This gene locus includes two genes named after dwarf 61 and OsBRI1 of Oryza sativa Japonica Group, whose length is 3,866 bpsand 1,122 amino acidsrespectively with a putative role that is systemin receptor SR160 precursor.
The mutants of d61 of Oryza sativa are not sensitive to BR(brassinosteroid) addition due to the prohibition of BR signal transduction in their bodies. This kind of mutant are characterized by the limit of the longitudinal elongation of some specific internodes, bending of the lamina joints, shorter leaf sheaths nut longer neck panicles than wide type, and undesirable skotomorphogenesis. In addition, the phenotypes of the mutants of d61 are closely related to the mutations of OsBRI1.
Brassinosteroids (BRs) are plant growth–promoting natural products required for plant growth and development. Physiological studies have demonstrated that exogenous BR, alone or in combination with auxin, enhance bending of the lamina joint of rice.
(A) Schematic representation of the various elongation patterns of internodes in the wild type (N) and various rice dwarf mutants (dn-, dm-, d6-, nl- and sh-types), adapted from Takeda (1977).
(B) Gross morphology of a wild-type plant (left); d61-1 mutant (center), a weak allele; and d61-2 mutant (right), a strong allele.
(C) Elongation pattern of internodes. The wild-type plant (left) shows the N-type of the elongation pattern, whereas the d61-1 (center) and d61-2 (right) mutants show typical dm- and d6-type patterns, respectively. The number of each internode is indicated.
(D) Panicle structure. The wild-type plant (left) has a short panicle; the d61-1 (center) and d61-2 (right) mutants have longer panicles. The arrows indicate the nodes.
(E) Leaf morphology. The leaf of the wild-type plant (left) is bent at the lamina joint indicated by the white arrow, whereas the leaves of d61-1 (center) and d61-2 (right) mutants are more erect.
(F) Leaf sheath morphology. The leaf sheath in the d61-1 (center) and d61-2 (right) mutants is shorter than in the wild-type plant (left).
OsBRI1 has extensive sequence similarity to that of the Arabidopsis BRI gene, which encodes a putative BR receptor kinase. Linkage analyses showed that the OsBRI1 gene is closely linked to the d61 locus. Single nucleotide substitutions found at different sites of the d61 alleles would give rise to amino acid changes in the corresponding polypeptides. Furthermore, introduction of the entire OsBRI1 coding region, including the 5' and 3' flanking sequences, into d61 plants complemented the mutation to display the wild-type phenotype. Transgenic plants carrying the antisense strand of the OsBRI1 transcript showed similar or even more severe phenotypes than those of the d61mutants. OsBRI1 functions in various growth and developmental processes in rice, including (1) internode elongation, by inducing the formation of the intercalary meristem and the longitudinal elongation of internode cells; (2) bending of the lamina joint; and (3) skotomorphogenesis.
The phytohormones auxins and brassinosteroids are both essential regulators of physiological and developmental processes, and it has been suggested that they act inter‐dependently and synergistically. auxins stimulate brassinosteroid perception by regulating the level of brassinosteroid receptor. Auxin treatment increased expression of the rice brassinosteroid receptor gene OsBRI1. The promoter of OsBRI1 contains an auxin-response element (AuxRE) that is targeted by auxin-response factor (ARF) transcription factors. An AuxRE mutation abolished the induction of OsBRI1 expression by auxins, and OsBRI1 expression was down‐regulated in an arf mutant. The AuxRE motif in the OsBRI1 promoter, and thus the transient up‐regulation of OsBRI1 expression caused by treatment with indole-3-acetic acid, is essential for the indole-3-acetic acid-induced increase in sensitivity to brassinosteroids.
Auxin-response factor transcription factors participate in the regulation of OsBRI1 expression
Auxin-induced BU1 expression depends on the presence of the auxin-response element in the OsBRI1 promoter
The expression of OsBRI1 showed in Figure 1 varies in different organs, different parts of elongating culms and even different in the upper four internodes of the elongation zones of elongating culms. This suggest that the sensitivity to BRs also differs among these parts of the plant.
In different organs, the expression of OsBRI1 varies markedly, which is rich in vegatative shoot apices, weak in flowers, rachis, roots and expanded leaf sheaths, but no expression in leaf blades either seeds. This suggest that the sensitivity to BRs also differs among these organs.
