Chloroplast and Nuclear Transcripts for Plastid Proteins in Arabidopsis thaliana: Tissue Distribution in Mature Plants and During Seedling Development and Embryogenesis

Abstract

We have investigated the distribution of transcripts for chloroplast proteins involved in photosynthesis and carbon assimilation in Arabidopsis thaliana. The probes that were used for assessment of transcripts represent the chloroplast genes psbA (encoding the D1 protein of photosystem 11) and rbcL (for the large subunit of RubisCO) as well as the nuclear rbcS gene family for the RubisCO small subunit. With any of these probes we find a tissue-specific distribution of transcripts in mature plants, with the highest concentration in rosette leaves and almost undetectable amounts in roots. The transcripts detected by each of these probes reveal similar spatial distribution and temporal kinetics during seedling development, with maximal levels around 70 h after sowing. They each show higher levels under light-grown as compared to dark-grown conditions, indicating a coarse control that ensures roughly coordinated levels throughout plastid differentiation. Embryonic transcripts are present in much higher concentration within the ground meristem and protoderm than in the procambium, which seems to reflect the different degree of determination towards the subsequent function in photosynthesis. Transcripts were detected in all characteristic stages throughout embryogenesis including the (non-green) proembryo stage, suggesting that chlorophyll accumulation is not essential for expression of these genes at the RNA level. Both processes, hence, appear to be triggered by earlier molecular events that remain to be characterized.

Transcriptional Profiling of the Arabidopsis Embryo

“…This is in accordance with in situ hybridization studies that show an increasing abundance of chloroplastic gene transcripts progressively from the proembryo stage of embryogenesis, with the highest concentration observed in the cotyledons of the mature embryo (Degenhardt et al, 1991). The peak in transcript abundance in the mature embryo corresponds with the fate of the cotyledons as the initial photosynthetic organs of the seedling and allows photosynthesis to commence promptly postgermination (Degenhardt et al, 1991;Raghavan, 1997). Chloroplastic gene transcripts are also present as up-regulated in the basal time course, which corresponds with the constitutive pattern of expression observed in developing embryos of Gossypium hirsutum (Borroto and Dure, 1986), Glycine max Walling, 1991, 1992), and Arabidopsis (Degenhardt et al, 1991).…”

“…This is in accordance with in situ hybridization studies that show an increasing abundance of chloroplastic gene transcripts progressively from the proembryo stage of embryogenesis, with the highest concentration observed in the cotyledons of the mature embryo (Degenhardt et al, 1991). The peak in transcript abundance in the mature embryo corresponds with the fate of the cotyledons as the initial photosynthetic organs of the seedling and allows photosynthesis to commence promptly postgermination (Degenhardt et al, 1991;Raghavan, 1997).…”

“…The peak in transcript abundance in the mature embryo corresponds with the fate of the cotyledons as the initial photosynthetic organs of the seedling and allows photosynthesis to commence promptly postgermination (Degenhardt et al, 1991;Raghavan, 1997). Chloroplastic gene transcripts are also present as up-regulated in the basal time course, which corresponds with the constitutive pattern of expression observed in developing embryos of Gossypium hirsutum (Borroto and Dure, 1986), Glycine max Walling, 1991, 1992), and Arabidopsis (Degenhardt et al, 1991). Figure 9.…”

“…The most significant functional group represented in the root is that of metabolism, and this correlates well with the analysis performed by Yamada et al (2003) on mature root tissue, which showed metabolism to be the largest functional group, comprising approximately 13% (plus 5.5% designated as protein metabolism) of the 549 root-specific transcripts identified. The cotyledon shows up-regulation of a wide range of functional groups, including energy and protein synthesis, correlating with the increase in chloroplastic gene transcripts to reach a peak in the cotyledons of the mature embryo (Degenhardt et al, 1991). A number of the genes functionally classified as protein synthesis are chloroplast and plastid ribosomal protein precursors.…”

The Arabidopsis Embryo Mutant schlepperless Has a Defect in the Chaperonin-60α Gene

“…There are indications that nongreen proembryos could be actively synthesizing proteins that are eventually localized in the plastids. For example, mRNAs corresponding to chloroplast psbA gene (encodes the D1 protein of photosystem II) and nuclear rbcS gene (encodes the small subunit of Rubisco) begin to accumulate in the proembryo (Degenhardt et al, 1991). Because the folding of proteins encoded by these mRNAs is chaperonin mediated, functional chaperonin-60␣ protein is necessary at the very early stages of embryogenesis, even before the embryo becomes green.…”