Rice Immature Pollen 1 ( RIP1 ) is a Regulator of Late Pollen Development

We isolated a pollen-preferential gene, RICE IMMATURE POLLEN 1 (RIPI), from a T-DNA insertional population of japonica rice that was trapped by a promoterless β-glucuronidase (GUS) gene. Semi-quantitative reverse transcription-PCR (RT-PCR) analyses confirmed that the RIPI transcript was abundant at the late stages of pollen development. Transgenic plants carrying a T-DNA insertion in the RIPI gene displayed the phenotype of segregation distortion of the mutated ripl gene. Moreover, ripl/ripl homozygous progeny were not present. Reciprocal crosses between Ripl/ripl heterozygous plants and the wild type showed that the ripl allele could not be transmitted through the male. Microscopic analysis demonstrated that development in the ripl pollen was delayed, starting at the early vacuolated stage. Close examination of that pollen by transmission electron microscopy also showed delayed formation of starch granules and the intine layer. In addition, development of the mitochondria, Golgi apparatus, lipid bodies, plastids and endoplasmic reticulum was deferred in the mutant pollen. Under in vitro conditions, germination of this mutant pollen did not occur, whereas the rate for wild-type pollen was >90%. These results indicate that RIPI is necessary for pollen maturation and germination. This gene encodes a protein that shares significant homology with a group of proteins containing five WD40 repeat sequences. The green fluorescent protein (GFP)-RIP1 fusion protein is localized to the nucleus. Therefore, RIPI is probably a nuclear protein that may form a functional complex with other proteins and carry out essential cellular and developmental roles during the late stage of pollen formation.