Photosynthetic Capacity Is Differentially Affected by Reductions in Sedoheptulose-1,7-Bisphosphatase Activity during Leaf Development in Transgenic Tobacco Plants

The impact of reduced sedoheptulose-1,7-bisphosphatase (SBPase) activity on photosynthetic capacity and carbohydrate status was examined during leaf expansion and maturation in antisense transgenic tobacco (Nicotiana tabacum L. cv Samsun) plants. In wild-type plants, photosynthetic capacity was lowest in young expanding leaves and reached a maximum in the fully expanded, mature leaves. In contrast, the transgenic antisense SBPase plants had the highest photosynthetic rates in the young expanding leaves and lowest rates in the mature leaves. In the mature, fully expanded leaves of the transgenic plants photosynthetic capacity was closely correlated with the level of SBPase activity. However, in the youngest leaves of the SBPase antisense plants, photosynthetic rates were close to, or higher than, those observed in wild-type plants, despite having a lower SBPase activity than the equivalent wild-type leaves. Reductions in SBPase activity affected carbohydrate levels in both the mature and young developing leaves. The overall trend was for decreased SBPase activity to lead to reductions in carbohydrate levels, particularly in starch. However, these changes in carbohydrate content were also dependent on the developmental status of the leaf. For example, in young expanding leaves of plants with the smallest reductions in SBPase activity, the levels of starch were higher than in wild-type plants. These data suggest that the source status of the mature leaves is an important determinant of photosynthetic development.

本研究考察了景天庚酮糖-1,7-二磷酸酶（sedoheptulose-1,7-bisphosphatase, SBPase）活性降低对反义转基因烟草（Nicotiana tabacum L. cv Samsun）在叶片扩展与成熟阶段的光合能力及碳水化合物状态的影响。野生型植株的光合能力在幼嫩展开叶中最低，在完全展开的成熟叶中达到最大值。与之形成鲜明对比的是，反义SBPase转基因植株的光合速率在幼嫩展开叶中最高，而在成熟叶中最低。在转基因植株的完全展开成熟叶中，光合能力与SBPase活性水平密切相关。然而，尽管SBPase反义植株的幼嫩展开叶的SBPase活性低于对应野生型叶片，但其光合速率接近甚至高于野生型植株的对应叶片。SBPase活性降低会对成熟叶与幼嫩发育叶的碳水化合物水平产生影响。整体趋势为SBPase活性降低会导致碳水化合物含量下降，尤以淀粉含量的降低最为显著。但碳水化合物含量的变化同时也取决于叶片的发育阶段。例如，在SBPase活性降幅最小的植株的幼嫩展开叶中，淀粉含量反而高于野生型植株。上述数据表明，成熟叶的源状态是调控光合发育的重要决定因素。