25mRNA translation is a critical step in gene expression, but our understanding of the 26 landscape and control of translation in diverse crops remains lacking. Here, we combined de 27 novo transcriptome assembly and ribosome profiling to study global mRNA translation in tomato 28 roots. Taking advantage of the 3-nucleotide periodicity displayed by translating ribosomes, we 29 identified 354 novel small ORFs (sORFs) translated from previously unannotated transcripts, as 30 well as 1329 upstream ORFs (uORFs) translated within the 5' UTRs of annotated protein-coding 31 genes. Proteomic analysis confirmed that some of these novel uORFs and sORFs generate 32 stable proteins in planta. Compared with the annotated ORFs, the uORFs use more flexible 33 Kozak sequences around translation start sites. Interestingly, uORF-containing genes are 34 enriched for protein phosphorylation/dephosphorylation and signaling transduction pathways, 35 suggesting a regulatory role for uORFs in these processes. We also demonstrated that 36 ribosome profiling is useful to facilitate the annotation of translated ORFs and noncanonical 37 translation initiation sites. In addition to defining the translatome, our results revealed the global 38 control of mRNA translation by uORFs and microRNAs in tomato. In summary, our approach 39 provides a high-throughput method to discover unannotated ORFs, elucidates evolutionarily 40 conserved translational features, and identifies new regulatory mechanisms hidden in a crop 41 genome. 42 43 48 transcriptome assembly and ribosome profiling, we mapped and quantified translating 49 ribosomes across the entire transcriptome in tomato roots. This is the first experiment-based 50 survey to systematically identify actively translated ORFs in a crop. Our results reveal numerous 51 unannotated translation events and uncover new regulatory mechanisms of gene expression in 52 tomato. Our approach not only facilitates our understanding of the tomato translational 53 landscape but also provides a practical strategy to study the translatomes of other species. 54 55 56 Introduction 57Besides being an essential step in gene expression, mRNA translation directly shapes 58 the proteome, which contributes to cellular structure, function, and activity in all organisms. The 59 characterization of translational regulation has enabled crop improvement, including increasing 60 tomato sweetness, rice immunity and lettuce resistance to oxidative stress (1-3). However, due 61 to limited genomic resources and methods, most crop translatomes remain understudied.
62Ribosome profiling, or Ribo-seq, has emerged as a high-throughput technique to study 63 global translation (4-6). In a Ribo-seq experiment, ribosomes in the sample of interest are 64 immobilized, and the lysate is treated with nucleases to obtain ribosome-protected mRNA 65 fragments (i.e. ribosome footprints). Finally, sequencing of the ribosome footprints reveals the 66 quantity and positions of ribosomes on a given transcript. Features corresponding to active 67 translat...