Ó Springer 2005
Plant Molecular Biology (2005) 59:75–84 DOI 10.1007/s11103-004-4038-x
The plant architecture of rice (Oryza sativa) Yonghong Wang and Jiayang Li* State Key Laboratory of Plant Genomics and National Center for Plant Gene Research. Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing, China (*author for correspondence; e-mail
[email protected]) Received 8 May 2004; accepted in revised form 30 September 2004
Key words: apical dominance, axillary meristem, phytohormone, plant architecture, plant height, rice (Oryza sativa), tillering
Abstract Plant architecture, a collection of the important agronomic traits that determine grain production in rice, is mainly affected by factors including tillering, plant height and panicle morphology. Recently, significant progress has been made in isolating and collecting of mutants that are defective in rice plant architecture. Although our understanding of the molecular mechanisms that control rice tillering, panicle development and plant height are still limited, new findings have begun to emerge. This review, therefore, summarizes the recent progress in exploring the mechanisms that control rice plant architecture.
Introduction Flowering plants display a variety of architectures that are defined by the degree of branching, internodal elongation and shoot determinancy (Ward and Leyser, 2004). The study of plant architectures of crops has for centuries attracted attentions since they were found to be correlative with the plant survival ability when plants suffer environmental stress, for example, wind and/or rain damage that may severely affect plant seed production (Peng et al., 1999). Although plant architectures are to some extent influenced by environmental factors such as light, temperature, plant density and humidity, their species-specific characteristics indicate the genetic regulatory mechanisms. During the past decades, studies on the model plants Antirrhinum majus and Arabidopsis thaliana and on crop plants such as maize and barley have begun to elucidate the molecular genetic basis of plant architectures (Peng et al., 1999; Reinhardt and Kuhlemeier, 2002; Babb and Muehlbauer, 2003). Rice (Oryza sativa) is one of the most important food crops that feeds more than half of the
world’s population. In Asia, Africa and Latin America, the demand for rice will increase dramatically because of the steady increase in population. To meet this increasing demand, new elite varieties with ideal plant architectures that can produce much higher grain yields need to be developed, as the case of ‘‘green revolution’’ in which the grain yields have been significantly increased by growing lodging-resistant semi-dwarf varieties of wheat and rice (Peng et al., 1999). Therefore, the understanding of the mechanism that underlies the rice plant architecture will facilitate to breed so-called super-rice varieties. Rice is an annual grass with round, hollow and jointed stems (culms) that bear panicles. A mature rice plant normally has a main culm and a number of side branches (tillers). Generally, rice plant growth is divided into three stages: vegetative stage (from germination to panicle initiation), reproductive stage (from panicle initiation to heading) and grain filling or ripening stage (from heading to maturity). In the vegetative stage, the shoot apical meristem (SAM) produces leaves and tillers arise from the axils of leaves (Li, 1979). Although rice
76 tillering ability is affected by environmental conditions such as light, temperature, plant density, and nutrient or water supply, the tiller number of a given rice variety is mainly determined by its genetic background. In the reproductive stage, the rice plant goes through a series of processes including culm elongation, panicle initiation and differentiation, heading, and flowering. When a rice plant enters the ripening stage, the rice grains increase in size and weight to accumulate sugars, starches, storage proteins and other storage compounds. The rice plant architecture is mainly determined by tiller number, tiller angle, plant height, and panicle morphology, and their representative mutants are shown in Figure 1. In contrast to the model dicotyledonous plant A. thaliana, our knowledge of the molecular mechanism underlying the plant architecture of rice plants is still limited. However, in the recent years, significant progress has been made in elucidating the molecular mechanisms that control rice plant architecture. For example, genes that act as key regulators for the initiation and outgrowth of rice tiller buds or leaves during the vegetative stage have been identified (Komatsu et al., 2003a; Li et al., 2003b; Takeda et al., 2003; Miyoshi
et al., 2004) and phytohormones such as gibberellin (GA) and brassinosteroid (BR) have been found to play crucial roles in controlling rice plant height (Yamamuro et al., 2000; Monna et al., 2002; Sasaki et al., 2002; Spielmeyer et al., 2002; Hong et al., 2003). In this review, we will focus on the latest progress in the molecular genetic understanding of the components that shape the rice plant architecture. For general plant architecture, readers are referred to the recent reviews (Sussex and Kerk, 2001; Reinhardt and Kuhlemeier, 2002; Ward and Leyser, 2004).
