To identify genes related to initiation of storage root development in sweetpotato, a cDNA library was constructed with early stage storage roots (0.3^1 cm in diameter). Singlepass sequences of the 5P P ends of 2859 sweetpotato cDNA clones were assembled into 483 clusters and 442 singletons. Comparison of sweetpotato expressed sequence tags (ESTs) to nodulation/tumorigenesis-related sequence databases (nodule-, tumor-, potato tuber-and development-related sequences) revealed that homologs of 39 sweetpotato EST sequences potentially involved in gene regulation, signal transduction and development were present in at least one of the nodulation/tumorigenesis-related sequence databases. Northern blot analyses of these 39 sequences identi¢ed 22 di¡erentially expressed genes in early stage storage root and ¢brous root. These di¡erentially expressed genes will be potential candidates for research to elucidate the molecular processes related to sweetpotato storage root induction. ß

The interaction between proliferating cell nuclear antigen (PCNA) and DNA polymerase ␦ is essential for processive DNA synthesis during DNA replication/repair; however, the identity of the subunit of DNA polymerase ␦ that directly interacts with PCNA has not been resolved until now. In the present study we have used reciprocal co-immunoprecipitation experiments to determine which of the two subunits of core DNA polymerase ␦, the 125-kDa catalytic subunit or the 50-kDa small subunit, directly interacts with PCNA. We found that PCNA co-immunoprecipitated with human p50, as well as calf thymus DNA polymerase ␦ heterodimer, but not with p125 alone, suggesting that PCNA directly interacts with p50 but not with p125. A PCNA-binding motif, similar to the sliding clamp-binding motif of bacteriophage RB69 DNA polymerase, was identified in the N terminus of p50. A 22-amino acid oligopeptide containing this sequence (MRPFL) was shown to bind PCNA by far Western analysis and to compete with p50 for binding to PCNA in co-immunoprecipitation experiments. The binding of p50 to PCNA was inhibited by p21, suggesting that the two proteins compete for the same binding site on PCNA. These results establish that the interaction of PCNA with DNA polymerase ␦ is mediated through the small subunit of the enzyme.

Screening of the expressed sequence tag library of the wild rice species Oryza minuta revealed an unknown gene that was rapidly and strongly induced in response to attack by a rice fungal pathogen (Magnaporthe oryzae) and an insect (Nilaparvata lugens) and by wounding, abscisic acid (ABA), and methyl jasmonate treatments. Its recombinant protein was identified as a bifunctional nuclease with both RNase and DNase activities in vitro. This gene was designated OmBBD (for O. minuta bifunctional nuclease in basal defense response). Overexpression of OmBBD in an Arabidopsis (Arabidopsis thaliana) model system caused the constitutive expression of the PDF1.2, ABA1, and AtSAC1 genes, which are involved in priming ABA-mediated callose deposition. This activation of defense responses led to an increased resistance against Botrytis cinerea. atbbd1, the knockout mutant of the Arabidopsis ortholog AtBBD1, was susceptible to attack by B. cinerea and had deficient callose deposition. Overexpression of either OmBBD or AtBBD1 in atbbd1 plants complemented the susceptible phenotype of atbbd1 against B. cinerea as well as the deficiency of callose deposition. We suggest that OmBBD and AtBBD1 have a novel regulatory role in ABA-mediated callose deposition.

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