Main conclusion

Downregulation in the expression of the signal recognition particle 43 (SRP43) gene in tobacco conferred a truncated photosynthetic light-harvesting antenna (TLA property), and resulted in plants with a greater leaf-to-stem ratio, improved photosynthetic productivity and canopy biomass accumulation under high-density cultivation conditions. Evolution of sizable arrays of light-harvesting antennae in all photosynthetic systems confers a survival advantage for the organism in the wild, where sunlight is often the growth-limiting factor. In crop monocultures, however, this property is strongly counterproductive, when growth takes place under direct and excess sunlight. The large arrays of light-harvesting antennae in crop plants cause the surface of the canopies to over-absorb solar irradiance, far in excess of what is needed to saturate photosynthesis and forcing them to engage in wasteful dissipation of the excess energy. Evidence in this work showed that downregulation by RNA-interference approaches of the Nicotiana tabacum signal recognition particle 43 (SRP43), a nuclear gene encoding a chloroplast-localized component of the photosynthetic light-harvesting assembly pathway, caused a decrease in the light-harvesting antenna size of the photosystems, a corresponding increase in the photosynthetic productivity of chlorophyll in the leaves, and improved tobacco plant canopy biomass accumulation under high-density cultivation conditions. Importantly, the resulting TLA transgenic plants had a substantially greater leaf-to-stem biomass ratio, compared to those of the wild type, grown under identical agronomic conditions. The results are discussed in terms of the potential benefit that could accrue to agriculture upon application of the TLA-technology to crop plants, entailing higher density planting with plants having a greater biomass and leaf-to-stem ratio, translating into greater crop yields per plant with canopies in a novel agronomic configuration.

Main conclusion

The role of six alkaloid biosynthesis genes in the process of nicotine accumulation in tobacco was investigated. Downregulation of ornithine decarboxylase, arginine decarboxylase, and aspartate oxidase resulted in viable plants with a significantly lower nicotine content. Attenuation of nicotine accumulation in Nicotiana tabacum was addressed upon the application of RNAi technologies. The approach entailed a downregulation in the expression of six different alkaloid biosynthesis genes encoding upstream enzymes that are thought to function in the pathway of alkaloid and nicotine biosynthesis. Nine different RNAi constructs were designed to lower the expression level of the genes that encode the enzymes arginine decarboxylase, agmatine deiminase, aspartate oxidase, arginase, ornithine decarboxylase, and SAM synthase. Agrobacterium-based transformation of tobacco leaves was applied, and upon kanamycin selection, T0 and subsequently T1 generation seeds were produced. Mature T1 plants in the greenhouse were topped to prevent flowering and leaf nos. 3 and 4 below the topping point were tested for transcript levels and product accumulation. Down-regulation in arginine decarboxylase, aspartate oxidase, and ornithine decarboxylase consistently resulted in lower levels of nicotine in the leaves of the corresponding plants. Transformants with the aspartate oxidase RNAi construct showed the lowest nicotine level in the leaves, which varied from below the limit of quantification (20 μg per g dry leaf weight) to 1.3 mg per g dry leaf weight. The amount of putrescine, the main polyamine related to nicotine biosynthesis, showed a qualitative correlation with the nicotine content in the arginine decarboxylase and ornithine decarboxylase RNAi-expressing transformants. A putative early senescence phenotype and lower viability of the older leaves was observed in some of the transformant lines. The results are discussed in terms of the role of the above-mentioned genes in the alkaloid biosynthetic pathway and may serve to guide efforts to attenuate nicotine content in tobacco leaves.