Tillering ability Branching plays a central role in the elaboration of plant architecture. Branches of seed plants develop from the axillary meristems of stems. Formation of an axillary meristem normally undergoes two distinct steps, the initiation of a new axillary meristem at the axil of a leaf and subsequently outgrowth of the axillary meristem. Mutants defective in axillary meristem initiation and/or outgrowth have been well characterized in various species such as Arabidopsis, tomato, pea
Figure 1. Representative mutants defective in rice plant architecture. (A) A mutant plant with reduced tiller number (left) and its wild type (right), (B) A mutant plant with a spreading phenotype (left) and its wild type (right), (C) A panicle of the small panicle mutant (left) and a wild-type panicle with a normal size (right), and (D) A dwarf mutant plant with increased tiller number (left) and its wild type (right).
77 and maize (Schumacher et al., 1999; Stirnberg et al., 1999, 2002; McSteen and Hake, 2001; Otsuga et al., 2001; Sorefan et al., 2003; Booker et al., 2004). However, the molecular factors that govern branching control are still largely unknown. Tillers are grain-bearing branches in monocotyledonous plants. Normally, a tiller bud arises from the axial of each leaf on its mother stem of a rice plant, but only those on the unelongated basal internodes have potentials to develop into tillers, whereas, those formed on the elongated upper internodes become arrested when the mother stems begin to differentiate their own panicles (Li, 1979; Hanada, 1993). Figure 2 shows the successive steps of the formation of tillers. The rice tillering pattern is obviously different from that of A. thaliana in which the lateral branches cannot be observed until the SAM carries out the transition
from vegetative to reproductive growth stage (Gribic and Bleecker, 1996). Although rice tillering mutants showing increased culm number (icn) or reduced culm number (rcn) were reported many years ago (Iwata et al., 1995), their molecular mechanisms remain totally unknown until the recent studies on the rice mutant monoculm 1 (moc1) (Li et al., 2003b) and functional characterization of OsTB1 (Takeda et al., 2003), a rice ortholog of maize Teosinte branched 1 (TB1) involved in maize apical dominance (Doebley et al., 1995; Hubbard et al., 2002). The moc1 mutant plant typically produces a main culm with no or a limited number of tillers, due to the loss of ability to initiate tiller buds. Sequence analysis indicates that MOC1 is a rice ortholog of tomato Lateral suppressor (Ls) gene and Arabidopsis LATERAL SUPPRESSOR (LAS) gene (Schumacher et al., 1999; Greb et al.,
Figure 2. Development of primary tillers at the shoot apex of the main stem in a rice plant. The arrows indicate (A) an axillary meristem initiated from the axial of a leaf, (B) a tiller bud formed from the axillary meristem, (C) a tiller bud with the first leaf primordium, (D) the mature tiller buds with several young leaves, and (E) tillers outgrown from mature tiller buds.
78 2003; Li et al., 2003b). MOC1 is a member of the plant-specific GRAS family proteins that function in diverse aspects of plant development including signal transduction, meristem maintenance and development (Richards et al., 2000; Bolle, 2004). GRAS family proteins have been proposed to function as transcription factors (Pysh et al., 1999; Richards et al., 2000). Although MOC1 has no typical nuclear localization sequences (NLS) like other members such as RGA and SLR1, MOC1 was mainly found in the nucleus, consistent with the hypothesis that MOC1 might function as a transcription factor (Li et al., 2003b). The MOC1 spatial and temporal expression patterns revealed by RNA in situ hybridization are consistent with the function of MOC1 for axillary meristem initiation and tiller bud formation. MOC1 expression is detectable in a small number of epidermal or subepidermal cells at the leaf axils before any visible morphological changes at the position where axillary meristems will initiate. Thereafter, MOC1 is mainly expressed in the protuberance and axillary meristem and extended to the entire tiller bud including the axillary leaf primordia and young leaves, whereas no signal could be observed in SAM. Moreover, transgenic studies demonstrate that MOC1 also functions in promoting tiller bud outgrowth (Li et al., 2003b). Based on these findings, MOC1 has been suggested as a major regulatory gene controlling rice branching. The rice OsTB1 gene has been cloned recently based on the sequence similarity