Light-independent synthesis of LHC IIb polypeptides and assembly of the major pigmented complexes during the initial stages of Pinus palustris seedling development

Abstract

Pinus palustris has a greatly reduced need for light to initiate chloroplast development in comparison to angiosperms. Light is not required for chlorophyll synthesis in dark-grown Pinus palustris seedlings. However, embryos do not contain chlorophyll, and synthesis is limited to seedlings having cotyledon lengths between about 0.5 cm and 2.0 cm. The final amount of chlorophyll accumulated by dark-grown seedlings is about one fifth of that in light-grown seedlingsat the same stage. The major light-harvesting chlorophyll a/b-polypeptides of Photosystem II (LHC IIb) are absent in the embryos but begin to accumulate in seedlings of 0.5 cm cotyledon length, irrespective of the light conditions. Although dark-grown seedlings accumulate most of the pigmented complexes seen in light-grown seedlings, there are differences in the subunit structure of some of them. These findings suggest that the majority of the components of the photosynthetic membrane do not require light for induction of synthesis or assembly into complexes, but that the final forms seen in light-grown seedlings may require light.

Developmental and Light-Regulated Expression of Individual Members of the Light-Harvesting Complex b Gene Family in Pinus palustris

“…The P. palustris Lhcb sequences were also closely related to G. biloba type 1 and type 2 Lhcb sequences, showing between 85 and 97% identity at the amino acid level (Chinn and Silverthorne, 1993;Chinn et al, 1995). Since a previous study of LHCIIb accumulation in P. palustris indicated that the major polypeptides were expressed in the absence of light (Canovas et al, 1993), sequence-specific probes were constructed to determine whether individual Lhcb mRNAs are also expressed in the absence of light. The 3' UT sequences of each clone (Fig.…”

“…Neither sequence was expressed in embryos allowed to imbibe either in darkness or in light. However, both sequences were already expressed in 0.5-cm cotyledons that had initiated chlorophyll synthesis and chloroplast development (Canovas et al, 1993). The expression levels of both sequences increased during development and were approximately 2-fold higher in the light than in darkness.…”

“…Dark-grown seedlings used for light treatments were of 2.0-cm cotyledon length and were already green (Canovas et al, 1993). Red light was supplied from a 100-W warmwhite fluorescent tube (Sylvania) filtered with two layers of Shinkalite no.…”

“…Dark-grown seedlings having cotyledons of about 2 cm in length were chosen, since these seedlings have been found to express Lhcb sequences (Fig. 6) and actively accumulate chlorophylls a and b and LHCIIb complexes (Canovas et al, 1993). A fluence-response curve was determined for the expression of each mRNA in response to red light of fluence between 10p4 and 104 pmol m-' (Fig.…”

“…It should be pointed out that light is also not required for expression of Lkcb mRNAs in some monocots and dicots, even though they lack chlorophyll (Baumgartner et al, 1989;Sullivan et al, 1989;Brusslan and Tobin, 1992;Rapp and Mullet, 1991). Conifer seedlings can accumulate LHCIIb polypeptides and at least partially assemble photosynthetic complexes in complete darkness (Kukidome et al, 1986;Canovas et al, 1993). Furthermore, recent studies have shown that LHCIIb polypeptides and mRNA accumulate in dark-grown Douglas fir (Pseudotsuga menziesii; Alosi et al, 1990) and Pinus tkunbergii seedlings (Yamamoto et al, 1991).…”