Taiyo Toriba

Distinct Regulation of Adaxial-Abaxial Polarity in Anther Patterning in Rice

This study demonstrates that an RNA-dependent RNA polymerase involved in trans-acting small interfering RNA production plays a role in establishing adaxial-abaxial polarity in rice floral organs. A model is presented for anther patterning in rice. Establishment of adaxial-abaxial polarity is essential for lateral organ development. The mechanisms underlying the polarity establishment in the stamen remain unclear, whereas those in the leaf are well understood. Here, we investigated a rod-like lemma (rol) mutant of rice (Oryza sativa), in which the development of the stamen and lemma is severely compromised. We found that the rod-like structure of the lemma and disturbed anther patterning resulted from defects in the regulation of adaxial-abaxial polarity. Gene isolation indicated that the rol phenotype was caused by a weak mutation in SHOOTLESS2 (SHL2), which encodes an RNA-dependent RNA polymerase and functions in trans-acting small interfering RNA (ta-siRNA) production. Thus, ta-siRNA likely plays an important role in regulating the adaxial-abaxial polarity of floral organs in rice. Furthermore, we found that the spatial expression patterns of marker genes for adaxial-abaxial polarity are rearranged during anther development in the wild type. After this rearrangement, a newly formed polarity is likely to be established in a new developmental unit, the theca primordium. This idea is supported by observations of abnormal stamen development in the shl2-rol mutant. By contrast, the stamen filament is likely formed by abaxialization. Thus, a unique regulatory mechanism may be involved in regulating adaxial-abaxial polarity in stamen development.

The YABBY Gene TONGARI-BOUSHI1 Is Involved in Lateral Organ Development and Maintenance of Meristem Organization in the Rice Spikelet

This work reports that mutation in a YABBY gene, TOB1, causes pleiotropic phenotypes, such as formation of a cone-shaped organ and premature termination of the meristem in rice spikelets. Molecular genetic analyses show that TOB1 regulates the initiation and growth of the lateral organs in the spikelet and acts non-cell autonomously to maintain activity and proper organization of the meristem. The meristem initiates lateral organs in a regular manner, and proper communication between the meristem and the lateral organs ensures the normal development of plants. Here, we show that mutation of the rice (Oryza sativa) gene TONGARI-BOUSHI1 (TOB1) results in pleiotropic phenotypes in spikelets, such as the formation of a cone-shaped organ instead of the lemma or palea, the development of two florets in a spikelet, or premature termination of the floret meristem, in addition to reduced growth of the lemma or palea and elongation of the awn. These phenotypes seem to result from not only failure in growth of the lateral organs, but also defects in maintenance and organization of the meristem. For example, the cone-shaped organ develops as a ring-like primordium from an initial stage, suggesting that regulation of organ initiation in the meristem may be compromised. TOB1 encodes a YABBY protein, which is closely related to FILAMENTOUS FLOWER in Arabidopsis thaliana, and is expressed in the lateral organ primordia without any patterns of polarization. No TOB1 expression is detected in the meristem, so TOB1 may act non–cell autonomously to maintain proper meristem organization and is therefore likely to play an important role in rice spikelet development.

Molecular characterization the YABBY gene family in Oryza sativa and expression analysis of OsYABBY1

Members of the YABBY gene family have a general role that promotes abaxial cell fate in a model eudicot, Arabidopsis thaliana. To understand the function of YABBY genes in monocots, we have isolated all YABBY genes in Oryza sativa (rice), and revealed the spatial and temporal expression pattern of one of these genes, OsYABBY1. In rice, eight YABBY genes constitute a small gene family and are classified into four groups according to sequence similarity, exon–intron structure, and organ-specific expression patterns. OsYABBY1 shows unique spatial expression patterns that have not previously been reported for other YABBY genes, so far. OsYABBY1 is expressed in putative precursor cells of both the mestome sheath in the large vascular bundle and the abaxial sclerenchyma in the leaves. In the flower, OsYABBY1 is specifically expressed in the palea and lemma from their inception, and is confined to several cell layers of these organs in the later developmental stages. The OsYABBY1-expressing domains are closely associated with cells that subsequently differentiate into sclerenchymatous cells. These findings suggest that the function of OsYABBY1 is involved in regulating the differentiation of a few specific cell types and is unrelated to polar regulation of lateral organ development.

