Would reducing chlorophyll content result in a higher photosynthesis nitrogen use efficiency in crops?

Decreasing antenna size is considered a potential option for improving photosynthesis and increasing yield potential. Reducing chlorophyll content has been employed as a strategy to decrease antenna size. One of the commonly mentioned advantages of this approach is its ability to enhance crop nitrogen use efficiency (NUE); however, there is limited field evidence supporting this claim. In this study, we utilized a rice mutant called p35s-Ami-YGL1, which exhibits lower chlorophyll content and smaller antenna size, to investigate the effects of modifying leaf chlorophyll content on tissue nitrogen content and NUE. Our results demonstrate that the nitrogen contents in various tissues, including seed tissue, increased on a weight basis in p35s-Ami-YGL1 mutants while exhibiting a decrease in C:N ratio. Simultaneously, we observed a reduction in tissue carbon content along with an increase in the levels of chlorophyll precursors such as Proto IX. Specifically, we observed an upregulation in the expression of genes associated with photosynthetic light reactions and chlorophyll metabolism, while there was no increase in the expression of genes involved in the CBB cycle and nitrogen metabolism. In addition, p35s-Ami-YGL1 experienced increased photodamage. These findings suggest that the alterations in the C:N ratio and nitrogen content in plants may be attributed to Proto IX-mediated photodamage and chloroplast reverse transduction signaling. Besides, these results suggest that the observed increase in tissue nitrogen content in p35s-Ami-YGL1 does not reflect an increase in plant nitrogen absorption or use efficiency, rather it is a result of stunted carbon fixation capacity.