Submitted to: Physiological and Molecular Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/15/2001
Publication Date: 10/1/2001
Citation: Swiech, R., Browning, S., Milsen, D., Stenger, D.C., Holbrook, G.P. 2001. Photosynthetic responses of sugarbeet and nicotiana benthamiana domin. infected with beet curly top virus. Physiological And Molecular Plant Pathology 58:43-52. Interpretive Summary: Physiological and structural changes associated with beet curly top virus (BCTV) infection of sugar beet and the experimental host Nicotiana benthamiana were examined. Sugar retention in leaves was greatly elevated in BCTV infected leaves and transport of sugar from leaves to other tissues of the plant was impaired. Photosynthetic activity was also reduced in BCTV infected plants, relative to uninfected plants. Changes in the cellular structure of infected leaves was observed. The size of leaf cells was increased, most probably due to higher osmotic pressures resulting from retention of sugar in infected leaves. These results indicate that BCTV infection induces profound changes in both structure and function plants that not only reduce yields, but also may be directly responsible for the type of symptoms expressed.
Technical Abstract: This study examined photosynthesis and assimilate partitioning in sugar beet (Beta vulgaris L.) and Nicotiana benthamiana Domin. plants systemically infected with either the Logan or CFH strains of Beet curly top virus (BCTV). Leaves of both susceptible (Z-10) and resistant (9BB6090) beet varieties exhibited a striking accumulation of sucrose as much as 10- fold greater than controls, and 38-fold higher than that seen in N. benthamiana per dm2 leaf area. [U-14C]-sucrose uptake experiments showed a severe decline in the export of this sugar, particularly from young leaves of the Z-10 variety. This directly confirms that BCTV impairs phloem transport in sugar beet leaves, and can cause excessive retention of sucrose. Starch levels were also elevated, but remained substantially lower than sucrose. Anatomical changes included notable increases in mesophyll cell size, indicating high cell turgor pressure due to the elevated sugar content. There was an associated 20-50% reduction in photosynthetic rates compared to mock-inoculated plants, and a decrease in ribulose 1,5 bisphosphate carboxylase activity and protein levels. Similar reductions in the activity of cytoplasmic and chloroplastic fructose 1,6, bisphosphatase were consistent with a possible down-regulation of genes encoding key photosynthetic enzymes initiated by the buildup of sugars. BCTV-infected plants may therefore constitute a useful naturally occurring system for investigating end-product control of carbon assimilation.