to maize TB1 (Takeda et al., 2003). The TB1 gene encodes a putative transcription factor of the TCP protein family. Impairment of TB1 leading to enhance lateral branching in maize suggests its negative regulatory role in controlling the axillary bud outgrowth (Doebley et al., 1995, 1997; Kosugi and Ohashi, 2002). Consistent with the function of TB1 in maize, overexpression of OsTB1 reduces rice tillers severely while its loss-of-function mutation in the classical mutant fine culm (fcn1) promotes the outgrowth of rice tillers. However, the initiation of tiller buds is not affected in OsTB1 overexpression transgenic lines. These findings indicate that the pivotal role of OsTB1 is to control the outgrowth of rice tiller buds rather than the initiation of tiller buds (Takeda et al., 2003), suggesting that OsTB1 is a real counterpart of maize TB1. Interestingly, OsTB1 expression was found to be
down regulated in moc1, consistent with the possible regulatory role downstream of MOC1 (Li et al., 2003b). In many plant species, the axillary buds become dormant due to the inhibiting effects of the primary shoot on the outgrowth of axillary buds, a phenomenon known as ‘‘apical dominance’’. The activity of these axillary meristems is fine regulated by genetic and environmental factors, which in turn are mediated by phytohormones. Thimann and Skoog (1934) first hypothesized that auxin might play a direct role in this process. However, the late investigation revealed that a second messenger is required for the action of auxin (Hillman et al., 1977; Morris, 1977; Pilate et al., 1989; Prasad et al., 1993). Cytokinin has been proposed as a second messenger that may mediate the action of auxin. Cytokinin is synthesized in roots and transported into axillary buds to break the dormancy of arrested buds; in the mean time, auxin modulates the cytokinin concentration thus mediates bud outgrowth (Palni et al., 1988; Bangerth, 1994; Nordstrom et al., 2004). In addition, ABA was also found to act as an inhibitor to axillary bud growth (Beveridge, 2000; Stirnberg et al., 2002; Sorefan et al., 2003). However, recent studies on the plant architecture mutants, more axillary branching (max) in Arabidopsis and ramosous (rms) in pea, revealed that in higher plants there may exist a novel shoot branching regulatory pathway mediated by non-phytohormones such as carotenoid-derived compounds (Beveridge, 2000; Stirnberg et al., 2002; Sorefan et al., 2003; Booker et al., 2004). Although it is unclear whether MOC1 is involved in any hormonal regulatory pathway, some hints could be traced from the features of its orthologous gene Ls in tomato (Schumacher et al., 1999; Li et al., 2003b). The ls mutant plant is characterized by a branchless phenotype and severe imbalance of major phytohormones at the vegetative stage (Schumacher et al., 1999). The ls mutant plant also exhibits a phenotype of reduced seed germination and lack of petal development, a phenomenon related to the defect in the GA regulatory pathway. In fact, the GA contents in different organs of ls mutant plants are dramatically increased, supporting the hypothesis that Ls protein may participate in the negative regulation of the GA signal transduction pathway (Tucker, 1976). The finding that the moc1 mutant is also
79 impaired in seed germination and fertilization to some extent implies that the rice MOC1 protein is likely involved in a GA signaling pathway (Li et al., unpublished data).
Tillering angle Tiller angle, the angle between the main culm and its tillers, is another important agronomic trait that contributes to the rice plant architecture (Xu et al., 1998). Neither the extremely spreading nor the compact plant type is beneficial to rice grain production. Relatively spreading rice plants may escape from some diseases such as infection of Rhizoctonia solani ku¨hn (Qian et al., 2001), but the extremely spreading rice plants occupy too much space, thus reduce the grain production within a given area. In contrast, the extremely compact rice plants are less efficient in harvesting light and more susceptible to pathogen attacks (Xu et al., 1998; Qian et al., 2001). Two single recessive mutations, la (lazy) and er (erecta), were reported to confer spreading and compact phenotype, respectively (Takahashi et al., 1963, 1968; Li et al., 2003a). Both the LA and ER genes have been isolated through a map-based cloning approach (Li et al., unpublished data). However, they are novel proteins and the investigation of their functions in controlling the tiller angle is under way.
