Environmental stresses (UV-B, temperature, drought, flood, salt, etc.) are among the major factors in plant productivity losses in natural and agricultural conditions. Adding to the magnitude of this problem is the accumulating evidence of increases in trace gases in the atmosphere, projecting global shifts in the distribution of temperature and precipitation patterns, depletions in the stratospheric ozone layer and increases in UV-B radiation reaching the earth. Since plants in a natural environment are unavoidably exposed to long hours of UV-B, it is often a source of considerable stress to them. The impact of this stress is reflected in observations from many laboratories which indicate that of the roughly 300 species and varieties of plants investigated, nearly one-third to one-half show physiological damage and/or growth reduction in response to UV-B, with Fabaceae, Cucurbitaceae and Brassicaceae being among the most sensitive plant families. These plant families are also of considerable agricultural importance, with Fabaceae ranking second only to Poaceae in importance to human society.

Our lab studies the mechanisms involved in protection of molecular targets from UV-B damage and the contribution of interacting environmental factors to the overall effect of elevated UV-B radiation. The plant of our study is soybean, which belongs to the Fabaceae family

One of the protection mechanisms utilized by all higher plants operates via accumulation of flavonoid pigments in leaf epidermal cells so that a major fraction of the damaging wavelengths are filtered out before reaching the mesophyll cells where photosynthesis takes place. The key enzyme of flavonoid biosynthesis is chalcone synthase (CHS) which catalyzes the stepwise condensation of the coumaroyl moiety of p-coumaroyl-coenzyme A (CoA) derived from the phenylpropanoid pathway, with three acetate moieties derived from malonyl-CoA to give rise to the C15 flavonoid skeleton, naringenin chalcone. Isomerization and further substitution of this central intermediate leads to the synthesis of a class of molecules collectively known as flavonoids. We have cloned, sequenced, and characterized the CHS gene family in soybean, Glycine max (Gm), one of the key crops of worldwide importance which has been identified as being sensitive to UV-B radiation. Our work has demonstrated that the Gm chs gene family consists of at least eight members and shows two categories of expression: dark expression and expression in response to light, with up-regulation by UV-B. The dark expression begins within the first hours of initiating germination and points to the importance of flavonoids already early in plant life, when the seedlings are under the ground and light-independent expression of chs genes must provide the means for flavonoid biosynthesis. The light-responsive expression appears later, almost coincidental with the time-point when the head of the seedling is pushed up above ground by stem elongation, suggesting the possibility of a developmental switch modulating the response. We have identified two classes of genes in the Gm chs gene family: (a) a larger class consisting of genes which respond to broad spectrum light and show up-regulation by UV-B and (b) a smaller class responding to broad spectrum light but up-regulated preferentially by UV-B. This latter class, consisting of genes Gm chs 5 and Gm chs 6, is of particular interest because it provides a specific probe with which to examine the question of intra-specific variation in sensitivity to UV-B in soybean. Experiments to determine the role of these genes in UV-B protection are in progress. Our experiments examining the effect of UV-B on soybean's photosynthetic ability in a pair of soybean cultivars of contrasting UV-B sensitivities have demonstrated that the UV-B-sensitive cultivar CNS is more susceptible to photodegradation of D1-D2 photosystem II (PSII) reaction center heterodimer than the UV-B-resistant cultivar, Williams. These results draw further attention to the possibility that the observed reductions in photosynthetic carbon assimilation and seed yields in UV-B sensitive plants may, at least, in part be due to increased photodegradation of the PSII heterodimers. Further work on identifying genes whose expression is differentially affected by UV-B in UV-B sensitive and UV-B resistant cultivars is in progress.

DNA sequences deposited in the GenBank database:

Genomic clones:

Rice 4-Coumarate:CoA Ligase 1, Os 4-cl.1 (X52623)
Soybean Chalcone Synthase 1, Gm chs1 (X54644)
Soybean Chalcone Synthase 2, Gm chs2 (X65636)
Soybean Chalcone Synthase 3, Gm chs3 (X53958)
Soybean Chalcone Synthase 4, Gm chs4 (X52097)
Soybean Chalcone Synthase 5, Gm chs5 (L07647)
Soybean Chalcone Synthase 6, Gm chs6 (L03352)
Soybean Chalcone Synthase 7, Gm chs7 (M98871)
Soybean Chalcone Synthase 8, Gm chs8 (AY237728)
Soybean Chalcone Synthase 1-3, intergenic (X56749)
Sorghum XbaI Repetitive DNA, Sha XSR3 (X54623)
Sorghum XbaI Repetitive DNA, Sha XSR3 (X54624)
Sorghum XbaI Repetitive DNA, Sha XSR6 (X54625)
Rat fibroblast growth factor, Rn Fgf2 (EF030430)

cDNA clones:

Tobacco PSII Oxygen-Evolving Enhancer protein 2, Nt oee2-A (X58910)
Tobacco PSII Oxygen-Evolving Enhancer protein 2, Nt oee2 23-A (X58909)
Tobacco PSII Oxygen-Evolving Enhancer protein 2, Nt oee2 23-B (X62426)
Tobacco PSII Oxygen-Evolving Enhancer protein 2, Nt oee2 23-BA(X62425)
Tobacco PSII Oxygen-Evolving Enhancer protein 2, Nt oee2 23-F (X55354)
Tobacco PSII Oxygen-Evolving Enhancer protein 2, Nt oee2 23-FA(X62427)
Soybean 1-Aminocyclopropane-1-Carboxylate Synthase, Gm acs1 (X67100)
Rat S100 calcium-binding protein A4 (NM_012618)

RNA sequences:

E.coli f-Met-tRNA (M35184)
E. coli initiator transfer RNA, Methionine-tRNA-f, (K00305)
E coli CUG decoding transfer RNA, tRNA-Leu, (K01550)