Alleviation of Phytophthora infestans Mediated Necrotic Stress in the Transgenic Potato (Solanum tuberosum L.) with Enhanced Ascorbic acid Accumulation

Potato is the most widely cultivated non-cereal crop in the world, and like any other crop, it is susceptible to yield losses because of various factors, including pathogen attacks. Among the various diseases of potato, late blight caused by the oomycete Phytophthora infestans is considered as the most devastating disease worldwide. In this study, transgenic potato plants overexpressing the D-galacturonic acid reductase (GalUR) gene with an enhanced level of cellular L-ascorbate (L-AsA) were challenged with Phytophthora infestans to determine the level of stress tolerance induced in those plants. With the onset of pathogen infection, necrotic lesions progressively expanded and became necrotic in the control plants. The transgenic potato lines with enhanced ascorbic acid showed reduced necrotic lesions. Hydrogen peroxide (H2O2) and malondialdehyde (MDA) levels were relatively lower in transgenic plants compared to the untransformed control (UT) plants. The mRNA expressions of pathogenesis-related (PR) genes, such as pathogenesis related 1 (PR1) and phenylalanine ammonia-lyase (PAL) were slightly higher in GalUR overexpressing transgenic lines as compared to the untransformed control plants. Pathogen infection also altered the mRNA expression of genes associated with gibberellic acid (GA) and abscisic acid (ABA) biosynthesis. Furthermore, the increase in various antioxidant enzymes was also observed in the gene expression analysis with the transgenic plants. The complete loss of the pathogen growth and disease occurrence was not observed in our study; however, the findings indicated that an increase in the level of cellular L-ascorbate in the transgenic potato leads to enhanced cellular antioxidants, PR genes and plant defense hormones, such as GA and ABA resulting in the reduction of the disease symptoms caused by the Phytophthora infestans.

Regulation of Starch Degradation in Mature Leaves of Transgenic Apple Trees with Decreased Sorbitol Synthesis

Considering starch synthesis was enhanced in leaves of transgenic apple trees with decreased sorbitol synthesis, we hypothesized that starch degradation must be up-regulated correspondingly to maintain carbon supply to sink tissues. Compared with the untransformed control, mature leaves of the transgenic plants had a larger drop in starch concentration between dusk and pre-dawn, higher maltose concentration, and higher activities of two key enzymes in starch degradation: -amylase and cytosolic glucosyltransferase during the day and night. 14C-maltose and 14C-glucose were fed to the apple leaves to study the fate of starch breakdown products in the synthesis of sorbitol and sucrose. Under light, a larger proportion of both 14C-maltose and 14C-glucose were converted to sorbitol than to sucrose in the untransformed control, whereas conversion of 14C-maltose and 14C-glucose to sucrose predominated over that to sorbitol in the transgenic apple leaves. The leaf samples fed with 14C-maltose and 14C-glucose in the dark are still being analyzed, but it appears that sucrose is the main product in both the untransformed control and the transgenic plants. These results support the hypothesis that starch degradation is up-regulated in the transgenic plants.

Bromoxynil resistance in transgenic potato clones expressing thebxngene

To broaden the spectrum of herbicides useful in potato production, thebxngene for bromoxynil resistance, which encodes a nitrilase specific for bromoxynil, was introduced into ‘Lemhi Russet’ potato byAgrobacterium tumefaciens-mediated transformation. In GR50studies, transformed potato clones were at least 70-fold more resistant to bromoxynil than the untransformed control. Resistance was due to rapid metabolism of bromoxynil to 3,5-dibromo-4-hydroxybenzoic acid, followed by conjugation to polar compounds. In yield trials, the best performing transgenic clones had total tuber yields equal to the untreated, untransformed control, but U.S. No. 1 tuber yields were 15 to 30% lower than the untreated, untransformed control. Tubers from three out of four transgenic clones had specific gravities, percent solids, and fry color similar to or better than the untreated, untransformed control. The data suggest that lower U.S. No. 1 yields in the transgenic clones were due to somaclonal variation and that expression of thebxntransgene had no consistent, detrimental effect on internal tuber quality.

Altered transcriptional activity of c-fos promoter plasmids in v-raf-transformed NIH 3T3 cells.

In cells transformed by v-raf, an oncogenic counterpart of the serine/threonine kinase Raf-1, regulatory elements of the c-fos promoter were active under conditions of cell growth or stimulation for which they were inactive in untransformed control cells. This suggests that v-raf transforms by deregulating transcription of early response genes.

Altered transcriptional activity of c-fos promoter plasmids in v-raf-transformed NIH 3T3 cells

In cells transformed by v-raf, an oncogenic counterpart of the serine/threonine kinase Raf-1, regulatory elements of the c-fos promoter were active under conditions of cell growth or stimulation for which they were inactive in untransformed control cells. This suggests that v-raf transforms by deregulating transcription of early response genes.

Increased secretion of type beta transforming growth factor accompanies viral transformation of cells

Cells transformed by Harvey or Moloney sarcoma virus secrete at least 40 times as much type beta transforming growth factor as their respective untransformed control cells. The transformed cells bind only 20 to 50% as much type beta transforming growth factor as the control cells, suggesting that transformation causes down-regulation of the type beta transforming growth factor receptor.

Increased secretion of type beta transforming growth factor accompanies viral transformation of cells.

Cells transformed by Harvey or Moloney sarcoma virus secrete at least 40 times as much type beta transforming growth factor as their respective untransformed control cells. The transformed cells bind only 20 to 50% as much type beta transforming growth factor as the control cells, suggesting that transformation causes down-regulation of the type beta transforming growth factor receptor.