Panicle morphology The aerial organs are all originated from the SAM. When the SAM undergoes the developmental transition from the vegetative to reproductive stage, a complex organ system termed inflorescence is generated. The architecture of inflorescences is mainly determined by the basic pattern of the floral branches and the position of flowers (Coen and Nugent, 1994). In spite of detailed descriptions of inflorescence development in various species, our knowledge about their control mechanisms is still limited, especially the development of the inflorescences of monocotyledonous crops such as maize and rice. Unlike Arabidopsis that produces floral meristems directly from the inflorescence meristem with indeterminate growth habit, the rice and maize inflorescence generate branches and spikelet meristems before producing
floral meristems in a determinate pattern. Figure 3 illustrates the major steps in the rice panicle development: (1) formation of the first bract primordium indicating the transition from the vegetative phase to reproductive phase; (2) formation of the primordia of the primary branches from the base of bracts; (3) formation of the primordia of the secondary branches from the base of each primary branch primordium; (4) formation of the terminal and lateral spikelet meristem primordia on the rachis-branches (primary and/or secondary branches) and differentiation of the spikelet primordia (Takeoka et al., 1993; Komatsu et al., 2001). Therefore, the rice panicle develops three types of axillary meristems, the rachis-branch meristems, the lateral spikelet meristems and the terminal spikelet meristems. The initiation and outgrowth of these meristems determine the rice panicle morphology. Recently, several mutants altered in the panicle architecture have been identified (Mackill et al., 1991; Komatsu et al., 2001, 2003b). In the frizzy panicle (fzp) mutant plants, spikelets are replaced by branches in severe fzp alleles, but in the weak alleles they are arisen at the apices of ectopic branches. In both severe and weak fzp alleles, the spikelet meristems repeatedly produce extra bracts with axillary buds, leading to a mass of proliferating meristems (Komatsu et al., 2001, 2003b). The FZP gene, encoding an ethylene-responsive element-binding factor (ERF), is the ortholog of maize BRANCHED SILKLESS1 (BD1) gene and its duplicate BD1B (Chuck et al., 2002; Komatsu et al., 2003b). In situ hybridization analysis showed that FZP is expressed within a very short period of time during the inflorescence development in the half-ring area where the rudimentary glume primordium initiates, sharing a similar expression pattern as BD1 in maize. Based on these observations, Kyozuka and her coworkers suggested that FZP might first suppress the formation of axillary meristems of rice spikelets and then act as a positive regulator of floral meristem identity (Komatsu et al., 2003b). Similar FZP/ BD1 genes were cloned from other grass species and all these genes share the conserved ERF domain and have an overall identity of 45–75%, suggesting the conserved function in grass (Chuck et al., 2002). Another mutant with altered panicle architecture is lax panicle (lax), which is defective in
80
Figure 3. The panicle development. (A) The SAM undergoes the transition from the vegetative to reproductive phase, showing the formation of the first bract primordium (B), (B) Formation of the primordia of the primary branches (PB) in a spiral pattern, showing the apical meristem (AM) that become degenerated after the formation of the primary branches and the bract hair cells (H) that are the degenerated forms of the bracts except the first one, (C) Formation of the primordia of the secondary branches (SB) in an oppositely paired pattern, generated from the base of each primary branch (PB), and (D) Formation of spikelets. Lateral spikelets are formed on rachis-branches (the primary and/or secondary branches) and terminal spikelets are produced from the end of each rachis-branch. A spikelet primordium differentiates in order the primordia of a rudimentary glume (RG), an empty glume (EG), a lemma (L), a palea (P), six stamens (S) and a pistil (O).
