Inflorescence branching is a major yield trait in crop plants controlled by the developmental fate of axillary shoot meristems. Variations in branching patterns lead to diversity in flower-bearing architectures (inflorescences) and affect crop yield by influencing seed number or harvesting ability. Several growth regulators such as auxins, cytokinins and carotenoid derivatives regulate branching architectures. Inflorescence branching in maize is regulated by three RAMOSA genes. Here we show that one of these genes, RAMOSA3 (RA3), encodes a trehalose-6-phosphate phosphatase expressed in discrete domains subtending axillary inflorescence meristems. Genetic and molecular data indicate that RA3 functions through the predicted transcriptional regulator RAMOSA1 (RA1). We propose that RA3 regulates inflorescence branching by modification of a sugar signal that moves into axillary meristems. Alternatively, the fact that RA3 acts upstream of RA1 supports a hypothesis that RA3 itself may have a transcriptional regulatory function.

The role of polyploidy, particularly allopolyploidy, in plant diversification is a subject of debate. Whole-genome duplications precede the origins of many major clades (e.g., angiosperms, Brassicaceae, Poaceae), suggesting that polyploidy drives diversification. However, theoretical arguments and empirical studies suggest that polyploid lineages may actually have lower speciation rates and higher extinction rates than diploid lineages. We focus here on the grass tribe Andropogoneae, an economically and ecologically important group of C 4 species with a high frequency of polyploids. A phylogeny was constructed for ca. 10% of the species of the clade, based on sequences of four concatenated low-copy nuclear loci. Genetic allopolyploidy was documented using the characteristic pattern of double-labeled gene trees. At least 32% of the species sampled are the result of genetic allopolyploidy and result from 28 distinct tetraploidy events plus an additional six hexaploidy events. This number is a minimum, and the actual frequency could be considerably higher. The parental genomes of most Andropogoneae polyploids diverged in the Late Miocene coincident with the expansion of the major C 4 grasslands that dominate the earth today. The well-documented whole-genome duplication in Zea mays ssp. mays occurred after the divergence of Zea and Sorghum. We find no evidence that polyploidization is followed by an increase in net diversification rate; nonetheless, allopolyploidy itself is a major mode of speciation. P olyploidy (whole-genome duplication) is often linked with the acquisition of new traits and subsequent species diversification, particularly in plants (1, 2). Ancient polyploidy correlates with major land-plant radiations (3) and the origins of orders, large families, and major clades (4-7) although, in many cases, sharp changes in diversification rates are delayed for millions of years after the polyploidization event (1). This phylogenetic pattern has led to the hypothesis that polyploidy causes or promotes diversification. Good mechanistic reasons support such a hypothesis. Studies of naturally occurring and synthetic polyploids find changes in gene expression, gene loss, release of transposons, and changes in morphology and physiology immediately after polyploidy (4,(8)(9)(10)(11)(12)). This pattern is particularly true for allopolyploids, which originate from a cross between genetically distinct parents often representing different species; in such cases, the biological changes seen after polyploidization may not reflect the effects of genome doubling per se, but rather the effect of hybridization between distantly related progenitors (4).Despite the appeal of the hypothesis that polyploidy causes diversification, there is evidence to the contrary. As noted by Stebbins (13), "polyploidy has been important in the diversification of species and genera within families, but not in the origin of the families and orders themselves," implying that polyploidy is only a minor force in diversification (see also ref. 14). ...

The plant growth hormone auxin plays a critical role in the initiation of lateral organs and meristems. Here, we identify and characterize a mutant, sparse inflorescence1 (spi1), which has defects in the initiation of axillary meristems and lateral organs during vegetative and inflorescence development in maize. Positional cloning shows that spi1 encodes a flavin monooxygenase similar to the YUCCA (YUC) genes of Arabidopsis, which are involved in local auxin biosynthesis in various plant tissues. In Arabidopsis, loss of function of single members of the YUC family has no obvious effect, but in maize the mutation of a single yuc locus causes severe developmental defects. Phylogenetic analysis of the different members of the YUC family in moss, monocot, and eudicot species shows that there have been independent expansions of the family in monocots and eudicots. spi1 belongs to a monocot-specific clade, within which the role of individual YUC genes has diversified. These observations, together with expression and functional data, suggest that spi1 has evolved a dominant role in auxin biosynthesis that is essential for normal maize inflorescence development. Analysis of the interaction between spi1 and genes regulating auxin transport indicate that auxin transport and biosynthesis function synergistically to regulate the formation of axillary meristems and lateral organs in maize.auxin biosynthesis ͉ auxin transport ͉ yucca ͉ meristem T he plant hormone auxin is required for initiation and polar growth of all organ primordia. Auxin is synthesized by a number of pathways in the cell and is transported from cell to cell by diffusion and by the activity of influx and efflux carriers (1). During vegetative development, auxin is required for many developmental processes, including leaf and lateral root initiation, whereas during inflorescence development, auxin is required for the initiation of floral meristems (FMs) and floral organs (2-5). Extensive research in Arabidopsis has shown the importance of auxin transport during lateral organ and axillary meristem initiation (3-5). In addition, recent work has highlighted the role of localized auxin biosynthesis in all aspects of plant development (6-10). The role of auxin in monocots such as maize is not as well understood. Although some aspects of the control of auxin transport seem to be conserved between monocots and eudicots (11-16), there are also key differences (17).Maize plants produce separate male and female inflorescences (18). The male inflorescence, the tassel, is situated at the shoot apex, whereas the female inflorescence, the ear, is produced from an axillary meristem several nodes below the tassel. The tassel consists of a main spike with several long lateral branches at the base ( Fig. 1 A and B). Both the main spike and branches are covered with short branches, each of which bears a pair of spikelets. Each spikelet produces two leaf-like glumes that enclose a pair of florets. Florets consist of a lemma and palea (outer whorl structures derived from bracts...

Auxin plays a fundamental role in organogenesis in plants. Multiple pathways for auxin biosynthesis have been proposed, but none of the predicted pathways are completely understood. Here, we report the positional cloning and characterization of the vanishing tassel2 (vt2) gene of maize (Zea mays). Phylogenetic analyses indicate that vt2 is a co-ortholog of TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1 (TAA1), which converts Trp to indole-3-pyruvic acid in one of four hypothesized Trp-dependent auxin biosynthesis pathways. Unlike single mutations in TAA1, which cause subtle morphological phenotypes in Arabidopsis thaliana, vt2 mutants have dramatic effects on vegetative and reproductive development. vt2 mutants share many similarities with sparse inflorescence1 (spi1) mutants in maize. spi1 is proposed to encode an enzyme in the tryptamine pathway for Trp-dependent auxin biosynthesis, although this biochemical activity has recently been questioned. Surprisingly, spi1 vt2 double mutants had only a slightly more severe phenotype than vt2 single mutants. Furthermore, both spi1 and vt2 single mutants exhibited a reduction in free auxin levels, but the spi1 vt2 double mutants did not have a further reduction compared with vt2 single mutants. Therefore, both spi1 and vt2 function in auxin biosynthesis in maize, possibly in the same pathway rather than independently as previously proposed.