Cytoplasmic ribonucleoprotein (RNP) granules are membraneless structures composed of various RNAs and proteins that play important roles in post-transcriptional regulation. While RNP granules are known to regulate the meiotic entry in some organisms, little is known about their roles in plants.
In this study, we observed the cytoplasmic granular structures of rice RNA-binding protein MEL2, which contributes to the control of meiotic entry timing, in leaf protoplasts and spore mother cells. Our results indicated that MEL2 granules colocalized with processing body and stress granule factors. The maintenance of granule properties modulated by LOTUS domain and the intrinsically disordered region (IDR) is essential for proper MEL2 function in meiosis progression (Figure). MEL2-like proteins widely found in plant kingdom conserved LOTUS domain followed by the IDR despite their diverse domain structures, suggesting the functional conservation of these domains among plant species.
Figure: Roles of MEL2 functional domains in granule formation (a) Genomic fusion constructs of MEL2 and GFP genes, and its derivatives with deletions (dashed lines) or mutations (asterisks) in MEL2 domains. (b) MEL2 granule formation in spore mother cells in rice anthers transformed with the constructs in (a). The wildtype MEL2 (FL) formed granules (green) at premeiosis (0.3-0.4mm anthers), and disappeared by meiosis (0.5mm anthers). The mRING failed granule formation, and the granules of ∆LOTUS and ∆IDR were aberrantly carried over to meiocytes, all resulting in defective meiosis and pollen development.
