Experimental Farm・Nonomura Group

The genome of cultivated rice contains 32,000 genes, and more than 20,000 are expressed in developing anther and pollen. However, there are few genes whose function is evidenced in pollen development. In this study, we succeeded to identify the rice gene with its function specifically in pollen formation, and named COLLAPSED ABNORMAL POLLEN1 (CAP1).

In angiosperms, the pollen has a special, tricellular structure of a pair of sperm cells involved within the vegetative cell. The CAP1 gene is strongly expressed in anthers at the bicellular pollen stage (Fig.1). The pollen grains lacking CAP1 function lose most cellular components except for outer pollen wall, exine (Fig.2), and are unable to elongate the pollen tube at all. Homozygous cap1 mutant plants exhibit no remarkable aberration in other developmental stages, indicating the specific function of CAP1 in pollen development.

The amino acid alignment of rice CAP1 protein is similar to that of the plant L-arabinokinase. The function of this enzyme is supposed in phosphorylation of free L-arabinoses, generated during the cell-wall metabolism, for reuse in de novo wall formation. It is likely in rice cap1 mutant that the arabinoses unable to be reused accumulate aberrantly or the cell-wall metabolism is disrupted in pollen grains. One of Arabidopsis L-arabinokinase-like genes shows a similar expression pattern to rice CAP1-gene expression. This result suggests that the CAP1 function is conserved broadly among angiosperm species, and plays an important role in pollen development.

This is a collaborative work with the Akita Prefectural University and the National Institute of Agribiological Sciences, Japan, and is supported by the NIG Collaborative Research Funding.