Understanding the Role of PIN Auxin Carrier Genes under Biotic and Abiotic Stresses in Olea europaea L.

SIMPLE SUMMARY: The plant hormone auxin is involved in the majority of the processes related to plant development and growth and response to environmental constraints. The PIN proteins are a group of auxin transporters that are also involved in plant responses to stresses. This study describes the PIN gene family in the domesticated olive tree (Olea europaea subsp. europaea var. europaea) and its wild relative (O. europaea subsp. europaea var. sylvestris). Twelve and 17 PIN genes were identified for vars. sylvestris and europaea, respectively, being distributed across 6 subfamilies. Differences in the patterns of gene diversification and subfamilies expansion were observed across subfamilies. Genes encoding canonical OePIN proteins comprise six exons, while genes encoding non-canonical OePINs are composed of five exons, with implications for protein specificities and functionality. Expression analysis of OePINs from RNA-seq data showed that members from the subfamilies 1, 2, and 3 responded to abiotic and biotic stress factors. Our study shows the diversification of PINs in an important species such as the olive tree and highlights the importance of PIN genes on stress responses, contributing to a holistic understanding of the role of auxins in plants. ABSTRACT: The PIN-FORMED (PIN) proteins represent the most important polar auxin transporters in plants. Here, we characterized the PIN gene family in two olive genotypes, the Olea europaea subsp. europaea var. sylvestris and the var. europaea (cv. ‘Farga’). Twelve and 17 PIN genes were identified for vars. sylvestris and europaea, respectively, being distributed across 6 subfamilies. Genes encoding canonical OePINs consist of six exons, while genes encoding non-canonical OePINs are composed of five exons, with implications at protein specificities and functionality. A copia-LTR retrotransposon located in intron 4 of OePIN2b of var. europaea and the exaptation of partial sequences of that element as exons of the OePIN2b of var. sylvestris reveals such kind of event as a driving force in the olive PIN evolution. RNA-seq data showed that members from the subfamilies 1, 2, and 3 responded to abiotic and biotic stress factors. Co-expression of OePINs with genes involved in stress signaling and oxidative stress homeostasis were identified. This study highlights the importance of PIN genes on stress responses, contributing for a holistic understanding of the role of auxins in plants.