panicle lateral meristem initiation whereas the vegetative branching is comparable to the wildtype plant (Komatsu et al., 2001). LAX encodes a putative transcriptional regulator that contains a basic helix-loop-helix (bHLH) domain. The LAX transcripts accumulate when a new meristem initiates in the boundary between SAM and the new meristem, including primary and secondary panicle branches as well as lateral spikelets. However, the LAX transcripts disappeared gradually when the new meristem starts to elongate, suggesting that LAX is a regulator controlling axillary meristem initiation and/or maintenance during the rice reproductive development. Further study on the double mutant of lax and a panicle branching mutant spa support the hypothesis that the LAX gene may function redundantly with SPA in the same genetic pathway during the development of rice axillary meristem. However, the molecular nature of SPA is still unclear. Identification and characterization of SPA will help us to figure out the roles of SPA and LAX in the genetic
pathway that controls rice branching (Komatsu et al., 2003a). Two hypotheses have been proposed to explain the regulatory mechanisms of axillary meristem formation at the vegetative and reproductive stages. One hypothesis is that the plants turn on a different regulatory pathway in controlling the inflorescence branching when they transit from the vegetative phase to reproductive phase. Characterization of mutants that are altered specifically in the vegetative branching or inflorescence branching in various species, such as rice lax, fzp and tomato ls, supports this hypothesis (Schumacher et al., 1999; Komatsu et al., 2001, 2003a, b). On the other hand, the maize bif2 and tomato blind mutants are defective in all types of axillary meristems, supporting the other hypothesis that the axillary bud formation shares a common controlling mechanism at both the vegetative and reproductive developmental stages (McSteen and Hake, 2001; Schmitz et al., 2002). To unveil the genetic network that controls the rice axillary
81 meristem development, new mutants and their corresponding genes need to be identified. It is of particular importance to identify the components of the signal transduction pathways that are upstream or downstream of the cloned genes, for example, MOC1 and LAX1.
Plant height Rice plant height, an important agronomic trait associated with rice yield, is determined by the total number of nodes and the length of each internode and varies depending on varieties and environmental conditions. The rice plant height gradually increases in the vegetative developmental stage but rapidly elongates in the reproductive stage prior to heading. It is clear that the rice stem elongation starts at the beginning of panicle initiation and is mainly ascribed to the rapid elongation of cells in the top 4–6 internodes. Rice plant height is regulated by qualitative genes and quantitative loci (Huang et al., 1996). Using conditional QTL mapping approaches, a number of major QTLs that affect the rice plant height in a developmental stage-specific manner have been identified (Yan et al., 1998a). Up to date, more than 60 rice dwarf mutants have been identified and some genes conferring the dwarf phenotype have been cloned and characterized (Futsuhara and Kikuchi, 1997; Hong et al., 2003; Sakamoto et al., 2004). GAs and BR have been revealed to play major roles in modulating rice plant height. The GAs form a large family of tetracyclic diterpenoid phytohormones that regulate many aspects of plant growth and development, such as seed germination, stem elongation, leaf expansion, flowering, and fruit development (Hooley, 1994; Harberd et al., 1998). Some GA-related mutants with dwarf phenotype have been identified in a variety of species (Herberd and Freeling, 1989; Winkler and Freeling, 1994; Borner et al., 1996; Peng et al., 1997). The genetically improved crop varieties such as semi-dwarf cultivars of wheat and rice have produced remarkably increased grain yields since the 1960s, so-called the ‘‘green revolution’’ (Peng et al., 1999). Elite rice varieties carrying a recessive semidwarf (sd-1) gene have been applied in the rice production for nearly half century, because they can
produce much higher grain yields due to their enhanced absorbance of sunlight and strong resistance to lodging. Recently, three laboratories independently reported their results of the elucidation of the molecular basis of the ‘‘green revolution’’ rice (Monna et al., 2002; Sasaki et al., 2002; Spielmeyer et al., 2002). Their findings indicate that the reduced height of the sd-1 rice plants is resulted from the defect in the GA biosynthetic pathway, unlike the wheat Rht-B1/Rht-D1mutant plants in which the GA signal transduction pathway is impaired (Peng et al., 1999). In the rice genome, there are at least two GA20ox genes, GA20ox1 and GA20ox2, and it is GA20ox2 that corresponds to the Sd-1 locus (Sasaki et al., 2002). Strong expression of GA20ox2 is detected in rice leaves, stems and opened flowers, whereas GA20ox1 mainly expressed in the unopened flowers. Although GA20ox1 has been proved irrelevant to the sd-1 phenotype, it might be a counterpart of GA20ox2 that maintains normal fertilization in the sd-1 mutant (Sasaki et al., 2002). Up to date, a total of 18 GA-deficient mutants corresponding