Rice beta-tubulins mRNA levels are modulated during flower development and in response to external stimuli

Rice beta-tubulin cDNA clones have been recently isolated by different laboratories (Kang et al. (1994) Plant Mel. Biol. 26, 1975-1979; Breviario et al. (1995) Plant Physiol. 108, 823-824; Koga-Ban et al. (1995) DNA Res. 2, 21-26). Analysis of their deduced amino acid sequences shows the conservation of all those structural motifs typical of plant beta-tubulins. Putative sequences for autoregulation and tubulin mRNA stability, for GTP binding and exchange as well as for herbicides and Ca2+ binding are present. With the use of a generic or isotype specific beta-tubulin (OS-TUB16) probe we assayed the level of transcriptional accumulation in different rice tissues, during flower development, cell elongation and in response to anoxia. We found enhanced levels of total or isotype-16 specific beta-tubulin mRNA at early stages of flower development in spikelets at 8-10 days before anthesis. The lowest levels were observed in adult leaves. Total beta-tubulin as well as isotype-16 specific transcript levels were strongly decreased (10-fold) in rice coleoptiles treated with 50 mu M abscisic acid (ABA); a more modest decrease (2-fold) in the total level of rice alpha-tubulin mRNA was instead observed in these samples. We also show a strict correlation between growth under anoxia and beta-tubulin transcriptional abundance. Rice roots, unable to grow in anoxia, showed very low amount of total beta-tubulin mRNA whereas rice coleoptiles, capable of elongating under anaerobiosis, maintained high levels of total beta-tubulin transcripts. However, with the use of a 3' non-coding probe specific for beta-tubulin isotype-16, we found that the amount of this transcript is always decreased by anoxia regardless the type of tissue analyzed.