ESTIMATION OF UBIQUITIN AND UBIQUITIN MESSENGER-RNA CONTENT IN DARK SENESCING WHEAT LEAVES

Wheat leaves (Triticum aestivum L. cv. San Agustin INTA) were detached at the moment they had reached maximum expansion, put in tubes contai ning water and left in darkness. Under these conditions, leaf protein content decreased mainly as consequence of an increased rate of breakd own. In the range of 0 to 72 h after detachment, western blot analysis of leaf protein extracts displayed both similar proportions of total protein and quality of ubiquitin conjugates. Northern blot analysis of leaf RNA extracts revealed a 1.6 kb ubiquitin mRNA transcript which i ncreased 3.5-fold after 48 h of treatment. Thus wheat leaves maintain both their ability for the ubiquitination of proteins and the transcri ption of ubiquitin mRNA at stages of senescence in which rates of prot ein breakdown are increased. The results suggest that the ubiquitin-de pendent proteolytic pathway contributes to leaf protein breakdown duri ng senescence.