Correlative changes in proteases and protease inhibitors during mobilisation of protein from potato (Solanum tuberosum) seed tubers

Abstract

Potato tubers (Solanum tuberosum L.) contain protease inhibitors that function in plant defence and as storage proteins. A multi-domain cysteine protease inhibitor, potato multicystatin (PMC), has also been implicated in regulating protein accumulation in developing tubers by inhibiting proteases. Unlike developing tubers, sprouting tubers mobilise protein reserves to support growth of developing plants and, therefore, show an increase in protease activity. Using single-eye containing cores (seedcores) from seed tubers, we characterised the relative changes in patatin, PMC, proteases and serine (Ser) protease inhibitors, as a prerequisite to further research on their potential roles in protein mobilisation from tubers during plant establishment. Approximately 63% of seedcore dry matter was mobilised over a 29-day period of plant establishment (1.7 mg seedcore dry matter mobilised for every mg increase in plant dry matter). The gelatinolytic protease isoforms induced in seedcores during plant establishment differed from those characterised previously in developing tubers. Total protease activity increased progressively in seedcores and reached a maximum 23 days after planting. Conversely, seedcore soluble protein content declined, with patatin accounting for the greatest decrease in the soluble protein fraction during plant establishment. PMC also decreased 44% and Ser (trypsin) protease inhibitors decreased to levels barely detectable in seedcores over the 29-day growth interval. Moreover, the temporal changes in PMC, protease activity and patatin content were highly correlated. As PMC decreased from 6 to 4 ng core–1, protease activity increased 9-fold, patatin decreased 2.6-fold and total soluble protein decreased by 58%. These results suggest that catabolism of protease inhibitors may facilitate protein mobilisation from seed tubers. Further work to define unequivocally the role of protease inhibitors in modulating the activity of proteases during protein mobilisation from tubers is warranted.