Pollination Drop Proteome and Reproductive Organ Transcriptome Comparison in Gnetum Reveals Entomophilous Adaptation

Abstract

Gnetum possesses morphologically bisexual but functionally unisexual reproductive structures that exude sugary pollination drops to attract insects. Previous studies have revealed that the arborescent species (G. gnemon L.) and the lianoid species (G. luofuense C.Y.Cheng) possess different pollination syndromes. This study compared the proteome in the pollination drops of these two species using label-free quantitative techniques. The transcriptomes of fertile reproductive units (FRUs) and sterile reproductive units (SRUs) for each species were furthermore compared using Illumina Hiseq sequencing, and integrated proteomic and transcriptomic analyses were subsequently performed. Our results show that the differentially expressed proteins between FRUs and SRUs were involved in carbohydrate metabolism, the biosynthesis of amino acids and ovule defense. In addition, the differentially expressed genes between the FRUs and SRUs (e.g., MADS-box genes) were engaged in reproductive development and the formation of pollination drops. The integrated protein-transcript analyses revealed that FRUs and their exudates were relatively conservative while the SRUs and their exudates were more diverse, probably functioning as pollinator attractants. The evolution of reproductive organs appears to be synchronized with changes in the pollination drop proteome of Gnetum, suggesting that insect-pollinated adaptations are not restricted to angiosperms but also occur in gymnosperms.

Comparative Analyses of Full-Length Transcriptomes Reveal Gnetum luofuense Stem Developmental Dynamics

“…These differences might be because apical stem tissues lack a physical barrier between them and so the cuticle is potentially vulnerable to external pathogen and fungi. This scenario is similar to the vulnerability seen in fertile and sterile G. luofuense and G. gnemon reproductive units during pollination periods ( Hou et al, 2019b ). In terms of detected TFs, the gene TnS000176189g04 encodes a MYB-related TF and was shown to be principally expressed at GLN01 using qRT-PCR analysis ( Figure 6 ).…”

Fleshy Structures Associated with Ovule Protection and Seed Dispersal in Gymnosperms: A Systematic and Evolutionary Overview

“…These additional envelopes differ from the two integuments of angiosperms for having different origin and developmental pattern, being probably derived from bracts (Rydin et al, 2010). The thin ovule integument extends beyond the seed envelope(s), forming the micropyle from which the pollination drop will be secreted to allow pollen capture (Ickert-Bond and Rydin, 2011;J€ orgensen and Rydin, 2015;Hou et al, 2019). Unlike to what happens in other gymnosperms, in Gnetum and Ephedra two simultaneous fertilization events occur.…”

PacBio Long-Read Sequencing Reveals the Transcriptomic Complexity and Aux/IAA Gene Evolution in Gnetum (Gnetales)

“…A final of 53,057 transcripts were assembled, and 26,305 genes from 3896 gene families were annotated against the seven databases (NR, SwissProt, KOG, COG, GO, Pfam and KEGG) ( Figure 3A). Among the ten KEGG pathways annotated ( Figure 3D), "starch and sucrose metabolism", "purine metabolism", "ribosome", "biosynthesis of amino acids", and "carbon metabolism" might participate in the secretion of sugary pollination drops in G. gnemon and G. luofuense as the response to insect pollination [80]. Besides, KEGG pathways in "phenylpropanoid biosynthesis", "plant hormone signal transduction", and "plant-pathogen interaction" might affect fertile reproductive unites development of the two species [80].…”

“…Besides this, MIKC c -group genes-e.g., GGM2, GGM3, GGM9 and GGM11-participate in the sexual determination and development of female strobili in G. gnemon [14,17,104]. A genome-wide research of MADS-box genes yielded a total of 35 MIKC c -group genes [105], the majority of which were detected to express female strobili and fertile reproductive ovules of G. luofuense [80].…”