Supplemental supply of phenylalanine to soybean seeds reduces damage by ultraviolet radiation in etiolated seedlings

Background/Question/Methods

Although it is known that plants respond to UV-radiation with a suite of developmental (photomorphogenic) and protection (induction of secondary metabolites and repair enzymes), we still have a poor understanding of how UV is perceived and of the signal-response mechanisms elicited by that perception early in development.  We examined germination rate, hypocotyl elongation, leaf pubescence and surface and subcellular responses of germinating and/or etiolated soybean (Glycine max (L.) Merr.) seedlings in response to treatment with UV-A or UV-B radiation.  Exposure was at a total fluence of 10⁴ mmol m^-2 and was controlled by varying exposure times of UV at 300 nm [18 min], 305 nm [16 min], 311 nm [14 min], 317 nm [11 min]), 325 nm [6 min], 332 nm [6 min] or 368 nm [4 min] calculated from lamp output spectra. We also examined the effect of exogenously-added phenylalanine (Phe), a key substrate in the phenylpropanoid pathway, on some responses to UV. 

Results/Conclusions

Exposure of seeds to wavelengths below 332 nm reduced germination compared to controls and was less than 50% germination by 3 d post-irradiation.  Hypocotyl length showed similar trends to that of seed germination where exposure to 368 nm radiation had no effect on hypocotyl elongation and elongation was progressively more impacted at shorter wavelengths.  SEM and TEM images revealed cellular damage at only the shorter wavelengths while the longer UV-A wavelengths stimilated the development of leaf hairs in the accumulation of UV-screening compounds.  Application of exogenous phenylalanine reduced or entirely prevented damage from exposure to UV radiation.  These results suggest that the available pool of Phe may be an important limiting factor in the development of UV-protection mechanisms.