FON2 SPARE1 Redundantly Regulates Floral Meristem Maintenance with FLORAL ORGAN NUMBER2 in Rice

CLAVATA signaling restricts stem cell identity in the shoot apical meristem (SAM) in Arabidopsis thaliana. In rice (Oryza sativa), FLORAL ORGAN NUMBER2 (FON2), closely related to CLV3, is involved as a signaling molecule in a similar pathway to negatively regulate stem cell proliferation in the floral meristem (FM). Here we show that the FON2 SPARE1 (FOS1) gene encoding a CLE protein functions along with FON2 in maintenance of the FM. In addition, FOS1 appears to be involved in maintenance of the SAM in the vegetative phase, because constitutive expression of FOS1 caused termination of the vegetative SAM. Genetic analysis revealed that FOS1 does not need FON1, the putative receptor of FON2, for its action, suggesting that FOS1 and FON2 may function in meristem maintenance as signaling molecules in independent pathways. Initially, we identified FOS1 as a suppressor that originates from O. sativa indica and suppresses the fon2 mutation in O. sativa japonica. FOS1 function in japonica appears to be compromised by a functional nucleotide polymorphism (FNP) at the putative processing site of the signal peptide. Sequence comparison of FOS1 in about 150 domesticated rice and wild rice species indicates that this FNP is present only in japonica, suggesting that redundant regulation by FOS1 and FON2 is commonplace in species in the Oryza genus. Distribution of the FNP also suggests that this mutation may have occurred during the divergence of japonica from its wild ancestor. Stem cell maintenance may be regulated by at least three negative pathways in rice, and each pathway may contribute differently to this regulation depending on the type of the meristem. This situation contrasts with that in Arabidopsis, where CLV signaling is the major single pathway in all meristems.

The DROOPING LEAF and OsETTIN2 genes promote awn development in rice

The awn is a long needle-like appendage that, in some grass species, is formed on the lemma that encloses floral organs together with the palea. In rice, most wild species and most strains of Oryza sativa ssp. indica generate an awn, whereas most strains of O. sativa ssp. japonica do not. In japonica, the long-awn characteristic appears to have been lost during domestication and breeding programs. Here, we found that the genes DROOPING LEAF (DL) and OsETTIN2 (OsETT2) are involved in awn development in the awned indica strain Kasalath. Genetic analyses and RNA-silencing experiments indicate that DL and OsETT2 act independently in awn formation, and that either gene alone is not sufficient for awn development. Scanning electron microscopy revealed that the top region of the lemma (a putative awn primordium) is larger in an awned floret than in an awnless floret. OsETT2 is expressed in the awn primordium in the awned indica floret, but not in the awnless japonica floret except in the provascular bundle. DL is expressed underneath the primordium at similar levels in both indica and japonica florets, suggesting non-cell-autonomous action. We hypothesize that loss of expression of OsETT2 in the awn primordium is probably associated with the failure of awn formation in japonica strains.

Grass flower development.

Grasses bear unique flowers lacking obvious petals and sepals in special inflorescence units, the florets and the spikelet. Despite this, grass floral organs such as stamens and lodicules (petal homologs) are specified by ABC homeotic genes encoding MADS domain transcription factors, suggesting that the ABC model of eudicot flower development is largely applicable to grass flowers. However, some modifications need to be made for the model to fit grasses well: for example, a YABBY gene plays an important role in carpel specification. In addition, a number of genes are involved in the development of the lateral organs that constitute the spikelet. In this review, we discuss recent progress in elucidating the genes required for flower and spikelet development in grasses, together with those involved in fate determination of the spikelet and flower meristems.

