Thioredoxins are ubiquitous small proteins with a redox-active disulfide bridge; they participate in redox regulation of selected target proteins, such as transcription factors, receptors and various metabolic and regulatory enzymes. In the present study, we isolated by PCR cDNA subtraction analysis a grapefruit 135-bp polymerase chain reaction (PCR) fragment whose expression was enhanced in the fruit peel tissue following the application of Candida oleophila yeast cells, an elicitation treatment that induces resistance towards the green mold pathogen Penicillium digitatum (Pers.:Fr.) Sacc., and that showed high sequence homology with various thioredoxin h (TRXh) genes. The full-length cDNA sequence of cTRXh (for citrus TRXh) was further isolated by screening a grapefruit flavedo cDNA library; it had a total length of 785 bp with an open reading frame of 369 bp, and encoded a predicted polypeptide of 123 amino acids with a molecular mass of 13.4 kDa. Expression analysis studies revealed that cTRXh mRNA levels increased in grapefruit peel tissue upon infection by P. digitatum, and following the elicitation of fruit disease resistance by application of C oleophila yeast cells, wounding, exposure to ethylene, and a hot water rinsing and brushing treatment. These results indicate that cTRXh may be part of the molecular mechanisms involved in the induction of pathogen defense responses in grapefruit, and may be involved in the signaling cascades leading to enhanced resistance. (c) 2005 Elsevier Ltd. All rights reserved.