Figure 7.

Effects of optimized amino acid allocation to and within Arabidopsis aap2 leaves at the leaf (A) and whole-plant (B) level. In general, improvements in aap2 versus wild-type plants were achieved independently of how much N was supplied. A, Effects on the photosynthetically active leaf area, components of the photosynthetic machinery, C fixation, and leaf export of C (Suc) are shown. Overall, increased amino acid allocation to leaves positively affects both the light and dark reactions of photosynthesis as well as the leaf area available for C fixation, leading to a significant increase in C capture and export in the leaf phloem. B, Effects on root-to-shoot N supply, N and C source-to-sink partitioning, N and C tissue status, plant productivity, and NUE are demonstrated. Increased amino acid allocation to leaves positively affects NUpE and root-to-shoot N transport, probably by a feedback regulatory mechanism. In leaves, the N is used effectively for photosynthesis (see A), resulting in increased PNUE and enhanced total C gain. Increased C transport to sinks and/or changes in the C-to-N ratio lead to increases in seed number and yield as well as overall NUE. Within seeds, N amounts are reduced while C levels are unchanged, which, in Arabidopsis, relate to a decrease in protein and an increase in oil levels, respectively (Zhang et al., 2010). NUtE or total harvestable seed N is unchanged, while total seed C yield per plant is improved. Overall, this work supports the notion that increasing N allocation to leaves provides an effective strategy for improving photosynthetic and plant NUE and harvestable seed C yield in a range of N conditions. Arrows with circles represent transporters for amino acids (black) or Suc (white). Arrows located to the left of features analyzed indicate significant changes in aap2 mutants compared with wild-type plants (up, increase; down, decrease; up and down, no change).