Plant Plastid Engineering

Wani, Shabir H.; Haider, Nadia; Kumar, Hitesh; Singh, N.B.

Among many strategies used for development of abiotic stress tolerance in plants, the over-expression of compatible osmolytes like glycinebetaine was found to be successful [24] . Initial attempts for producing transplastomic plants through the introduction of CMO (Choline monooxygenase) and betaine aldehyde dehydrogenase (BADH) pathway were made in tobacco. Tobacco plants were transformed with cDNA for BADH from spinach (Spinacia oleracea) and sugar beet (Beta vulgaris) under the control of CaMV 35 S promoter. The BADH was produced in plastids of tobacco. Betaine aldehyde was converted to betaine by BADH, thus conferring resistance to betaine aldehyde. In another attempt, cDNA for choline monooxygenase from Spinacia oleracea was introduced into tobacco and the enzyme thus synthesized was transported to its functional place i.e., plastids. But the leaves of tobacco accumulated betaine at a very low concentration i.e., 10-100 folds lower [96] . The reason for insufficient synthesis of betaine most probably was the absence of engineered BADH activity in plastids. Therefore, both CMO and BADH need to be present in the plastids for efficient synthesis of betaine in transgenic plants which do not accumulate glycinebetaine. In carrot (Daucus carota), homoplasmic transgenic plants exhibiting high levels of salt tolerance were regenerated from bombarded cell cultures via somatic embryogenesis [48] . BADH enzyme activity was enhanced 8-fold in transgenic carrot cell cultures, grew 7-fold more, and accumulated 50-to 54-fold more betaine than untransformed cells grown in liquid medium containing 100 mM NaCl. Transgenic carrot plants expressing BADH grew in the presence of high concentrations of NaCl (up to 400 mM), the highest level of salt tolerance reported so far among genetically modified crop plants. Further, a gene for CMO, cloned from spinach (Spinacia oleracea) was introduced into rice through Agrobacterium mediated transformation. The level of glycinebetaine in rice was low to the expectations. The author has given several reasons for the low productivity of rice and low glycinebetaine accumulation. Firstly, the position of spinach CMO and endogenous BADH might be different and secondly the catalytic activity of spinach CMO in rice plants might be lower than it was in spinach [97] . Transplastomic plants constitutively expressing BvCMO under the control of the ribosomal RNA operon promoter and a synthetic T7 gene G10 leader were able to accumulate glycinebetaine in leaves, roots and seeds, and exhibited improved tolerance to toxic level of choline and to salt/drought stress when compared to wild type plants. Transplastomic plants showed higher net photosynthetic rate and apparent quantum yield of photosynthesis in the presence of 150 mM NaCl [98] . Thus, it can be concluded that glycinebetaine has a role as compatible solute and its engineering into non-accumulations will be a success only if both CMO and BADH pathways are introduced and if the localization of both CMO and BADH is in plastids. Very recently, George et al. [99] demonstrated how a chloroplastlocalized and auxin-induced glutathione S-transferase from phreatophyte Prosopis juliflora conferred drought tolerance on tobacco. For more examples of conferring tolerance to abiotic stress to plants via plastid engineering, see review by [3] .