In different parts of elongating culms, the expression of OsBRI1 is also different. The elongating culms are divided into four parts: the node, the division, elongation and elongated zones of the internode. The strongest expression is in the division zone, lower expression in the elongation zone and the node, but no expression in the elongated internode. That is, each region of the elongation culm may have different sensitivities to BRs, with the most sensitive parts of the elongating culm being the division and elongation zones, where cells are actively dividing and elongating.
In different elongation zones of the upper four internodes, the expression of OsBRI1 is also examined at the stage when each internode was actively elongating. The strong expression is in the elongation zone of the uppermost (first) and the lowest (fourth) internodes, whereas relatively weak expression is in the second and third internodes. Therefore, the internodes also differ in sensitivityto BRs, such that the second and third internodes are the least sensitive. Interestingly,the expression of OsBRI1 was also high in the stem at the vegetative stage, in which the internodes do not elongate, indicating that high expression of OsBRI1 in the culm does not necessarily coincide with internode elongation.
A rice dm-type dwarf mutant d61 is isolated in 2000. These mutant shows specific shortening of second internodes (dm-type mutants) and is less sensitive to brassinosteroids (BRs) compared to the wild type. They have also isolated the D61 gene, named OsBRI1, which encodes a putative protein kinase with a high similarity to BRI1, a putative BR receptor in Arabidopsis.
The structure of the OsBRI1
The predicted OsBRI1 polypeptide contains several domains that are similar in Arabidopsis BRI1, including a putative signal peptide, two conservatively spaced cysteine pairs, a leucinerich repeat (LRR) domain, a transmembrane domain and a kinase domain. Figure 2 shows the structure model of the OsBRI1 gene.
The mutation of the OsBRI1
Figure 2 also shows the 10 mutation positions of the OsBRI1 and their mutants information respectively. They are divided into three groups accouding to the phenotype. The mutant d61-1, d61-2 and d61-4 are respectively mild, intermediate and severe type. Figure 3 shows the phenotype of them.
A novel allelic mutant of the D61 gene, Fn189 has been found in 2013, showing semi-dwarf stature and erect leaves.
Three homologous genes of BRI1 have been reported in Arabidopsis: BRL1, BRL2, and BRL3. Two of them, BRL1 and BRL3, can compensate for the BRI1 function in bri1 mutant plants. In rice, there are three homologous genes of OsBRI. Comparing these rice proteins to the Arabidopsis proteins and other BRI1 proteins, it is found that the OsBRI1 protein fell into a group that included Arabidopsis, tomato, pea, and barley BRI1 proteins. OsBRL1 and OsBRL3 were categorized into a group that included Arabidopsis BRL1 and BRL3, while OsBRL2 was independently grouped with Arabidopsis BRL2.
These homologous genes for OsBRI1, OsBRL1 and OsBRL3, were highly expressed in roots but weakly expressed in shoots, and their expression was higher in d61-4 than in the wild type. In addition, OsBRL1 and OsBRL3 are at least partly involved in BR perception in the roots. Taken together, these observations suggest that OsBRL1 and OsBRL3, but not OsBRL2, have the ability to function as the BR receptor.
Dwarf mutants and dwarf phenotype
Numerous dwarf mutants of rice have been accumulated and characterized because of their agronomic importance. In rice, each internode is numbered from top to bottom such that the uppermost internode just below the panicle is the first. Based on the elongation pattern of the upper four or five internodes, dwarf mutants were classified into six groups: N-, dn-, dm-, d6-, nl- and sh-type.
Figure 5 shows schematically the relative length of each internode to the total culm in dwarf and wild plants, which is denoted by Takeda(1977). As for the dn-type mutants, the length of each internode is almost uniformly shortened, resulting in an elongation pattern similar to that of the wild-type plant. For the sh-and d6-type mutants, only the uppermost internode or the internodes below it are shortened, respectively. In the dm-type mutants, it is characterized by reduced elongation specifically in the 2nd internode counted from the top. Several recessive dwarfing genes, such as d1, d2, d11 and a dominant gene Ssi1 are known to confer such a characteristic.
