Identification of Kunitz-Type Inhibitor Gene Family of Populus yunnanensis Reveals a Stress Tolerance Function in Inverted Cuttings

Plant protease inhibitors are a ubiquitous feature of plant species and exert a substantial influence on plant stress responses. However, the <i>KTI</i> (Kunitz trypsin inhibitor) family responding to abiotic stress has not been fully characterized in <i>Populus yunnanensis</i>. In this study, we conducted a genome-wide study of the <i>KTI</i> family and analyzed their gene structure, gene duplication, conserved motifs, cis-acting elements, and response to stress treatment. A total of 29 <i>KTI</i>s were identified in the <i>P. yunnanensis</i> genome. Based on phylogenetic analysis, the Py<i>KTI</i>s were divided into four groups (1,2, 3, and 4). Promoter sequence analysis showed that the Py<i>KTI</i>s contain many cis-acting elements related to light, plant growth, hormone, and stress responses, indicating that Py<i>KTI</i>s are widely involved in various biological regulatory processes. RNA sequencing and real-time quantitative polymerase chain reaction analysis showed that <i>KTI</i> genes were differentially expressed under the inverted cutting stress of <i>P. yunnanensis</i>. Transcriptome analysis of <i>P. yunnanensis</i> leaves revealed that <i>PyKTI16</i>, <i>PyKTI18</i>, and <i>PyKTI19</i> were highly upregulated after inverted cutting. Through the GEO query of <i>Populus transcriptome</i> data, <i>KTI</i> genes played a positive defense role in MeJa, drought, time series, and pathogen stress. This study provided comprehensive information for the <i>KTI</i> family in <i>P. yunnanensis</i>, which should be helpful for the functional characterization of <i>P. yunnanensis KTI</i> genes in the future.