Abstract: A suppression subtractive hybridization（SSH）library of Poncirus trifoliata was successfully constructed for screening the citrus root-specific genes by using the root RNA as Driver and the leaf one as Tester. Of 1 362 sequenced clones，1 177 ESTs（expressed sequence tags）in good quality were acquired from the SSH library and assembled into 455 Unigenes. Among them，245 Unigenes were homologous with the genes in GenBank database，which have biology function such as catalytic activity，transporter activity，carrier activity and so on. They play the important roles in biology process such as metabolic process，cellular process，developmental process，response to stimulus and so on. The 17 genes were further analyzed by real-time PCR and confirmed that they were highly expressed in the roots，but very low in the leaves. Bioinformatic analysis indicated that they are the major latex protein gene，the early nodulin-like protein gene，the ent-kaurenoic acid oxidase gene and etc.

Key words: Poncirus trifoliata, root-specific gene, the suppression subtractive hybridization library, quantitive PCR