| Summary: | <p>Abstract</p> <p>Background</p> <p>Wound-inducible Pin-II Proteinase inhibitors (PIs) are one of the important plant serine PIs which have been studied extensively for their structural and functional diversity and relevance in plant defense against insect pests. To explore the functional specialization of an array of <it>Capsicum annuum</it> (L.) proteinase inhibitor (<it>CanPIs</it>) genes, we studied their expression, processing and tissue-specific distribution under steady-state and induced conditions. Inductions were performed by subjecting <it>C. annuum</it> leaves to various treatments, namely aphid infestation or mechanical wounding followed by treatment with either oral secretion (OS) of <it>Helicoverpa armigera</it> or water.</p> <p>Results</p> <p>The elicitation treatments regulated the accumulation of <it>CanPIs</it> corresponding to 4-, 3-, and 2-inhibitory repeat domains (IRDs). Fourty seven different <it>CanPI</it> genes composed of 28 unique IRDs were identified in total along with those reported earlier. The <it>CanPI</it> gene pool either from uninduced or induced leaves was dominated by 3-IRD PIs and trypsin inhibitory domains. Also a major contribution by 4-IRD <it>CanPI</it> genes possessing trypsin and chymotrypsin inhibitor domains was specifically revealed in wounded leaves treated with OS. Wounding displayed the highest number of unique <it>CanPIs</it> while wounding with OS treatment resulted in the high accumulation of specifically <it>CanPI-4</it>, <it>-7</it> and −<it>10</it>. Characterization of the PI protein activity through two dimensional gel electrophoresis revealed tissue and induction specific patterns. Consistent with transcript abundance, wound plus OS or water treated <it>C. annuum</it> leaves exhibited significantly higher PI activity and isoform diversity contributed by 3- and 4-IRD <it>CanPIs</it>. <it>CanPI</it> accumulation and activity was weakly elicited by aphid infestation yet resulted in the higher expression of <it>CanPI</it>-26, <it>-41</it> and −<it>43</it>.</p> <p>Conclusions</p> <p>Plants can differentially perceive various kinds of insect attacks and respond appropriately through activating plant defenses including regulation of PIs at transcriptional and post-translational levels. Based on the differentially elicited <it>CanPI</it> accumulation patterns, it is intriguing to speculate that generating sequence diversity in the form of multi-IRD PIs is a part of elaborative plant defense strategy to obtain a diverse pool of functional units to confine insect attack.</p>
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