Fruit-Specific Expression of a Defensin-Type Gene Family in Bell Pepper (Upregulation during Ripening and upon Wounding)

We have isolated a 454-bp cDNA that encodes a novel fruit-specific defensin from bell pepper (Capsicum annuum). The encoded 75-amino-acid polypeptide contains an N-terminal domain characteristic of a signal peptide and a 48-amino-acid mature domain named J1. The mature protein, from which the N-terminal amino acid sequence was determined, contains eight cysteines that form four intramolecular disulfide bridges, suggesting a monomeric form for J1. In healthy fruits J1 is undetectable at the green stage but high levels accumulate during ripening. In wound areas of the green fruit the accumulation of J1 dramatically increased, suggesting a role for J1 in the plant9s defense response. Moreover, we have demonstrated that J1 possesses an antifungal activity. We have isolated and characterized the corresponding two homologous genes (j1–1 and j1–2) that exist in the bell pepper genome. Both genes are interrupted by the insertion, at the same position, of one intron of 853 bp for j1–1 and 4900 bp for j1–2. Northern blot and reverse transcriptase-polymerase chain reaction and restriction fragment length polymorphism analyses revealed that j1–1 transcripts are present only in fruits, only in trace amounts in mature green fruits, and that they accumulate to high levels in fully ripe fruits, whereas no j1–2 transcripts were detected in the samples monitored.