The rolD oncogene promotes axillary bud and adventitious root meristems in Arabidopsis

The expression of the Agrobacterium rhizogenes rolD oncogene induces precocious floral transition and strong flowering potential in tobacco and tomato. Here, we describe specific developmental effects induced by expression of rolD in Arabidopsis. We show that floral transition, as histologically monitored, occurred in rolD- plants earlier than in wild type, and this was coupled with a premature and enhanced formation of vegetative and reproductive axillary bud meristems. Furthermore, CYP79F1/SUPERSHOOT/BUSHY (SPS), a gene that negatively controls shoot branching in Arabidopsis and involved in glucosinolate metabolism and in cytokinin and auxin homeostasis, was down-regulated in rolD plants. The multiplication of post-embryonic meristems was also observed in the root system, with enhanced adventitious root formation. This result was confirmed by thin cell layer response in vitro, both under hormone-free and standard rooting conditions. However, the formation of lateral root meristems was not affected by rolD expression. Our results show that rolD accelerates and enhances specific post-embryonic meristems in Arabidopsis.     

Dynamic and compensatory responses of Arabidopsis shoot and floral meristems to CLV3 signaling

In Arabidopsis thaliana, the stem cell population of the shoot system is controlled by regulatory circuitry involving the WUSCHEL (WUS) and CLAVATA (CLV1-3) genes. WUS signals from the organizing center (OC) to promote stem cell fate at the meristem apex. Stem cells express the secreted peptide CLV3 that activates a signal transduction cascade to restrict WUS expression, thus providing a feedback mechanism. Stem cell homeostasis is proposed to be achieved by balancing these signals. We tested the dynamics of CLV3 signaling using an inducible gene expression system. We show here that increasing the CLV3 signal can very rapidly repress WUS expression during development, which in turn causes a fast reduction of CLV3 expression. We demonstrate that increased CLV3 signaling restricts meristem growth and promotes allocation of peripheral meristem cells into organ primordia. In addition, we extend the current model for stem cell control by showing that meristem homeostasis tolerates variation in CLV3 levels over a 10-fold range and that high-level CLV3 signaling can be partially compensated with time, indicating that the level of CLV3 expression communicates only limited information on stem cell number to the underlying OC cells.     

Termination of stem cell maintenance in Arabidopsis floral meristems by interactions between WUSCHEL and AGAMOUS.

Floral meristems and shoot apical meristems (SAMs) are homologous, self-maintaining stem cell systems. Unlike SAMs, floral meristems are determinate, and stem cell maintenance is abolished once all floral organs are initiated. To investigate the underlying regulatory mechanisms, we analyzed the interactions between WUSCHEL (WUS), which specifies stem cell identity, and AGAMOUS (AG), which is required for floral determinacy. Our results show that repression of WUS by AG is essential for terminating the floral meristem and that WUS can induce AG expression in developing flowers. Together, this suggests that floral determinacy depends on a negative autoregulatory mechanism involving WUS and AG, which terminates stem cell maintenance.     

Elusive but not hypothetical: axillary meristems in Wollemia nobilis

BACKGROUND: The branches of Wollemia nobilis are unbranched; however, it has been noted that new branches can form from the distal end of damaged ones, and branches can grow from axillary structures once a terminal strobilus has fallen. Tomlinson and Huggett (2011, Annals of Botany 107: 909-916) have recently investigated the formation of these reiterative branches and stated in the title of their paper that 'Partial shoot reiteration in Wollemia nobilis (Araucariaceae) does not arise from "axillary meristems"'. They go on to state 'Further research may reveal the presence of these elusive, but still only hypothetical, axillary meristems'. RESPONSE: In this Viewpoint, I argue that Tomlinson and Huggett do not refer to previously published information that indicates that axillary meristems are present in Wollemia nobilis branch leaf axils, and that their anatomical methods were probably not optimal for locating and examining these minute structures. Thus, whilst I would agree that the axillary meristems in branch leaf axils of Wollemia nobilis are elusive, I contend that they are not hypothetical.     

Localization of axillary meristems during different stages of radish ontogenesis

An analysis of axillary meristem (axillary bud) localization of radish (Raphanus sativus L. cv. Tetra-I?ówiecka) was undertaken on vernalized (flowering) and unvernalized (vegetative) plants. It has been shown that the localization of these meristems can be different on successive nodes of the same plant and is connected with the development stages of the plants. The axillary meristems can arise on the stem as well as in the leaf axil or on the base of the subtending leaf. The localization of axillary meristems has been discussed in relation to growth directions and growth correlations inside the meristematic region of the shoot apex.     

