Photosynthetic responses of sugar beet and Nicotiana benthamiana Domin. infected with beet curly top virus

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 an accumulation of sucrose, as much as 10-fold greater than controls, and 38-fold higher than that seen in infected N. benthamiana per dm2leaf area. [U-14C]-sucrose uptake experiments showed a 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 levels. Anatomical changes included 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