Expression of Protein L-Isoaspartyl Methyltransferase in sunflower seeds as affected by ageing and priming

In orthodox seeds, non-enzymatic protein damage occurring during dehydration and storage is hypothesized to be a main cause of deterioration. Therefore, for vigorous germination dry seeds should be equipped with protein repair mechanisms. One of these involves protein L-isoaspartyl methyltransferase (PIMT) which counteracts protein misfolding by catalyzing the conversion of abnormal L-isoaspartyl residues to their regular form. PIMT activity is primarily localized in seed tissues during and after maturation desiccation, suggesting its involvement in restoring the functional conformation of the seed proteome. This study was initiated to assess the putative role of PIMT repair pathway on seed vigor in Helianthus annuus whose seeds are rather sensitive to deterioration during storage. A sunflower homolog of Arabidopsis PIMT1 was isolated and characterized, and its expression in seeds was measured by qRT-PCR on a panel of 16 inbred lines subjected to ageing, priming, and priming after-ageing. The wide variability in the physiological response observed among the lines and the considerable interactions between factors produced a rather complex picture. Considered separately, ageing and priming had expected, although only modest, effects on final germination, i.e. negative the former (-2.4%) and positive the latter (+4.7%). More marked were the effects on germination rate, with an 8.3% increase in T50 after ageing and a 30.6% reduction after priming. On average PIMT transcript levels were highest in quiescent seeds and reached the minimum after ageing, with a 38% drop; also after priming PIMT mRNA decreased 26%, while in primed after-ageing seeds its abundance was variously affected. Therefore PIMT expression pattern in sunflower resembles that of other protein transcripts with protective role in dry seeds. Correlations between transcript levels and germination percentage and rate were not found in control and aged seeds; on the contrary, correlations were negative and significant when priming was applied, perhaps indicating either an attempted repair response or a reduced degradation in damaged tissues. For the complexity of the responses observed, further studies on gene expression and enzyme activity will be necessary to assess the role of this repair mechanism in maintaining seed vigor in sunflower.