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Lax panicle mutant in rice is characterized by a few rachis branches and spikelets on the panicle.

Annotated Information


LAX PANICLE (LAX) is involved in the formation of all types of axillary meristems throughout the ontogeny of a rice plant, and/or maintenance during rice reproductive development, and functions redundantly with SPA (SMALL PANICLE) in the same genetic pathway. LAX may transiently accumulate in the initiating apical meristem at the plastochron 4 stage, which strictly regulates mRNA expression and subsequent control of protein targeting[1]. Ectopic LAX expression in rice caused pleiotropic effects, including dwarfing, an altered pattern of stem elongation, darker color, bending of the lamina joint, absence of the midribs of leaves, and severe sterility[2].


In strong mutant alleles of the LAX locus, such as lax-2 and lax-3, the initiation of lateral spikelets is completely suppressed, and panicle branches are also severely reduced. On the other hand, the defects were observed only in the lateral spikelets in lax-1, lax-4, and lax-5, which are weak mutant alleles of LAX(figure 1).Recently, a novel recessive mutant allele of rice LAX1 gene, lax1-6, which showed abnormal panicle phenotypes with few numbers of elongated primary rachis branches. Beside typical lax mutant phenotype, abnormalities of lax1-6 mutant allele were observed with defect lemma and palea primordial in floral organs[3].

  • An insertion of a retro transposon was detected in the lax-1 allele[2].
  • The existence of a long deletion, which contains five predicted genes, PG1 to PG5, was identified in the lax-2 allele[2].
  • A 59-bp region was deleted in lax-3.
  • Amino acid substitutions were found in lax-4 and lax-5, R50D or A49T, respectively.
Figure 1. Phenotypic analysis of lax mutant[2]
  • The lax1-6 mutant allele was caused by a transversion mutation of nucleotide T to G substitution that resulted in an amino acid substitution from serine (S) to alanine (A) at the 117th position from amino terminus of a basic helix-loop-helix protein coded by LAX1 gene.


LAX cDNA is 1,080-bp in length cloned by screening a cDNA library prepared from very young inflorescences. Sequencing of the LAX cDNA revealed that the LAX gene is intronless and encodes an ORF of 215 aa. It encodes a basic helix–loop–helix transcription factor and is expressed in the boundary between the shoot apical meristem and the region of new meristem formation(figure 2). This pattern of LAX expression was repeatedly observed in every axillary meristem[2].

Figure 2. Expression pattern of the LAX gene. RNA in situ hybridization of LAX (A–D, F–H, and J) and OSH1 (E, I, and K)[2]


The LAX bHLH domain showed high sequence similarity to that of other plant bHLH proteins and bHLH proteins predicted from the rice genome sequence(figure 3). Outside of the bHLH region, however, no other conserved domains were identified. A database search failed to identify cognate homologs of LAX in the Arabidopsis genome, suggesting a possibility that LAX represents a grass-specific regulator of shoot branching[2].

Figure 3. Positional cloning of the LAX gene.[2]

Knowledge Extension

By map-based cloning, LAX2 was isolated from the lax2 (LAX PANICLE 2)mutant, which has a similar phenotype to lax1 in that it lacks an AM in most of the lateral branches of the panicle, and has a reduced number of AMs at the vegetative stage. LAX2 encodes a nuclear protein that physically interacts with LAX1, suggesting that the two LAXs may act together in regulating rice AM formation process[4].

Labs working on this gene

  • Graduate School of Agriculture and Life Science, University of Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo 113-8657, Japan
  • Research Institute for Bioresources,Okayama University, Chuou 2-20-1, Kurashiki, Okayama 710-0046, Japan
  • CREST, Japan Science and Technology Corporation, Honcho 4-1-8,Kawaguchi, Saitama 332-0012, Japan
  • Department of Bio-Science, Nara Institute of Science and Technology, Takayama 8916-5,Ikoma, Nara 630-0101, Japan
  • Crop Development Division, National Agriculture and Food Research Organization Agricultural Research Center, Niigata 943-0193, Japan
  • National Center for Plant Gene Research, Key Laboratory of Molecular and Developmental Biology, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China
  • Department of Biological Mechanisms and Functions, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan
  • Photosynthesis and Photobiology Research Unit, National Institute of Agrobiological Sciences, Tsukuba 305-8602, Japan
  • State Key Laboratory of Rice Biology, China National Rice Research Institute, Chinese Academy of Agricultural Sciences, Hangzhou 310006, People’s Republic of China
  • Institute of Radiation Breeding, National Institute of Agrobiological Sciences, Hitachi-ohmiya 319-2293, Japan
  • Bioscience and Biotechnology Center, Nagoya University, Nagoya 464-8601, Japan
  • Rice Research Division, National Institute of Crop Science, National Agriculture and Food Research Organization, Tsukuba 305-8518, Japan
  • Department of Agricultural and Environmental biology, Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo 113-8657, Japan
  • Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan


  1. Oikawa, T. and J. Kyozuka (2009). "Two-Step Regulation of LAX PANICLE1 Protein Accumulation in Axillary Meristem Formation in Rice." Plant Cell 21(4): 1095-1108.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Komatsu, K., et al. (2003). "LAX and SPA: major regulators of shoot branching in rice." Proc Natl Acad Sci U S A 100(20): 11765-11770.
  3. Matin, M. N. and S. G. Kang (2011). "Genetic and Phenotypic Analysis of lax1-6, a Mutant Allele of LAX PANICLE1 in Rice." Journal of Plant Biology 55(1): 50-63.
  4. Tabuchi, H., et al. (2011). "LAX PANICLE2 of rice encodes a novel nuclear protein and regulates the formation of axillary meristems." Plant Cell 23(9): 3276-3287.

Structured Information