Persistence and Protection of Mitochondrial DNA in the Generative Cell of Cucumber is Consistent with its Paternal Transmission

Plants predominantly show maternal transmission of mitochondrial DNA (mtDNA). One known exception is cucumber, in which the mtDNA is paternally inherited. However, the mechanisms regulating this unique mode of transmission are unclear. Here we monitored the amounts of mtDNA throughout the development of cucumber microspores into pollen and observed that mtDNA decreases in the vegetative cell, but persists in the generative cell that ultimately produces the sperm cells. We characterized the cucumber homolog (CsDPD1) of the Arabidopsis gene defective in pollen organelle DNA degradation 1 (AtDPD1), which plays a direct role in mtDNA degradation. CsDPD1 rescued an Arabidopsis AtDPD1 mutant, indicating the same function in both plants. Expression of CsDPD1 coincided with the decrease of mtDNA in pollen, except in the generative cell where both the expression of CsDPD1 and mtDNA levels remained high. Our cytological results confirmed that the persistence of mtDNA in the cucumber generative cell is consistent with its paternal transmission. Our molecular analyses suggest that protection of mtDNA in the generative cell may be the critical factor for paternal mtDNA transmission, rather than mtDNA degradation mediated by CsDPD1. Taken together, these findings indicate that a mechanism may protect paternal mtDNA from degradation and is likely to be the genetic basis of paternal mtDNA transmission.