Early Embryogenesis-Specific Expression of the Rice Transposon <i>Ping</i> Enhances Amplification of the MITE <i>mPing</i>

<div><p>Miniature inverted-repeat transposable elements (MITEs) are numerically predominant transposable elements in the rice genome, and their activities have influenced the evolution of genes. Very little is known about how MITEs can rapidly amplify to thousands in the genome. The rice MITE <i>mPing</i> is quiescent in most cultivars under natural growth conditions, although it is activated by various stresses, such as tissue culture, gamma-ray irradiation, and high hydrostatic pressure. Exceptionally in the temperate <i>japonica</i> rice strain EG4 (cultivar Gimbozu), <i>mPing</i> has reached over 1000 copies in the genome, and is amplifying owing to its active transposition even under natural growth conditions. Being the only active MITE, <i>mPing</i> in EG4 is an appropriate material to study how MITEs amplify in the genome. Here, we provide important findings regarding the transposition and amplification of <i>mPing</i> in EG4. Transposon display of <i>mPing</i> using various tissues of a single EG4 plant revealed that most <i>de novo mPing</i> insertions arise in embryogenesis during the period from 3 to 5 days after pollination (DAP), and a large majority of these insertions are transmissible to the next generation. Locus-specific PCR showed that <i>mPing</i> excisions and insertions arose at the same time (3 to 5 DAP). Moreover, expression analysis and <i>in situ</i> hybridization analysis revealed that <i>Ping</i>, an autonomous partner for <i>mPing</i>, was markedly up-regulated in the 3 DAP embryo of EG4, whereas such up-regulation of <i>Ping</i> was not observed in the <i>mPing</i>-inactive cultivar Nipponbare. These results demonstrate that the early embryogenesis-specific expression of <i>Ping</i> is responsible for the successful amplification of <i>mPing</i> in EG4. This study helps not only to elucidate the whole mechanism of <i>mPing</i> amplification but also to further understand the contribution of MITEs to genome evolution.</p></div>