Genetic architecture of the sieve element occlusion (SEO) family underlying potential regulatory mechanisms in citrus stress response.

The Sieve Element Occlusion (SEO) proteins are subunits that assemble into structural filamentous phloem proteins, commonly referred to as P-proteins. These proteins in Arabidopsis play a structural role and contribute to plant defense by reversible sieve plate (SP) sealing. This mechanism could be particularly important also in Citrus trees affected by Huanglongbing (HLB), as SEO genes are highly induced in susceptible species C. sinensis upon Candidatus Liberibacter asiaticus (CLas) colonization. Given the limited information on this gene family in Citrus, we analyzed 27 SEO genes within the Rutaceae family, with a focus on genera closely related to Citrus, to better understand their potential roles in HLB tolerance. Genomic sequences revealed conserved exon-intron structures similar to Arabidopsis thaliana, while promoter regions contained a higher number of light-responsive Cis-elements, along with elements associated with growth, development, stress responses, and phloem-specific gene expression. Subcellular localization identified the cytoplasm as the primary site, with additional predictions for the plasma membrane and mitochondria. Phylogenetic analysis categorized SEO proteins into five subgroups, refining their classification in Citrus. Lately, protein interaction networks indicated strong connections with proteins involved in coordinated immune responses. These findings improve the understanding of SEO protein dynamics and evolutionary conservation, highlighting their role in phloem biology. Further investigation of these SEO genes and their promoters in the plant response to HLB could help identify specific targets for developing disease-tolerant Citrus varieties through genetic engineering.