BELL1-like homeobox genes regulate inflorescence architecture and meristem maintenance in rice.

Inflorescence architecture is diverse in angiosperms, and is mainly determined by the arrangement of the branches and flowers, known as phyllotaxy. In rice (Oryza sativa), the main inflorescence axis, called the rachis, generates primary branches in a spiral phyllotaxy, and flowers (spikelets) are formed on these branches. Here, we have studied a classical mutant, named verticillate rachis (ri), which produces branches in a partially whorled phyllotaxy. Gene isolation revealed that RI encodes a BELL1-type homeodomain transcription factor, similar to Arabidopsis PENNYWISE/BELLRINGER/REPLUMLESS, and is expressed in the specific regions within the inflorescence and branch meristems where their descendant meristems would soon initiate. Genetic combination of an ri homozygote and a mutant allele of RI-LIKE1 (RIL1) (designated ri ril1/+ plant), a close paralog of RI, enhanced the ri inflorescence phenotype, including the abnormalities in branch phyllotaxy and rachis internode patterning. During early inflorescence development, the timing and arrangement of primary branch meristem (pBM) initiation were disturbed in both ri and ri ril1/+ plants. These findings suggest that RI and RIL1 were involved in regulating the phyllotactic pattern of the pBMs to form normal inflorescences. In addition, both RI and RIL1 seem to be involved in meristem maintenance, because the ri ril1 double-mutant failed to establish or maintain the shoot apical meristem during embryogenesis.

Epidermal identity is maintained by cell-cell communication via a universally active feedback loop in Arabidopsis thaliana.

The transcription factors ARABIDOPSIS THALIANA MERISTEM L1 (ATML1) and PROTODERMAL FACTOR2 (PDF2) are indispensable for epidermal cell-fate specification in Arabidopsis embryos. However, the mechanisms of regulation of these genes, particularly their relationship with cell-cell signalling pathways, although the subject of considerable speculation, remain unclear. Here we demonstrate that the receptor kinase ARABIDOPSIS CRINKLY4 (ACR4) positively affects the expression of ATML1 and PDF2 in seedlings. In contrast, ATML1- and PDF2-containing complexes directly and negatively affect both their own expression and that of ACR4. By modelling the resulting feedback loop, we demonstrate a network structure that is capable of maintaining robust epidermal cell identity post-germination. We show that a second seed-specific signalling pathway involving the subtilase ABNORMAL LEAFSHAPE1 (ALE1) and the receptor kinases GASSHO1 (GSO1) and GASSHO2 (GSO2) acts in parallel to the epidermal loop to control embryonic surface formation via an ATML1/PDF2-independent pathway. Genetic interactions between components of this linear pathway and the epidermal loop suggest that an intact embryo surface is necessary for initiation and/or stabilization of the epidermal loop, specifically during early embryogenesis.

BARREN STALK FASTIGIATE1 is an AT-hook protein required for the formation of maize ears.

Ears are the seed-bearing inflorescences of maize (Zea mays) plants and represent a crucial component of maize yield. The first step in the formation of ears is the initiation of axillary meristems in the axils of developing leaves. In the classic maize mutant barren stalk fastigiate1 (baf1), first discovered in the 1950s, ears either do not form or, if they do, are partially fused to the main stalk. We positionally cloned Baf1 and found that it encodes a transcriptional regulator containing an AT-hook DNA binding motif. Single coorthologs of Baf1 are found in syntenic regions of brachypodium (Brachypodium distachyon), rice (Oryza sativa), and sorghum (Sorghum bicolor), suggesting that the gene is likely present in all cereal species. Protein-protein interaction assays suggest that BAF1 is capable of forming homodimers and heterodimers with other members of the AT-hook family. Another transcriptional regulator required for ear initiation is the basic helix-loop-helix protein BARREN STALK1 (BA1). Genetic and expression analyses suggest that Baf1 is required to reach a threshold level of Ba1 expression for the initiation of maize ears. We propose that Baf1 functions in the demarcation of a boundary region essential for the specification of a stem cell niche.

[Morphogenetic lability of reproductive structures in Ruppia maritima (Ruppiaceae, Alismatales): from two lateral flowers to a terminal flower].

Flowers of Ruppia are normally arranged into an open two-flowered spike, but sometimes the two lateral flowers are congenitally united with each other and form a terminal flower-like structure. This developmental abnormality resembles those described in well-investigated mutants of model organisms of developmental genetics such as Arabidopsis Antirrhinum. A study of Ruppia allows investigating morphogenetic lability of this feature in natural populations. These data will be important for understanding evolutionary transitions between open and closed inflorescences. This paper presents first data on frequencies ofterminal flower-like structures in natural populations of Ruppia maritima and first observations of their development. Vascular supply of inflorescences with free and united flowers is compared for the first time. Strong differences in frequencies of occurrence of terminal flower-like structures among examined natural populations are revealed. Data on variation of organ numbers in flowers of plants from different populations allow hypothesizing that increased size of floral primordia is a factor that plays a role in their amalgamation into ajoint primordium of a terminal structure. Vascular system of inflorescences of R. maritima with united flowers is quite similar to the vascular system of a flower and nothing contradicts a hypothesis on terminal position ofthis structure. Transversally inserted stamens in inflorescences with united flowers are usually of inverted polarity. This appears to be the first documented example of an inversion of relative polarity of stamens and carpels in angiosperms.

[Interaction between the ABRUPTUS/PINOID and APETALA1 genes regulating the inflorescence development in Arabidopsis thaliana].

Plant morphology was analyzed in the abr and ap1-1 single mutants, the abr ap1-1 double mutant, and abr mutant plants with extopic expression of the AP1 gene under the control of the 35S RNA constitutive promoter of the cauliflower mosaic virus (abr 35S::AP1). The level of AP1 gene expression was examined in wildtype plants and the abr mutant. The ABR gene was found to interact with the AP1 via dominant epistasis when determining the time of a transition to the reproductive developmental stage and the floral meristem identity. The abr mutant displayed a higher level of AP1 transcription and extended regions of its transcription in leaves and internal whorls of the flowers. Based on these findings, the ABR gene was assumed to play an indirect role in restricting the level and spatial range of AP1 transcription. A complementary interaction of the dominant alleles was observed during floral development, implicating both of the genes in the process.

Involvement of ribosomal protein RPL27a in meristem activity and organ development.

The plant shoot apical meristem is established early during embryogenesis and subsequently gives rise to a shoot through reiterative generation of lateral organs and axillary meristems. In our recent manuscript we reported identification and characterization of a semi-dominant mutation in ribosomal protein RPL27a, which disrupts plant growth and shoot development.1 rpl27ac-1d effects on the shoot are evident from an early stage of embryo development. During embryogenesis rpl27-1d mutants are slow growing and are defective in apical patterning with a delay in establishment of the shoot meristem and outgrowth of cotyledons. Concomitant with this disturbed patterning, the shoot meristem genes SHOOT MERISTEMLESS (STM) and CUP-SHAPED COTYLEDON2 (CUC2) are misexpressed in outer cell layers of the rpl27ac-1d embryo and there is a delay in expression of the organ-patterning gene FILAMENTOUS FLOWER (FIL). Genetic interactions between rpl27ac-1d and other ribosomal protein mutants indicates rpl27ac-1d has reduced ribosome function. Our results highlight a role for ribosomal proteins in growth and development and we propose that the ribosome regulates specific patterning events during development.