BROTHER OF FT AND TFL1 (BFT) has TFL1‐like activity and functions redundantly with TFL1 in inflorescence meristem development in Arabidopsis

The FLOWERING LOCUS T (FT)/TERMINAL FLOWER 1 (TFL1) family is a small gene family that encodes important regulators that control flower development in Arabidopsis. Here, we investigated the biological role of the product of BROTHER OF FT AND TFL1 (BFT), a member of this family, whose function remains unknown. Comparison of the critical residues that play a role in distinguishing FT‐ or TFL1‐like activity revealed that BFT is more similar to FT. Similar to FT expression, BFT expression showed a diurnal oscillation pattern, peaking in the evening. In situ hybridization revealed BFT expression in the shoot apical meristem, young leaf and axillary inflorescence meristem. Transgenic plants over‐expressing BFT exhibited delayed flowering and severe floral defects (floral indeterminacy and compact inflorescences surrounded by serrate leaves), similar to 35S::TFL1 plants. LEAFY (LFY) and APETALA1 (AP1) expression was significantly reduced in 35S::BFT plants. BFT over‐expression failed to rescue the terminal flower phenotype of tfl1 mutants; however, it delayed both terminal flower formation in the primary inflorescence and axillary inflorescence development in the tfl1 mutant background. Consistent with this, the loss‐of‐function BFT alleles, bft‐2 and an BFT RNAi line, accelerated termination of the primary inflorescence and formation of axillary inflorescences in the tfl1 mutant background. Taken together, our results suggest that, despite similarities in the critical residues of BFT and FT, BFT possesses a TFL1‐like activity and functions redundantly with TFL1 in inflorescence meristem development, and possibly contributes to the regulation of plant architecture.     

Identification of a novel mutant spp1 that specifies the identity of inflorescence meristem in rice

Abstract

Homeotic mutations can affect the genetic architecture of flower structure. Some genes have been identified that specify shoot and floral meristem development. ABCDE and floral quartet are two widely accepted models that explain how genes interact to form all the whorls of a flower. In the current study, we identified an spp1 (SUPER PISTIL 1) mutant controlled by a single recessive nuclear gene mapping to chromosome 6 near RM50. Compared to wild type, spp1 mutants showed similar agronomic traits, except for panicle length and 1000-grain weight, which were significantly lower in the spp1 mutant plants. The mutation in the SPP1 gene induced complete homeotic transformations of paleae, lodicules, and stamens into carpelloid structures. Although lemmata were only marginally affected in the spp1 mutants, they developed a stigma-like structure on the top instead of an awn. Interestingly, stigma-like structures were also observed at the base of panicle branches. From the results, we propose that the SPP1 gene plays an important role in specifying the identity of lemmata/paleae, lodicules, stamens, and inflorescence meristem in rice. Identification of this mutant not only provides further evidence for validity of the ABCDE model in monocots, but also contributes to the understanding of meristem development.     

An APETALA3 homolog controls both petal identity and floral meristem patterning in Nigella damascena L. (Ranunculaceae)

Flower architecture mutants provide a unique opportunity to address the genetic origin of flower diversity. Here we study a naturally occurring floral dimorphism in Nigella damascena (Ranunculaceae), involving replacement of the petals by numerous sepal‐like and chimeric sepal/stamen organs. We performed a comparative study of floral morphology and floral development, and characterized the expression of APETALA3 and PISTILLATA homologs in both morphs. Segregation analyses and gene silencing were used to determine the involvement of an APETALA3 paralog (NdAP3–3) in the floral dimorphism. We demonstrate that the complex floral dimorphism is controlled by a single locus, which perfectly co‐segregates with the NdAP3–3 gene. This gene is not expressed in the apetalous morph and exhibits a particular expression dynamic during early floral development in the petalous morph. NdAP3–3 silencing in petalous plants perfectly phenocopies the apetalous morph. Our results show that NdAP3–3 is fully responsible for the complex N. damascena floral dimorphism, suggesting that it plays a role not only in petal identity but also in meristem patterning, possibly through regulation of perianth organ number and the perianth/stamen boundary.     

Populus CEN/TFL1 regulates first onset of flowering,axillary meristem identity and dormancy release in Populus

Members of the CENTRORADIALIS (CEN)/TERMINAL FLOWER 1 (TFL1) subfamily control shoot meristem identity, and loss‐of‐function mutations in both monopodial and sympodial herbaceous plants result in dramatic changes in plant architecture. We studied the degree of conservation between herbaceous and woody perennial plants in shoot system regulation by overexpression and RNA interference (RNAi)‐mediated suppression of poplar orthologs of CEN, and the related gene MOTHER OF FT AND TFL 1 (MFT). Field study of transgenic poplars (Populus spp.) for over 6 years showed that downregulation of PopCEN1 and its close paralog, PopCEN2, accelerated the onset of mature tree characteristics, including age of first flowering, number of inflorescences and proportion of short shoots. Surprisingly, terminal vegetative meristems remained indeterminate in PopCEN1‐RNAi trees, suggesting the possibility that florigen signals are transported to axillary mersitems rather than the shoot apex. However, the axillary inflorescences (catkins) of PopCEN1‐RNAi trees contained fewer flowers than did wild‐type catkins, suggesting a possible role in maintaining the indeterminacy of the inflorescence apex. Expression of PopCEN1 was significantly correlated with delayed spring bud flush in multiple years, and in controlled environment experiments, 35S::PopCEN1 and RNAi transgenics required different chilling times to release dormancy. Considered together, these results indicate that PopCEN1/PopCEN2 help to integrate shoot developmental transitions that recur during each seasonal cycle with the age‐related changes that occur over years of growth.     