Chapter Eight – Flower Development in Rice

Abstract In rice, the flower consisting of lodicules, stamens and carpels is enclosed by the lemma and palea to form the floret, which together with sterile lemmas and rudimentary glumes constitutes the spikelet. Thus, the flower and the inflorescence units of rice are distinct from those of eudicots. The ABC model, which explains the genetic mechanism underlying floral organ specification in eudicots, is largely applicable to the specialized flowers of rice. For instance, the function of class B genes is conserved to specify the lodicule (a petal homologue) and the stamen. Two class C genes are functionally diversified in rice: one specifies stamen identity together with class B genes, whereas the other is mainly responsible for the determinacy of the flower meristem. By contrast, carpel specification in rice is regulated by a YABBY gene, DROOPING LEAF (DL). Homeotic transformation of the stamen or carpel in loss-of-function mutants of class B genes or DL reveals a mutual repression mode of action for these genes. Additional genes responsible for the development of spikelet organs, such as the lemma, palea, sterile lemma and rudimentary glume, have been identified in rice. Mutations in some of these genes affect the development of only spikelet organs, whereas mutations in others affect the development of both flower and non-floral spikelet organs. In this review, we describe the genetic mechanism underlying flower and spikelet development in rice, and discuss the regulation of maintenance and fate of reproductive meristems, the activity of which is closely associated with flower and spikelet development.

Common and distinct mechanisms underlying the establishment of adaxial and abaxial polarity in stamen and leaf development

Establishment of adaxial-abaxial polarity is essential for lateral organ development. A stamen consists of a bilaterally symmetrical anther and a radial filament. Using a rice mutant, rod-like lemma, in which establishment of adaxial-abaxial polarity is compromised, we found that stamen patterning is likely to be achieved by a unique regulatory mechanism: rearrangement of adaxial-abaxial polarity in the anther, and abaxialization in the filament. These regulations are not found in leaf development. Here, we discuss similarities and differences between the stamen and the leaf in the mechanisms underlying the establishment of adaxial-abaxial polarity. In addition, we propose the idea that the process of establishing adaxial-abaxial polarity in lateral organs is likely to be divided into two phases: a meristem-dependent, followed by a meristem-independent phase. In stamen development, the transition between these two phases is clearly observed as the rearrangement of expression patterns of the adaxial and abaxial marker genes.

Formation of two florets within a single spikelet in the rice tongari-boushi1 mutant.

Communication between the meristem and lateral organs plays important roles in plant development. The TONGARI-BOUSHI1 (TOB1) gene that encodes a YABBY transcription factor is involved in the regulation of meristem maintenance and fate determination of the meristem in rice spikelets. TOB1 is likely to act non-cell autonomously on the meristem, because as this gene is expressed in the lateral organ primordia but not in the meristem. Mutation in of the TOB1 gene results in pleiotropic phenotypes in the spikelet, such as abnormal morphology, formation of the two florets and premature termination of the meristem. Among these phenotypes, the formation of the two florets within a single spikelet is very unique, because one -floret per spikelet is a characteristics of the spikelet of the Oryza genus and is strictly regulated. Here, we describes the phenotype of the two-floret type spikelets and discuss the formation of this type of the spikelet in relation to the regulation of the meristem.

Three TOB1‐related YABBY genes are required to maintain proper function of the spikelet and branch meristems in rice

YABBY genes play important roles in the development of lateral organs such as leaves and floral organs in Angiosperms. However, the function of YABBY genes is poorly understood in monocots. We focused on three rice (Oryza sativa) YABBY genes, TONGARI-BOUSHI (TOB1, TOB2, TOB3), which are closely related to Arabidopsis (Arabidopsis thaliana) FILAMENTOUS FLOWER (FIL). To elucidate the function of these YABBY genes, we employed a reverse genetic approach. TOB genes were expressed in bract and lateral organ primordia, but not in meristems. RNAi knockdown of TOB2 or TOB3 in the tob1 mutant caused abnormal spikelet development. Furthermore, simultaneous knockdown of both TOB2 and TOB3 in tob1 affected not only spikelet, but also inflorescence development. In severe cases, the inflorescences comprised naked branches without spikelets. Analysis of inflorescence development at an early stage showed that the observed phenotypic defects were closely associated with a failure to initiate and maintain reproductive meristems. These results indicate that the TOB genes regulate the maintenance and fate of all reproductive meristems. It is likely that the function of FIL/TOB clade YABBY genes has been conserved between Arabidopsis and rice to maintain the proper function of meristems, even though these genes are expressed in lateral organ primordia.