Mechanisms of floral repression in Arabidopsis

In the past two years, several early-flowering genes have been shown to encode putative chromatin-associated proteins in Arabidopsis. These proteins probably function as epigenetic silencers that repress the promotion of flowering and flower organ identity genes, and thereby maintain vegetative growth. As the plant matures, levels of the floral promoters increase despite the continued presence of floral repressors. High levels of the floral promoters are somehow able to overcome floral repression and to activate flower development. Further characterization of mutants that have impairments in either floral promoters or floral repressors revealed that these mutants not only display defects in flowering time but also have altered inflorescence architectures. These findings indicate that these flowering genes also regulate other aspects of shoot development and may be used to study the mechanism of shoot growth pattern.     

The origin, initiation and development of axillary shoot meristems in Lotus japonicus

BACKGROUND AND AIMS: Lotus japonicus 'Gifu' develops multiple axillary shoots in the cotyledonary node region throughout the growth of the plant. The origin, initiation and development of these axillary meristems were investigated. METHODS: Morphological, histological and mRNA in situ analyses were done to characterize the ontogeny of cotyledonary axillary shoot meristems in Lotus. Morphological characterization of a putative Lotus shoot branching mutant (super-accessory branches) sac, is presented. KEY RESULTS: By using expression of an L. japonicus STM-like gene as a marker for meristematic tissues, it was demonstrated that groups of cells maintained in the meristematic state at the cotyledonary axil region coincide with the sites where additional axillary meristems (accessory meristems) form. A Lotus shoot branching mutant, sac, is a putative Lotus branching mutant characterized by increased proliferation of accessory shoots in all leaf axils including the cotyledons. CONCLUSION: In Lotus, axillary shoot meristems continually develop at the cotyledonary node region throughout the growth of the plant. These cotyledonary primary and accessory axillaries arise from the position of a meristematic zone of tissue at the cotyledonary node axil region.     

Specification of reproductive meristems requires the combined function of SHOOT MERISTEMLESS and floral integrators FLOWERING LOCUS T and FD during Arabidopsis inflorescence development

In Arabidopsis floral meristems are specified on the periphery of the inflorescence meristem by the combined activities of the FLOWERING LOCUS T (FT)-FD complex and the flower meristem identity gene LEAFY. The floral specification activity of FT is dependent upon two related BELL1-like homeobox (BLH) genes PENNYWISE (PNY) and POUND-FOOLISH (PNF) which are required for floral evocation. PNY and PNF interact with a subset of KNOTTED1-LIKE homeobox proteins including SHOOT MERISTEMLESS (STM). Genetic analyses show that these BLH proteins function with STM to specify flowers and internodes during inflorescence development. In this study, experimental evidence demonstrates that the specification of flower and coflorescence meristems requires the combined activities of FT-FD and STM. FT and FD also regulate meristem maintenance during inflorescence development. In plants with reduced STM function, ectopic FT and FD promote the formation of axillary meristems during inflorescence development. Lastly, gene expression studies indicate that STM functions with FT-FD and AGAMOUS-LIKE 24 (AGL24)-SUPPRESSOR OF OVEREXPRESSION OF CONTANS1 (SOC1) complexes to up-regulate flower meristem identity genes during inflorescence development.     

Two lily SEPALLATA-like genes cause different effects on floral formation and floral transition in Arabidopsis

Two AGL2-like MADS-box genes, Lily MADS Box Gene (LMADS) 3 and LMADS4, with extensive homology of LMADS3 to the Arabidopsis SEPALLATA3 were characterized from the lily (Lilium longiflorum). Both LMADS3 and LMADS4 mRNA were detected in the inflorescence meristem, in floral buds of different developmental stages, and in all four whorls of the flower organ. LMADS4 mRNA is also expressed in vegetative leaf and in the inflorescence stem where LMADS3 expression is absent. Transgenic Arabidopsis, which ectopically expresses LMADS3, showed novel phenotypes by significantly reducing plant size, flowering extremely early, and loss of floral determinacy. By contrast, 35S::LMADS4 transgenic plants were morphologically indistinguishable from wild-type plants. The early-flowering phenotype in 35S::LMADS3 transgenic Arabidopsis plants was correlated with the up-regulation of flowering time genes FT, SUPPRESSOR OF OVEREXPRESSION OF CO 1, LUMINIDEPENDENS, and flower meristem identity genes LEAFY and APETALA1. This result was further supported by the ability of 35S::LMADS3 to rescue the late-flowering phenotype in gigantea-1 (gi-1), constans-3 (co-3), and luminidependens-1 but not for ft-1 or fwa-1 mutants. The activation of these flowering time genes is, however, indirect because their expression was unaffected in plants transformed with LMADS3 fused with rat glucocorticoid receptor in the presence of both dexamethasone and cycloheximide.     

