Hormonal and environmental responsiveness of a developmentally regulated protein repair L-isoaspartyl methyltransferase in wheat.

The L-isoaspartyl protein methyltransferase (EC 2.1.1.77) has been proposed to be involved in the repair of spontaneously damaged proteins by facilitating the conversion of abnormal L-isoaspartyl residues to normal L-aspartyl residues. Based on the abundance of this enzyme in the seeds of a variety of plants and its unique substrate specificity, it has been hypothesized that it functions to prevent the accumulation of abnormal aspartyl residues in the proteins of aging seeds that can limit the viability of the embryo or its chances for germination. In this work, we show that the expression of the L-isoaspartyl methyltransferase is under developmental regulation in the winter wheat, Triticum aestivum. Methyltransferase mRNA and active enzyme are first detected in seeds during the late stages (III-IV) of caryopsis development. As mature seeds germinate, methyltransferase mRNA levels decline and are nearly undetectable by 72 h post-imbibition. Enzyme activity remains constant for 24 h post-imbibition and then decreases rapidly following the reduction of its corresponding mRNA. Methyltransferase activity is very low or undetectable in wheat seedlings, including leaf and root tissues. We show, however, that the L-isoaspartyl methyltransferase can be induced in vegetative tissues in response to hormone treatment and environmental stress. Abscisic acid, a phytohormone involved in seed development and desiccation tolerance, induces both methyltransferase mRNA and enzyme activity in 4-day-old wheat seedlings. Dehydration and salt stress also induce its transcription and enzymatic activity in seedlings. The ability of a plant to regulate methyltransferase activity in its seeds and vegetative tissues in response to desiccation, aging, and environmental stress may allow the plant to efficiently repair protein damage associated with these physiological changes.