LEAF DEVELOPMENTAL AGE CONTROLS EXPRESSION OF GENES ENCODING ENZYMES OF CHLOROPHYLL AND HEME-BIOSYNTHESIS IN PEA (PISUM-SATIVUM L)

The effects of leaf developmental age on the expression of three nucle ar gene families in pea (Pisum sativum L.) coding for enzymes of chlor ophyll and heme biosynthesis have been examined. The steady-state leve ls of mRNAs encoding aminolevulinic acid (ALA) dehydratase, porphobili nogen (PBG) deaminase, and NAD PH:protochlorophyllide reductase were m easured by RNA gel blot and quantitative slot-blot analyses in the fol iar leaves of embryos that had imbibed for 12 to 18 h and leaves of de veloping seedlings grown either in total darkness or under continuous white light for up to 14 d after imbibition. Both ALA dehydratase and PBC deaminase mRNAs were detectable in embryonic leaves, whereas mRNA encoding the NADPH:protochlorophyllide reductase was not observed at t his early developmental stage. All three gene products were found to i ncrease to approximately the same extent in the primary leaves of pea seedlings during the first 6 to 8 d after imbibition (postgermination) regardless of whether the plants were grown in darkness or under cont inuous white-light illumination. In the leaves of dark-grown seedlings , the highest levels of message accumulation were observed at approxim ately 8 to 10 d postgermination, and, thereafter, a steady decline in mRNA levels was observed. In the leaves of light-grown seedlings, stea dy-state levels of mRNA encoding the three chlorophyll biosynthetic en zymes were inversely correlated with leaf age, with youngest, rapidly expanding leaves containing the highest message levels. A correspondin g increase in the three enzyme protein levels was also found during th e early stages of development in the light or darkness; however, maxim al accumulation of protein was delayed relative to peak levels of mRNA accumulation. We also found that although protochlorophyllide was det ectable in the leaves immediately after imbibition, the time course of accumulation of the phototransformable form of the molecule coincided with NADPH:protochlorophyllide reductase expression. In studies in wh ich dark-grown seedlings of various ages were subsequently transferred to light for 24 and 48 h, the effect of light on changes in steady-st ate mRNA levels was found to be more pronounced at later developmental stages. These results suggest that the expression of these three gene s and likely those genes encoding other chlorophyll biosynthetic pathw ay enzymes are under the control of a common regulatory mechanism. Fur thermore, it appears that not light, but rather as yet unidentified en dogenous factors, are the primary regulatory factors controlling gene expression early in leaf development.