals,⁶ few studies have been conducted on eco-dormancy. Similarly, studies of endo-dormancy have largely been restricted to low-temperature mediated growth-cessation of axillary buds of perennial plants.^7,8 To understand the regulation of dormancy and outgrowth of axillary buds in monocots, we initiated a study on the molecular mechanisms inhibiting bud outgrowth by shade and defoliation signals in sorghum. Our results published in the January 2010 issue of Plant, Cell & Environment indicate that different types of dormancy may be induced in axillary buds of annual grasses by various signals and there may be overlapping and independent molecular mechanisms mediating induction of axillary bud dormancy.