The history of brassinosteroids studying
Recent molecular genetic approaches have revealed that plant dwarfism is often caused by defects in the biosynthesis and perception of plant hormones such as gibberellin (GA) and brassinosteroids (BRs). BRs are one of plant hormones that have various effects on plant growth and development, including cell elongation, cell division, vascular development, abscission, and stress resistance (Clouse and Sasse, 1998; Sasse, 1999). Physiological studies have demonstrated that exogenous BR, alone or in combination with auxin, enhance bending of the lamina joint of rice.
The study of BRs began much later than that of other classical plant hormones, such as auxin, cytokinin, GA, abscisic acid, and ethylene. The effect of BR was first demonstrated in the 1960s, and the isolation of brassinolide (BL), the most active BR, was accomplished in 1979 (Mandava, 1988; Sasse, 1999). Since then, the study of BRs has rapidly progressed, coupled with successful molecular genetics approaches in Arabidopsis (Arabidopsis thaliana). The cloning of the BR receptor BRASSINOSTEROID INSENSITIVE1 (BRI1), the second plant hormone receptor ever to be cloned, was typical of the rapid progress in the study of BRs.
In Arabidopsis, for example, several mutants are BR-related mutants that have a distinctive dwarf phenotype with dark green rugose leaves. When grown in the dark, these mutants show a deetiolated (DET) phenotype with less hypocotyl elongation (Chory et al., 1991; Kauschmann et al., 1996). Through the characterization of these Arabidopsis mutants, BRs are shown to play important roles in normal growth and also in light and dark development during morphogenesis.
The BR signaling transduction pathway
The steroidal hormone brassinosteroids (BRs) play important roles in plant growth and development. Genetic, genomic and proteomic studies in Arabidopsis have identified major BR signaling components and elucidated the signal transduction pathway from the cell surface receptor kinase BRI1 to the BES1/BZR1 family of transcription factors. BRs interact with other plant hormones in coordinating gene expression and plant growth and development. The BR signal transduction pathway is showed in Figure 6.
Labs working on this gene
- Bioscience Center, and Graduate School of Bioagricultural Science, Nagoya University, Nagoya 464-8601, Japan
- Plant Functions Laboratory, Institute of Physical and Chemical Research RIKEN, Wako, Saitama 351–0198, Japan
- Research institute for bioresources, Okayama University, 2-20-1, Chuo, Kurashiki 710-0046, Japan
- Department of Chemistry, Joetsu University of Education, Joetsu, Niigata 943-8512, Japan
- Field Production Science Center, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Nishi-Tokyo, Tokyo 188–0002, Japan
- Plant Genetics Laboratory, National Institute of Genetics, Mishima, Shizuoka 411–8504, Japan
- National Key Facility for Crop Gene Resources and Genetic Improvement, Institute of Crop Science, Chinese Academy of Agricultural Sciences, Beijing 100081, China
- College of Life Science, Hebei Normal University, Shijiazhuang 050016, China
- Shandong Rice Research Institute, Jinan 250100, China
- Yamamuro C; Ihara Y; Wu ; et al.(2000) Loss of function of a rice brassinosteroid insensitive1 homolog prevents internode elongation and bending of the lamina joint. Plant Cell 12(9):1591-1605.
- Tomoaki Sakamoto;Yoichi Morinaka;Yoshiaki Inukai;Hidemi Kitano;Shozo Fujioka. (2013) Auxin signal transcription factor regulates expression of the brassinosteroid receptor gene in rice. The Plant Cell 12(9): 1591-1606.
- Nakamura A, Fujioka S, Sunohara H, et al.(2006) The role of OsBRI1 and its homologous genes, OsBRL1 and OsBRL3, in rice. Plant Physiology 2006;140(2):580-590.
- Zhao J, Wu C, Yuan S, et al.(2013) Kinase activity of OsBRI1 is essential for brassinosteroids to regulate rice growth and development. Plant Science 199:113-120.
- Xiong W, Ihara Y, Takeda K, Kitano H.(1999) New dm-type dwarf mutants varying in internode elongation patterns are controlled by different mutant genes at the same locus in rice (Oryza sativa L.). Breeding Science 49(3):147-153.
- Guo H, Li L, Aluru M, Aluru S, Yin Y.(2013) Mechanisms and networks for brassinosteroid regulated gene expression. Current Opinion in Plant Biology 16(5):545-553.
- Hao J, Yin Y, Fei S-z.(2013) Brassinosteroid signaling network: implications on yield and stress tolerance. Plant Cell Reports 32(7):1017-1030.