LEAFY controls floral meristem identity in Arabidopsis.

The first step in flower development is the generation of a floral meristem by the inflorescence meristem. We have analyzed how this process is affected by mutant alleles of the Arabidopsis gene LEAFY. We show that LEAFY interacts with another floral control gene, APETALA1, to promote the transition from inflorescence to floral meristem. We have cloned the LEAFY gene, and, consistent with the mutant phenotype, we find that LEAFY RNA is expressed strongly in young flower primordia. LEAFY expression procedes expression of the homeotic genes AGAMOUS and APETALA3, which specify organ identify within the flower. Furthermore, we demonstrate that LEAFY is the Arabidopsis homolog of the FLORICAULA gene, which controls floral meristem identity in the distantly related species Antirrhinum majus.     

Genes WHEAT FRIZZY PANICLE and SHAM RAMIFICATION 2 independently regulate differentiation of floral meristems in wheat

Background

Inflorescences of wheat species, spikes, are characteristically unbranched and bear one sessile spikelet at a spike rachis node. Development of supernumerary spikelets (SSs) at rachis nodes or on the extended rachillas is abnormal. Various wheat morphotypes with altered spike morphology, associated with the development of SSs, present an important genetic resource for studies on genetic regulation of wheat inflorescence development.

Results

Here we characterized diploid and tetraploid wheat lines of various non-standard spike morphotypes, which allowed for identification of a new mutant allele of the WHEAT FRIZZY PANICLE (WFZP) gene that determines spike branching in diploid wheat Ttiticum monococcum L. Moreover, we found that the development of SSs and spike branching in wheat T. durum Desf. was a result of a wfzp-A/TtBH-A1 mutation that originated from spontaneous hybridization with T. turgidum convar. сompositum (L.f.) Filat. Detailed characterization of the false-true ramification phenotype controlled by the recessive sham ramification 2 (shr2) gene in tetraploid wheat T. turgidum L. allowed us to suggest putative functions of the SHR2 gene that may be involved in the regulation of spikelet meristem fate and in specification of floret meristems. The results of a gene interaction test suggested that genes WFZP and SHR2 function independently in different processes during spikelet development, whereas another spike ramification gene(s) interact(s) with SHR2 and share(s) common functions.

Conclusions

SS mutants represent an important genetic tool for research on the development of the wheat spikelet and for identification of genes that control meristem activities. Further studies on different non-standard SS morphotypes and wheat lines with altered spike morphology will allow researchers to identify new genes that control meristem identity and determinacy, to elucidate the interaction between the genes, and to understand how these genes, acting in concert, regulate the development of the wheat spike.

     

Presumable role of plasmodesmata in floral signal transduction in shoot apical meristems of rudbeckia and perilla plants

The number of plasmodesmata was calculated per 1 μm of cell wall length in the central and medullar zones of shoot apical meristems (SAM) in the course of floral transition in a long-day (LD) plant Rudbeckia bicolor Nutt. and a short-day plant Perilla nankinensis Lour. Under the day length unfavorable for flowering (control), the numbers of plasmodesmata differed in the central and medullar zones of SAM, which produce the reproductive organs and stems, respectively. Besides, the numbers of plasmodesmata in the central zone of perilla SAM considerably differed between the anticlinal and periclinal cell walls of the first and second cell layers. Following the photoperiodic induction (PI) with eight LD in rudbeckia and twelve SD in perilla favorable for floral transition, the numbers of plasmodesmata considerably increased in the anticlinal and periclinal cell walls of the first and second cell layers of the central zone; meanwhile in the medullar zone, the numbers of plasmodesmata dropped down following PI. These data show that floral transition presumably involves the activation of cell-to-cell interactions and enhances the signal transduction in SAM.