Greening in Pea Seedlings

The effect of gibberellic acid (GA) on light-induced greening of etiolated pea plants (Pisum sativum [L.] cultivars Alaska and Progress) was characterized. Progress, a GA-deficient dwarf of Alaska, was found to accumulate chlorophyll and light harvesting chlorophyll protein associated with photosystem 11 (LHC-11) more rapidly than Alaska, Alaska treated with GA, or Progress treated with GA. A slightly lower chlorophyll content was noted after 24 hours of light induced greening for Alaska treated with GA relative to untreated Alaska. GA-treated Progress, Alaska, and GAtreated Alaska all gave essentially identical pattems for LHC-11 accumulation. Similar pattems of LHC-11 mRNA induction were found in all four treatments indicating that differences in mRNA induction did not cause differences in LHC-11 accumulation. Chlorophyll and LHC-11 accumulation in each treatment followed the same pattems of accumulation and a significant correlation (at the 0.01 level of significance) was found between chlorophyll and LHC-11 content. Since Progress treated with GA accumulated LHC11 and chlorophyll in a manner similar to that of Alaska, it is clear that GA alters the process of greening either directly or indirectly. The LHC-II' complex is an integral component of the antenna Chl complexes in the thylakoid membranes of higher plants (26) and is coded by a nuclear, multigene family (6, 8, 21). LHC-II genes are regulated by light (4, 11, 25) through the phytochrome system. The biosynthetic pathway for Chl similarly depends on phytochrome activation (1). Chl has been implicated to be a limiting factor in the formation of LHC-II complexes (15, 17, 18). Recently using light intensity as a means of varying the rate of Chl accumulation it was shown that LHC-II accumulation mimics Chl accumulation (18). Other investigators have also shown LHC-II accumulation mimicking Chl accumulation (2, 3, 12). In this report two pea cultivars, Alaska and Progress (a GA-deficient dwarf of Alaska), are shown to green at different rates. Previously, addition of exogenous GA has been shown to cause changes in populations of translatable mRNAs and accumulated polypeptides in dwarfs of maize and pea (7). The tall/dwarf phenotype in Progress pea is controlled by a single Mendelian gene designated le and acts during a single enzymatic step in GA biosynthesis (22). Addition of exogenous GA to cv Progress has been shown to produce internode lengths similar to 'Abbreviations: LHC-II, the major light harvesting Chl-protein complex associated with photosystem II; DMF, N, N dimethylformamide. the GA-sufficient cv Alaska. Exogenous GA was applied to Progress and Alaska pea cultivars to determine its effect on greening. Chl content, LHC-II content, and LHC-II mRNA induction were determined for etiolated pea seedlings greened for a 24-h period. MATERIALS AND METHODS Plant Material and Growth Conditions Seeds for Pisum sativum [L.] cvs Progress and Alaska were germinated in moist vermiculite at 25°C in the dark for 7 d. The manipulation of the peas in the dark was done using a green safelight constructed from a combination of 2.75-W tungsten illuminator and green filter (Bausch and Lomb 3135-61). The etiolated plants were placed in continuous white light at 27°C. The light was from fluorescent and incandescent lamps that produced an intensity of 275 to 300 Umol.m-2. s-' at the top ofthe pea seedlings. GA obtained from Carolina Biological Supply was applied each day after planting to one set of Alaska and Progress plants. The GA was applied as a spray in a concentration of2 mm. Three separate sets ofplants were germinated and greened in this manner. The internode lengths were measured on five seedlings from each treatment to check for a similar GA effect during each of the greening experiments.