SQUAMOSA‐PROMOTER BINDING PROTEIN 1 initiates flowering in Antirrhinum majus through the activation of meristem identity genes

The degree to which developmental genetic pathways are conserved across distantly related organisms is a major question in biology. In Arabidopsis thaliana (L.) Heynh., inflorescence development is initiated in response to a combination of external and internal floral inductive signals that are perceived across the whole plant, but are integrated within the shoot apical meristem. Recently, it was demonstrated that SQUAMOSA‐PROMOTER BINDING PROTEIN (SBP)‐box proteins regulate A. thaliana flowering time by mediating signals from the autonomous and photoperiod pathways, and by directly activating key genes involved in inflorescence and floral meristem identity, including FRUITFULL (FUL), APETALA1 (AP1) and LEAFY (LFY). In the distantly related core eudicot species Antirrhinum majus L., paralogous SBP‐box proteins SBP1 and SBP2 have likewise been implicated in regulating the AP1 ortholog SQUAMOSA (SQUA). To test the hypothesis that SBP‐box genes are also involved in the floral induction of A. majus, we used a reverse genetic approach to silence SBP1. SBP1‐silenced lines are late to nonflowering, and show reduced apical dominance. Furthermore, expression and sequence analyses suggest that the SBP1‐mediated transition to flowering occurs through the positive regulation of FUL/LFY homologs. Together, these data outline the utility of virus‐induced gene silencing in A. majus, and provide new insight into the conservation of flowering time genetic pathways across core eudicots.     

Dwarf and deformed flower 1, encoding an F‐box protein,is critical for vegetative and floral development in rice (Oryza sativa L.)

Recent studies have shown that F‐box proteins constitute a large family in eukaryotes, and play pivotal roles in regulating various developmental processes in plants. However, their functions in monocots are still obscure. In this study, we characterized a recessive mutant dwarf and deformed flower 1‐1 (ddf1‐1) in Oryza sativa (rice). The mutant is abnormal in both vegetative and reproductive development, with significant size reduction in all organs except the spikelet. DDF1 controls organ size by regulating both cell division and cell expansion. In the ddf1‐1 spikelet, the specification of floral organs in whorls 2 and 3 is altered, with most lodicules and stamens being transformed into glume‐like organs and pistil‐like organs, respectively, but the specification of lemma/palea and pistil in whorls 1 and 4 is not affected. DDF1 encodes an F‐box protein anchored in the nucleolus, and is expressed in almost all vegetative and reproductive tissues. Consistent with the mutant floral phenotype, DDF1 positively regulates B‐class genes OsMADS4 and OsMADS16, and negatively regulates pistil specification gene DL. In addition, DDF1 also negatively regulates the Arabidopsis LFY ortholog APO2, implying a functional connection between DDF1 and APO2. Collectively, these results revealed that DDF1, as a newly identified F‐box gene, is a crucial genetic factor with pleiotropic functions for both vegetative growth and floral organ specification in rice. These findings provide additional insights into the molecular mechanism controlling monocot vegetative and reproductive development.     

A VIGS screen identifies immunity in the Arabidopsis Pla‐1 accession to viruses in two different genera of the Geminiviridae

Geminiviruses are DNA viruses that cause severe crop losses in different parts of the world, and there is a need for genetic sources of resistance to help combat them. Arabidopsis has been used as a source for virus‐resistant genes that derive from alterations in essential host factors. We used a virus‐induced gene silencing (VIGS) vector derived from the geminivirus Cabbage leaf curl virus (CaLCuV) to assess natural variation in virus–host interactions in 190 Arabidopsis accessions. Silencing of CH‐42, encoding a protein needed to make chlorophyll, was used as a visible marker to discriminate asymptomatic accessions from those showing resistance. There was a wide range in symptom severity and extent of silencing in different accessions, but two correlations could be made. Lines with severe symptoms uniformly lacked extensive VIGS, and lines that showed attenuated symptoms over time (recovery) showed a concomitant increase in the extent of VIGS. One accession, Pla‐1, lacked both symptoms and silencing, and was immune to wild‐type infectious clones corresponding to CaLCuV or Beet curly top virus (BCTV), which are classified in different genera in the Geminiviridae. It also showed resistance to the agronomically important Tomato yellow leaf curl virus (TYLCV). Quantitative trait locus mapping of a Pla‐1 X Col‐0 F₂ population was used to detect a major peak on chromosome 1, which is designated gip‐1 (geminivirus immunity Pla‐1‐1). The recessive nature of resistance to CaLCuV and the lack of obvious candidate genes near the gip‐1 locus suggest that a novel resistance gene(s) confers immunity.     

