Convergent evolution leading to the appearance of furanocoumarins in citrus plants.

Furanocoumarins are defense molecules mainly described in four plant families that are phylogenetically distant. Molecular characterization of the biosynthetic pathway has been started for many years in Apiaceae and Rutaceae. The results obtained thus far in Apiaceae indicated a major role of cytochromes P450 (P450s) in the CYP71 family. In the present work, we describe the importance of another subfamily of P450s, CYP82D, identified by using a deep analysis of the citrus (Rutaceae) genome and microarray database. CYP82D64 is able to hydroxylate xanthotoxin to generate 5-OH-xanthotoxin. Minor and limited amino acid changes in the CYP82D64 coding sequence between Citrus paradisi and Citrus hystrix provide the enzyme in the latter with the ability to hydroxylate herniarin, but with low efficiency. The kinetic constants of the enzyme are consistent with those of other enzymes of this type in plants and indicate that it may be the physiological substrate. The activity of the enzyme is identical to that of CYP71AZ6 identified in parsnip, showing possible evolutionary convergence between these two families of plants. It is highly possible that these molecules are derived from the synthesis of ubiquitous coumarins throughout the plant kingdom.

Nitrogen nutrition of tomato plant alters leafminer dietary intake dynamics.

The leafminer Tuta absoluta (Meyrick) is a major pest of the tomato crop and its development rate is known to decline when nitrogen availability for crop growth is limited. Because N limitation reduces plant primary metabolism but enhances secondary metabolism, one can infer that the slow larval development arises from lower leaf nutritive value and/or higher plant defence. As an attempt to study the first alternative, we examined the tomato-T. absoluta interaction in terms of resource supply by leaves and intake by larvae. Tomato plants were raised under controlled conditions on N-sufficient vs. N-limited complete nutrient solutions. Plants were kept healthy or artificially inoculated with larvae for seven days. Serial harvests were taken and the N, C, dry mass and water contents were determined in roots, stems and leaves. Leaf and mine areas were also measured and the N, C, dry mass and water surface densities were calculated in order to characterize the diet of the larvae. The infestation of a specific leaf lessened its local biomass by 8-26%, but this effect was undetectable at the whole plant scale. Infestation markedly increased resource density per unit leaf area (water, dry mass, C and N) suggesting that the insect induced changes in leaf composition. Nitrogen limitation lessened whole plant growth (by 50%) and infested leaflet growth (by 32-44%). It produced opposite effects on specific resource density per unit area, increasing that of dry mass and C while decreasing water and N. These changes were ineffective on insect mining activity, but slowed down larval development. Under N limitation, T. absoluta consumed less water and N but more dry mass and C. The resulting consequences were a 50-70% increase of C:N stoichiometry in their diet and the doubling of faeces excretion. The observed limitation of larval development is therefore consistent with a trophic explanation caused by low N and/or water intakes.