Shoot branching is a major determinant of plant architecture and is highly regulated by endogenous and environmental cues. Two classes of hormones, auxin and cytokinin, have long been known to have an important involvement in controlling shoot branching. Previous studies using a series of mutants with enhanced shoot branching suggested the existence of a third class of hormone(s) that is derived from carotenoids, but its chemical identity has been unknown. Here we show that levels of strigolactones, a group of terpenoid lactones, are significantly reduced in some of the branching mutants. Furthermore, application of strigolactones inhibits shoot branching in these mutants. Strigolactones were previously found in root exudates acting as communication chemicals with parasitic weeds and symbiotic arbuscular mycorrhizal fungi. Thus, we propose that strigolactones act as a new hormone class-or their biosynthetic precursors-in regulating above-ground plant architecture, and also have a function in underground communication with other neighbouring organisms.

The evolution of land flora transformed the terrestrial environment. Land plants evolved from an ancestral charophycean alga from which they inherited developmental, biochemical, and cell biological attributes. Additional biochemical and physiological adaptations to land, and a life cycle with an alternation between multicellular haploid and diploid generations that facilitated efficient dispersal of desiccation tolerant spores, evolved in the ancestral land plant. We analyzed the genome of the liverwort Marchantia polymorpha, a member of a basal land plant lineage. Relative to charophycean algae, land plant genomes are characterized by genes encoding novel biochemical pathways, new phytohormone signaling pathways (notably auxin), expanded repertoires of signaling pathways, and increased diversity in some transcription factor families. Compared with other sequenced land plants, M. polymorpha exhibits low genetic redundancy in most regulatory pathways, with this portion of its genome resembling that predicted for the ancestral land plant. PAPERCLIP.

The growth of plants depends on continuous function of the meristems. Shoot meristems are responsible for all the post-embryonic aerial organs, such as leaves, stems and flowers. It has been assumed that the phytohormone cytokinin has a positive role in shoot meristem function. A severe reduction in the size of meristems in a mutant that is defective in all of its cytokinin receptors has provided compelling evidence that cytokinin is required for meristem activity. Here, we report a novel regulation of meristem activity, which is executed by the meristem-specific activation of cytokinins. The LONELY GUY (LOG) gene of rice is required to maintain meristem activity and its loss of function causes premature termination of the shoot meristem. LOG encodes a novel cytokinin-activating enzyme that works in the final step of bioactive cytokinin synthesis. Revising the long-held idea of multistep reactions, LOG directly converts inactive cytokinin nucleotides to the free-base forms, which are biologically active, by its cytokinin-specific phosphoribohydrolase activity. LOG messenger RNA is specifically localized in shoot meristem tips, indicating the activation of cytokinins in a specific developmental domain. We propose the fine-tuning of concentrations and the spatial distribution of bioactive cytokinins by a cytokinin-activating enzyme as a mechanism that regulates meristem activity.

SummaryA rice gene encoding a calcium-dependent protein kinase (CDPK), OsCDPK7, was induced by cold and salt stresses. To elucidate the physiological function of OsCDPK7, we generated transgenic rice plants with altered levels of the protein. The extent of tolerance to cold and salt/drought stresses of these plants correlated well with the level of OsCDPK7 expression. Therefore, OsCDPK7 was shown to be a positive regulator commonly involved in the tolerance to both stresses in rice. Over-expression of OsCDPK7 enhanced induction of some stress-responsive genes in response to salinity/drought, but not to cold. Thus, it was suggested that the downstream pathways leading to the cold and salt/drought tolerance are different from each other. It seems likely that at least two distinct pathways commonly use a single CDPK, maintaining the signalling speci®city through unknown post-translational regulation mechanisms. These results demonstrate that simple manipulation of CDPK activity has great potential with regard to plant improvement.

SummaryPlant architecture is mostly determined by shoot branching patterns. Apical dominance is a well-known control mechanism in the development of branching patterns, but little is known regarding its role in monocots such as rice. Here, we show that the concept of apical dominance can be applied to tiller bud outgrowth of rice. In dwarf10 (d10), an enhanced branching mutant of rice, apical dominance can be observed, but the inhibitory effects of the apical meristem was reduced. D10 is a rice ortholog of MAX4/RMS1/DAD1 that encodes a carotenoid cleavage dioxygenase 8 and is supposed to be involved in the synthesis of an unidentified inhibitor of shoot branching. D10 expression predominantly occurs in vascular cells in most organs. Real-time polymerase chain reaction analysis revealed that accumulation of D10 mRNA is induced by exogenous auxin. Moreover, D10 expression is upregulated in six branching mutants, d3, d10, d14, d17, d27 and high tillering dwarf (htd1). No such effects were found for D3 or HTD1, the MAX2 and MAX3 orthologs, respectively, of rice. These findings imply that D10 transcription might be a critical step in the regulation of the branching inhibitor pathway. In addition, we present observations that suggest that FINE CULM1 (FC1), a rice ortholog of teosinte branched 1 (tb1), possibly works independently of the branching inhibitor pathway.

Recent studies using highly branched mutants of pea, Arabidopsis and rice have demonstrated that strigolactones, a group of terpenoid lactones, act as a new hormone class, or its biosynthetic precursors, in inhibiting shoot branching. Here, we provide evidence that DWARF14 (D14) inhibits rice tillering and may act as a new compo-nent of the strigolactone-dependent branching inhibition pathway. The d14 mutant exhibits increased shoot branch-ing with reduced plant height like the previously characterized strigolactone-deficient and -insensitive mutants d10 and d3, respectively. The d10-1 d14-1 double mutant is phenotypically indistinguishable from the d10-1 and d14-1 single mutants, consistent with the idea that D10 and D14 function in the same pathway. However, unlike with d10, the d14 branching phenotype could not be rescued by exogenous strigolactones. In addition, the d14 mutant contained a higher level of 2'-epi-5-deoxystrigol than the wild type. Positional cloning revealed that D14 encodes a protein of the alpha/beta-fold hydrolase superfamily, some members of which play a role in metabolism or signaling of plant hormones. We propose that D14 functions downstream of strigolactone synthesis, as a component of hormone signaling or as an enzyme that participates in the conversion of strigolactones to the bioactive form.

In this study, we analyzed five tillering dwarf mutants that exhibit reduction of plant stature and an increase in tiller numbers. We show that, in the mutants, axillary meristems are normally established but the suppression of tiller bud activity is weakened. The phenotypes of tillering dwarf mutants suggest that they play roles in the control of tiller bud dormancy to suppress bud activity. However, tillering dwarf mutants show the dependence of both node position and planting density on their growth, which implies that the functions of tillering dwarf genes are independent of the developmental and environmental control of bud activity. Map-based cloning of the D3 gene revealed that it encodes an F-box leucine-trich repeat (LRR) protein orthologous to Arabidopsis MAX2/ORE9. This indicates the conservation of mechanisms controlling axillary bud activity between monocots and eudicots. We suggest that tillering dwarf mutants are suitable for the study of bud activity control in rice and believe that future molecular and genetic studies using them may enable significant progress in understanding the control of tillering and shoot branching.

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...