sparse inflorescence1 encodes a monocot-specific YUCCA-like gene required for vegetative and reproductive development in maize

Abstract

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.

Fig. 1.

Characterization of the spi1 inflorescence. (A) Schematic of the different types of axillary meristem initiated during maize inflorescence development (Left) and the structures they produce (Right). (B) Tassels of normal (Left) and spi1 (Right), showing reduced numbers of branches and spikelets in the spi1 mutant. (C) Ears of normal and spi1, showing reduced kernel number as well as production of kernels over the tip (arrowhead) of spi1 ears. (D–H) Scanning electron microscope images of developing spi1 tassels and ears. (D) Normal tassel. (E) spi1 tassel showing fewer SPMs. (F) Close-up of tip of spi1 tassel later in development showing defective apical inflorescence meristem with SMs initiating over the tip (arrowhead). (G) Normal ear. (H) spi1 ear showing fewer SPMs. The tip is fasciated and produces SMs (arrowhead). (Scale bars: 100 μm.)

Fig. 2.

Cloning of spi1. (A) Schematic representation of the positional cloning of the spi1 gene. Black bars represent chromosomal segments of rice, sorghum, and maize (not to scale). Predicted genes for both rice and sorghum are indicated. In maize, the region encompasses two BAC contigs (FPC contigs 146 and 147). The number of recombinants (R) is shown below each maize marker. Empty rectangles represent BAC clones, and the filled rectangle indicates the BAC clone containing spi1. (B) spi1 gene structure. Exons are represented as rectangles, mRNA sequence is indicated between the arrow and the vertical bar. The putative enzymatic sites of the YUC proteins are shown in gray. Upward and downward triangles symbolize deletions and insertion, respectively, in the corresponding spi1 mutant alleles.

Fig. 3.

spi1 expression analysis. (A) spi1 qualitative RT-PCR in different tissue samples. ubq, ubiquitin as control. (B–F) spi1 RNA in situ hybridization in young inflorescences. (B) spi1 expression marks the site of new SPM formation (arrowhead). (C and D) Developing SPMs. (E) An SM giving rise to the lower FM (arrowhead). (F) Floral meristem forming stamen primordia (arrowheads). (Scale bars: 20 μm.)

Fig. 4.

Phylogenetic analysis of YUC-like genes from diverse land plants. Bayesian consensus phylogram of 62 YUC-like genes from land plants rooted using two fungal sequences (Debaryomyces DhCBS767 and Pichia PsCBS6054). Thick branches supported by >0.95 CC. Fungi (red): SACC = Saccharomycetaceae; Moss (green): FUNA = Funariaceae; Angiosperm-eudicot (black): BRAS = Brassicaceae, SALI = Salicaceae, SOLA = Solanaceae, VITA = Vitaceae; Angiosperm-monocot (blue): JOIN = Joinvilleaceae, POAC = Poaceae. Maize spi1 and orthologues in rice (OsYUC1), sorghum (Sb03g029440), and Joinvillea (JaSPI1) in bold.

Fig. 5.

Interaction between spi1 and genes required for auxin transport. (A) Mature tassel phenotype showing all phenotypic classes in spi1;bif2 family. (B) Whole plant phenotype of all phenotypic classes of spi1;bif2 family. (C and D) ZmPIN1a-YFP expression in young tassels. (C) In normal plants, ZmPIN1a-YFP expression marks the emerging axillary meristems (arrowheads). (D) In spi1 mutants, ZmPIN1a-YFP expression is absent on the flanks of the inflorescence when axillary meristems do not form. Expression is detected when axillary meristems have initiated (arrowhead). (Scale bars: 50 μm.)