to 6 genetic loci have been identified in rice and these mutants are categorized into three groups, the severe dwarfs with aborted flower or seed development, severe dwarfs with normal seedsetting, and semi-dwarfs with good production which are promising for the improvement of rice varieties (Sakamoto et al., 2004). In contrast, the slender rice (slr1) plant was found to be a constitutive GA responsive mutant (Ikeda et al., 2001), showing much higher plant height at the seedling stage compared to the wildtype plant. Molecular analysis of the SLR gene revealed that it is an ortholog of the height-related genes, namely, GAI and RGA in Arabidopsis, RHT in wheat and D8 in maize, and functions negatively in the GA signal transduction pathway mediated by the DELLA domain (Herberd and Freeling, 1989; Winkler and Freeling, 1994; Borner et al., 1996; Peng et al., 1997; Ikeda et al., 2001). Therefore, the rice height-related genes modulate rice plant height by both the GA biosynthetic and signaling pathways. The understanding of the roles of BRs in controlling rice plant height comes from the characterization of a novel dwarf mutant d61 and a classic rice dwarf mutant d2 (Yamamuro et al., 2000; Hong et al., 2003). The d61 mutant plant is less sensitive to BR compared to the wild-type plant and is defective in the internode elongation
82 and lamina joint bending. Analysis of the cloned gene revealed that the rice OsBRI1 gene, a homolog of the Arabidopsis BRI1 gene (Li and Chory, 1997), is the D61 gene, indicating that OsBRI1 mediates the BR signaling pathway by which the internode elongation and lamina joint bending are regulated. However, the d2 mutant, known as ebisu dwarf, showing a similar phenotype to d61, was characterized as a BR-deficient mutant (Hong et al., 2003). The D2 gene encodes a new member of the P450 gene family, CYP90D2, which is highly homologous to a BR biosynthetic enzyme that catalyzes the reactions from 6-deoxoteasterone to 3-dehydro-6-deoxoteasterone and from teasterone to 3-dehydroteasterone. Consistent with the d2 mutant phenotype of the abnormal leaf structure and the shortened stems, the D2 gene is mainly expressed in leaves and elongating stems (Hong et al., 2003). It is well known that rice plant height is negatively correlated to tiller number (Huang et al., 1996; Yan et al., 1998b; Asai et al., 2002; Hong et al., 2003). However, the molecular mechanisms that underlie the cross talk between the plant height and branching are poorly understood. The finding that the transgenic rice plants harboring three MOC1 transgenes are dwarf but produce more tillers compared to the wild-type plants (Li et al., 2003b) will prompt us to investigate the control network. A large number of rice dwarf mutants have been documented and characterized so far, but few of them have been successfully applied to improve rice varieties due to their severe dwarfism and sterility (Spielmeyer et al., 2002). However, the recent advance in genetic manipulation of the GA biosynthetic and/or signaling pathways has shed light on the modulation of plant height of rice and other cereal crops to yield higher grain products (Sakamoto et al., 2003).
Arabidopsis and Petunia and crops including tomato, maize, and rice. A large number of mutants that alter in the various aspects of plant architecture have been collected and some key regulatory genes, have been cloned and studied. Although our present knowledge about plant architecture is still fragmentary, we can expect a rapid progress in the field due to the recent achievements in sequencing the genomes of Arabidopsis, rice, maize, wheat and other plants and in creating numerous mutants by T-DNA or transposon/retrotransposon tagging approaches. It will become relatively easy to identify and clone genes that play important roles in the control of plant architecture. More importantly, advances in biotechnologies including microarray analysis, yeast two-hybrid method and proteomics approaches will help us to identify the components associated with known proteins, such as MOC1, LAX, FZP and OsTB1 in rice plants. Understanding of the mechanisms underlying each individual aspect of tillering, plant height and panicle morphology will ultimately uncover the genetic network that controls the rice plant architecture. Finally, the progress in studying of plant architecture in other plant species such as Arabidopsis, maize, tomato, barley, and Petunia will certainly facilitate our understanding of the plant architecture of rice.
Acknowledgements We are grateful to Dr Qian Qian for providing the plant materials of rice mutants, Peijin Li, Renxiao Wang and Shengben Li for providing the pictures of rice plants. We thank Dr Zhukuan Cheng and Guosheng Xiong for assistance on photography. This work was supported by grants from the Ministry of Sciences and Technology of China (J02-A-001 and G19990116) and National Natural Science Foundation of China (30221002 and 30330040).
Perspective Plant breeders have paid special attention to plant architecture for centuries due to its agronomic importance. However, only recently have the mechanisms that underlie plant architecture been exploded and studied at the molecular level. Our knowledge about plant architecture is mainly accumulated from studies on model plants such as
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