The Content of Small 18S rRNA Fragments Is Regulated Developmentally and in Response to Stress in Plants

Protein synthesis is a crucial biosynthetic process in all organisms, including plants. The integrity of the translational machinery, especially ribosomes, can be compromised during rapid cell division in ontogenesis or in response to environmental stress. In this study, Northern blotting was employed to analyze total RNA from various angiosperms, focusing on small 5'- and 3'-terminal 18S rRNA fragments. Stem-loop array RT-PCR was employed to map the cleavage sites within the target regions. Severe stress, such as extreme drought, induced the accumulation of three distinct 18S rRNA fragments across diverse angiosperm taxa, indicating that this phenomenon is likely universal. In rapidly dividing cells, such as those found in in vitro callus cultures and germinating wheat embryos, high levels of discrete 5'-terminal fragments were observed, while 3'-terminal fragments were absent. The stem-loop array RT-PCR mapping identified specific sites of 18S rRNA strand breaks. Structural annotation of the 3D model of the plant 40S subunit revealed spatial clustering of these sites in proximity to the RPS6 binding region. Notably, wheat cultivars that are tolerant to osmotic stress exhibited significantly higher levels of 18S rRNA fragmentation than sensitive cultivars. This suggests a regulatory mechanism rather than a mere byproduct of apoptotic-like regulated cell death. Additionally, fragmented ribosomes were gradually eliminated during embryo maturation, indicating a process of programmed functional ribophagy. Our findings suggest that a potential inability of plant tissues to selectively retain functional ribosomes might contribute to a decline in generative potential. Monitoring the integrity of the translational machinery could improve breeding efficiency and aid in preserving long-term stored germplasm.