Protease inhibitors have been associated with resistance to insect infestation in multiple crops and have been used successfully to confer resistance to insect pests in transgenic plants. We are reporting on the cDNAs encoding seven protease inhibitor-like peptides (PIs) isolated from a grapefruit immature fruit flavedo cDNA library. The cDNA clones belonged to the following PI families: legume Kuntiz inhibitors (LkiL-1, LkiL-2, LkiL-3), potato trypsin inhibitor I (PtiIL-1), serpins (SerpL-1), cystatins (CysL-1), and gamma thionins (GthL-1).LkiL-2 clone was the most abundantly cloned sequence and SerpL-1 was the least abundant being represented 93 and 2 times respectively out of 34273 EST sequences.Response of steady state transcript abundance to fruit development and leaf wounding was determined for all but LkiL-3 using real-time RT-PCR. Immature leaves had the highest transcript levels for all but CystL-1 which was equally represented in immature leaves and young fruit. Comparing all clones and tissues, the highest transcript level was observed for GthL-1 in immature sink leaves. In fruit flavedo, transcripts for all PIs were most abundant in the youngest fruit (<15mm dia. fruit). Mechanical or Diaprepes root weevil feeding damage caused a less than 10-fold reduction or had no effect on transcript pool size with the exception of GthL-1 which increased at least 50-fold as a result of mature leaf damage and decreased >1400 fold in damaged immature leaves. Changes in citrus PI transcript levels suggest diverse roles for the products of these genes in citrus growth and development, with only the GthL-1 functioning in an induced defense response role.
Keywords: serine protease inhibitor, gamma thionin
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