Plant Molecular Biology 39: 823–833, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 823 Degradation of oat mRNAs during seed development Russell R. Johnson 1 , Marta E. Chaverra 2 , Harwood J. Cranston 2 , Tzili Pleban 2 and William E. Dyer 2,∗ 1 Department of Biology, Colby College, Waterville, ME 04901-8857, USA; 2 Department of Plant Sciences, Montana State University, Bozeman, MT 59717-0312, USA ( ∗ author for correspondence) Received 9 July 1998; accepted in revised form 10 November 1998 Key words: avenin, oats, protein Z, puroindoline, RNA degradation Abstract The genes AV1, AV10, and Z1 encode proteins that accumulate during oat seed development. In developing endosperm of Avena sativa (cultivated oat), AV1, AV10 and Z1 mRNAs reach maximal levels midway through seed development but fall to very low levels in mature seeds. Similarly, mRNAs for these proteins peak during en- dosperm development of Avena fatua (wild oat) and are later degraded. However, during late maturation of A. fatua seeds, populations of mRNA fragments shorter than the intact transcripts accumulate as the full-length transcripts decline in abundance. The smaller RNA molecules, which are apparently long-lived decay intermediates, are derived randomly from the entire transcripts and are most likely not generated by cleavage at precisely defined sites. Other A. fatua endosperm mRNAs that are degraded during late seed development, such as those for ADP glucose pyrophosphorylase and starch synthase, do not produce detectable decay intermediates. Decay intermediates of AV1 and Z1 mRNAs persist at high levels during late seed development of two other undomesticated oat species, Avena strigosa and Avena barbata. The persistence of decay intermediates for these endosperm mRNAs in wild grass species may represent a model system for studying RNA decay process in plant tissues. Introduction Control of mRNA stability plays a critical role in the overall regulation of gene expression, in coopera- tion with transcriptional and translational controls [1]. Degradation of most mRNA transcripts is tightly reg- ulated and is often mediated by interactions between specific nucleotide sequences and factors that bind to these elements [3, 38]. In plants, most mRNAs have half-lives on the order of several hours and are be- lieved to be inherently stable unless they are actively destabilized [55, 56]. Several recent studies have focused on the mech- anisms that control mRNA stability in plants. These efforts have identified specific sequences that target short-lived mRNAs for rapid degradation [33, 38, 39]. The nucleotide sequence data reported will appear in the EMBL, GenBank and DDBJ Nucleotide Sequence Databases un- der the accession numbers AF118559 (AV1), AF118560 (Z1), and AF117889 (AFD5). Stability-conferring motifs have been characterized in several animal systems [28, 29] but very little is known about specific sequences or factors that may confer high stability to plant mRNAs. The poly(A) tail and 5 ′ cap have been shown to have stabilizing effects on some plant transcripts [16] but so far no internal plant mRNA motifs have been identified that protect against degradation. In almost all cases, once mRNA degrada- tion is initiated, full-length transcripts are very rapidly converted into fragments too small to be detected on RNA blots, indicating that mRNA decay intermediates are extremely transient [1]. During seed development a number of mRNAs accumulate and then decline in developmentally reg- ulated patterns, including those for important seed proteins like storage proteins [46], enzymes for starch biosynthesis [4], and late embryogenesis-abundant (LEA) proteins [24]. mRNAs for certain storage pro- teins such as prolamines [11], protein Z [6], and puroindolines [17] are expressed specifically in cereal