The Plant Ovule Secretome: A Different View toward Pollen–Pistil Interactions

During plant sexual reproduction,
continuous exchange of signals
between the pollen and the pistil (stigma, style, and ovary) plays
important roles in pollen recognition and selection, establishing
breeding barriers and, ultimately, leading to optimal seed set. After
navigating through the stigma and the style, pollen tubes (PTs) reach
their final destination, the ovule. This ultimate step is also regulated
by numerous signals emanating from the embryo sac (ES) of the ovule.
These signals encompass a wide variety of molecules, but species-specificity
of the pollen–ovule interaction relies mainly on secreted proteins
and their receptors. Isolation of candidate genes involved in pollen–pistil
interactions has mainly relied on transcriptomic approaches, overlooking
potential post-transcriptional regulation. To address this issue,
ovule exudates were collected from the wild potato species Solanum chacoense using a tissue-free gravity-extraction
method (tf-GEM). Combined RNA-seq and mass spectrometry-based proteomics
led to the identification of 305 secreted proteins, of which 58% were
ovule-specific. Comparative analyses using mature ovules (attracting
PTs) and immature ovules (not attracting PTs) revealed that the last
maturation step of ES development affected almost half of the ovule
secretome. Of 128 upregulated proteins in anthesis stage, 106 were
not regulated at the mRNA level, emphasizing the importance of post-transcriptional
regulation in reproductive development.