Responses of soybean seedlings to supplemental phenylalanine and its implications for tolerance to UV-B radiation

Background/Question/Methods

One of the more common responses of plants to UV-radiation is the induction of secondary metabolites that serve as selective filters to absorb UV and protect potential targets such as DNA, photosynthesis proteins or other sensitive molecules.  However, we still have a poor understanding of how UV is perceived and of the controls and limiting factors in this response.  We have shown in previous studies that the addition of phenylalanine (Phe) to seeds of Arabidopsis and soybean can enhance the synthesis of potential screening compounds and increase UV tolerance in etiolated soybean seedlings.  In this study we examined the changes in phenolics in primary and trifoliate leaves of three soybean (Glycine max (L.) Merr.) cultivars in response to supplemental phenylalanine added to seeds or seedlings and grown in a glasshouse under a full solar spectrum.  Seedlings were exposed to either, 3, 5 or 7 kJ of UV when weighted with a common UV weighting function. 

Results/Conclusions

Seed or foliar treatment with 1 mmol supplemental Phe led to increased flavonoid or overall phenolic levels in primary and trifoliate leaves in all three soybean cultivars including the Clark-magenta isoline of Clark, which does not produce flavonoids.  However, the magnitude of this response decreased with leaf age suggesting that Phe levels were initially limiting the response but the combination of exhausting the supply of supplemental Phe and or the development of other limitations (e.g. enzyme activity) may have become more important as the plants developed.  UV-tolerance in terms of general growth parameters such as leaf area and plant height or biomass was also generally enhanced by supplemental Phe, although this response varied among cultivars.  These results suggest that the available pool of Phe may be an important limiting factor in the development of UV-protection mechanisms but further studies are needed to determine the allocation of the supplemental Phe and the long-term responses under field conditions.