Roles of BdUNICULME4 and BdLAXATUM‐A in the non‐domesticated grass Brachypodium distachyon

In cultivated grasses, tillering, spike architecture and seed shattering represent major agronomical traits. In barley, maize and rice, the NOOT‐BOP‐COCH‐LIKE (NBCL) genes play important roles in development, especially in ligule development, tillering and flower identity. However, compared with dicots, the role of grass NBCL genes is underinvestigated. To better understand the role of grass NBCLs and to overcome any effects of domestication that might conceal their original functions, we studied TILLING nbcl mutants in the non‐domesticated grass Brachypodium distachyon. In B. distachyon, the NBCL genes BdUNICULME4 (CUL4) and BdLAXATUM‐A (LAXA) are orthologous, respectively, to the barley HvUniculme4 and HvLaxatum‐a, to the maize Zmtassels replace upper ears1 and Zmtassels replace upper ears2 and to the rice OsBLADE‐ON‐PETIOLE1 and OsBLADE‐ON‐PETIOLE2/3. In B. distachyon, our reverse genetics study shows that CUL4 is not essential for the establishment of the blade–sheath boundary but is necessary for the development of the ligule and auricles. We report that CUL4 also exerts a positive role in tillering and a negative role in spikelet meristem activity. On the other hand, we demonstrate that LAXA plays a negative role in tillering, positively participates in spikelet development and contributes to the control of floral organ number and identity. In this work, we functionally characterized two new NBCL genes in a context of non‐domesticated grass and highlighted original roles for grass NBCL genes that are related to important agronomical traits.     

Subcellular dynamics of proteins and metabolites under abiotic stress reveal deferred response of the Arabidopsis thaliana hexokinase‐1 mutant gin2‐1 to high light

Stress responses in plants imply spatio‐temporal changes in enzymes and metabolites, including subcellular compartment‐specific re‐allocation processes triggered by sudden changes in environmental parameters. To investigate interactions of primary metabolism with abiotic stress, the gin2‐1 mutant, defective in the sugar sensor hexokinase 1 (HXK1) was compared with its wildtype Landsberg erecta (Ler) based on time resolved, compartment‐specific metabolome and proteome data obtained over a full diurnal cycle. The high light sensitive gin2‐1 mutant was substantially delayed in subcellular re‐distribution of metabolites upon stress, and this correlated with a massive reduction in proteins belonging to the ATP producing electron transport chain under high light, while fewer changes occurred in the cold. In the wildtype, compounds specifically protecting individual compartments could be identified, e.g., maltose and raffinose in plastids, myo‐inositol in mitochondria, but gin2‐1 failed to recruit these substances to the respective compartments, or responded only slowly to high irradiance. No such delay was obtained in the cold. At the whole cell level, concentrations of the amino acids, glycine and serine, provided strong evidence for an important role of the photorespiratory pathway during stress exposure, and different subcellular allocation of serine may contribute to the slow growth of the gin2‐1 mutant under high irradiance.     

Role of the gibberellin receptors GID1 during fruit‐set in Arabidopsis

Gibberellins (GAs) play a critical role in fruit‐set and fruit growth. Gibberellin is perceived by its nuclear receptors GA INSENSITIVE DWARF1s (GID1s), which then trigger degradation of downstream repressors DELLAs. To understand the role of the three GA receptor genes (GID1A, GID1B and GID1C) in Arabidopsis during fruit initiation, we have examined their temporal and spatial localization, in combination with analysis of mutant phenotypes. Distinct expression patterns are revealed for each GID1: GID1A is expressed throughout the whole pistil, while GID1B is expressed in ovules, and GID1C is expressed in valves. Functional study of gid1 mutant combinations confirms that GID1A plays a major role during fruit‐set and growth, whereas GID1B and GID1C have specific roles in seed development and pod elongation, respectively. Therefore, in ovules, GA perception is mediated by GID1A and GID1B, while GID1A and GID1C are involved in GA perception in valves. To identify tissue‐specific interactions between GID1s and DELLAs, we analyzed spatial expression patterns of four DELLA genes that have a role in fruit initiation (GAI, RGA, RGL1 and RGL2). Our data suggest that GID1A can interact with RGA and GAI in all tissues, whereas GID1C–RGL1 and GID1B–RGL2 interactions only occur in valves and ovules, respectively. These results uncover specific functions of each GID1–DELLA in the different GA‐dependent processes that occur upon fruit‐set. In addition, the distribution of GA receptors in valves along with lack of expression of GA biosynthesis genes in this tissue, strongly suggests transport of GAs from the developing seeds to promote fruit growth.