The BUD13 splicing regulator: transcript structure and expression in ovules of sexual and apomictic Paspalum notatum

Background and AimsPaspalum notatum reproduces through either sexuality or apomixis, two pathways that may coexist within the same individual and are regulated by interconnected molecular networks responsive to environmental cues. Here, we characterized the transcript structure and expression of BUD SITE SELECTION PROTEIN 13 (BUD13), a component of the RES spliceosomal complex previously reported as differentially expressed in florets of sexual and apomictic plants, as a first step toward testing its involvement in the molecular regulation of the apomixis-sexuality switch. MethodsPreviously generated floral and leaf transcriptomes from sexual and apomictic Paspalum notatum plants, including Oxford Nanopore long-read data, were mined to characterize BUD13 transcript structure and expression. Phylogenetic analyses and in silico mapping were conducted to infer evolutionary relationships and determine the origin of the transcripts. Differential expression was validated by RT-qPCR, while in situ hybridization was used to reveal cell-specific ovule expression patterns. Key resultsBUD13 is expressed in Paspalum notatum florets as a truncated isoform (SHORT) encoding a small protein lacking part of the herpes simplex virus regulatory protein (ICP4) domain. Two SHORT transcripts, SHORT1 and SHORT2, with different 5' untranslated region (UTR) regions, were identified in flowers. SHORT1 was consistently upregulated in apomictic ovules from premeiosis to anthesis. Both transcripts originated from a single genomic locus located in the subtelomeric region of the short arm of chromosome 6. SHORT isoforms with variable structures were detected in other monocots. In situ hybridization showed that, whereas BUD13 was expressed throughout sexual ovules, expression was absent from the female germline of apomictic ovules. A consistent expression was observed in somatic proembryos of aposporous embryo sacs. ConclusionsOur findings reveal structural, spatial and temporal divergence in BUD13 expression between sexual and apomictic reproductive programs, providing new insights into the molecular regulation of asexual seed formation.