Silencing of SbPPC3 reduces the germination capacity in salinity and decreases the nutritional value of sorghum seeds

Abstract

Sorghum (Sorghum bicolor L.) is the fifth most important cereal crop worldwide and tolerant to drought and salinity. Phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) is an enzyme playing key roles in seed development and germination. We have previously demonstrated that the silencing of the non-photosynthetic SbPPC3 gene affects plant growth and productivity, delaying flowering, and reducing seed production. In this work, knock-down lines (Ppc3 lines) were used for assessing the contribution of PPC3 to seed filling and germination. PEPC activity was greatly reduced in dry and germinating seeds although the germination capacity was not affected. This could be due to increased phosphorylation of PPC2, the only PEPC isoenzyme co-expressed in the dry seed stage with PPC3. In salinity, PPC2 does not increase its phosphorylation in Ppc3 lines, and silenced lines show lower germination rate. In addition, the survival of seedlings in salinity was reduced to 25% in Ppc3 plants, whereas it remains close to 80% in WT. Thereby, the importance of PPC3 isoenzyme during seed germination in salinity is stablished. The dry seeds of silenced lines show reduced weight, lower starch and fibers levels, and altered energetic state. Despite lower levels of protein compared to WT seeds, Ppc3 seeds showed lower C/N ratio and higher phytate content, indicating alterations in C, N and P metabolisms. These results show that PPC3 activity affects replenishment of seed reserves, thus altering its nutritional value. In addition, they corroborate the relevance of phosphorylation of a starch-storing-cereal seed PEPC during germination.