The aerial architecture of plants is determined primarily by the pattern of shoot branching. Although shoot apical meristem initiation during embryogenesis has been extensively studied by molecular genetic approaches using Arabidopsis, little is known about the genetic mechanisms controlling axillary meristem initiation, mainly because of the insufficient number of mutants that specifically alter it. We identified the LAX PANICLE (LAX) and SMALL PANICLE (SPA) genes as the main regulators of axillary meristem formation in rice. LAX 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. This pattern of LAX expression was repeatedly observed in every axillary meristem, consistent with our observation that LAX is involved in the formation of all types of axillary meristems throughout the ontogeny of a rice plant. 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. O rganogenesis occurs in plants throughout their lifetimes. The main axis of growth is determined by the production of two meristems: a primary shoot apical meristem (SAM) and a root meristem at embryogenesis. During postembryonic development, plants initiate a multitude of growth axes by forming new meristems called axillary meristems, which are generated in the axils of leaves and give rise to branch shoots and flowers (1). Therefore, the pattern of axillary meristem initiation and development is a key factor in determining plant architecture. Significant progress has been made on the molecular genetic analysis of SAM initiation during embryo development; however, little is known about the initiation of axillary meristems.The development of an axillary meristem is controlled by two distinctive steps, namely, the initiation of a new meristem in the axil of a leaf and its outgrowth. Mutations that exhibit an altered pattern of axillary bud outgrowth have been described for various plant species (1), e.g., Arabidopsis auxin resistant1 (2), supershoot (3), max1, and max2 (4) and maize teosinte branched (5). On the other hand, there are only a few mutants in which the axillary meristem initiation is specifically altered. Maize barren inflorescence 2 (bif2) (6) and barren stalk1 (ba1) (7), Arabidopsis revoluta (rev) (8, 9), tomato lateral suppressor (ls) (10) and lateral suppressor of Arabidopsis (las), its cognate ortholog in Arabidopsis (11), tomato blind (bl) (12), and rice lax panicle (lax) (13) and monoculm 1(moc1) (14) are categorized in this class of mutants. The bif2 and ba1 exhibit severe suppression of all types of axillary meristems, implying that they are involved in genetic pathways controlling the general steps of axillary meristem initiation. Similarly, defects are observed in all types of lateral branches in tomato bl and Arabidopsis rev; however, the expressivity of their mut...

Highly organized interphase cortical microtubule (MT) arrays are essential for anisotropic growth of plant cells, yet little is known about the molecular mechanisms that establish and maintain the order of these arrays. The Arabidopsis thaliana spiral1 (spr1) mutant shows right-handed helical growth in roots and etiolated hypocotyls. Characterization of the mutant phenotypes suggested that SPR1 may control anisotropic cell expansion through MT-dependent processes. SPR1 was identified by map-based cloning and found to encode a small protein with unknown function. Proteins homologous to SPR1 occur specifically and ubiquitously in plants. Genetic complementation with green fluorescent protein fusion proteins indicated that the SPR1 protein colocalizes with cortical MTs and that both MT localization and cell expansion control are conferred by the conserved N-and C-terminal regions. Strong SPR1 expression was found in tissues undergoing rapid cell elongation. Plants overexpressing SPR1 showed enhanced resistance to an MT drug and increased hypocotyl elongation. These observations suggest that SPR1 is a plant-specific MT-localized protein required for the maintenance of growth anisotropy in rapidly elongating cells.

SummaryThe basic structure of a rice inflorescence (the panicle) is determined by the pattern of branch formation, which is established at the early stages of panicle development. In this study we conducted global transcriptome profiling of the early stages of rice panicle development from phase transition to floral organ differentiation. To generate a meristem-specific gene-expression profile, shoot apical meristems (SAMs) and subsequently formed, very young panicles were collected manually and used for cDNA microarray analysis. We identified 357 out of 22 000 genes that are expressed differentially in the early stages of panicle development, and the 357 genes were classified into seven groups based on their temporal expression patterns. The most noticeable feature is that a fairly small number of genes, which are extensively enriched in transcription factors, are upregulated in the SAM immediately after phase transition. In situ hybridization analysis showed that each gene analysed exhibits a unique and interesting localization of mRNA. Remarkably, one of the transcription factors was proven to be a close downstream component of the pathway in which LAX, a major regulator of panicle branching, acts. These results suggest that our strategy -careful collection of meristems, global transcriptome analysis and subsequent in situ hybridization analysis -is useful not only to obtain a genomewide view of gene expression, but also to reveal genetic networks controlling rice panicle development.