Plastid DNA in Developing Maize Endosperm

Abstract

Amyloplasts in storage organs such as maize (Zea mays L.) endosperm are plastid-derived, nonphotosynthetic, starch-accumulating organelles. This study was initiated to characterize the plastid genome in maize endosperm cells containing differentiated amyloplasts and to determine whether plastid genes are transcribed during the period of amyloplast biogenesis in endosperm development. Four cosmid clones representing the total sequence diversity of the maize plastid genome were hybridized to restriction digests of total cellular DNA from isolated 16-day-old endosperms. The In contrast to chloroplasts, less is known about the molecular controls involved in development of nongreen plastids. Although synthesis of most plastid components is controlled by nuclear genes, plastids contain an independent genome and synthesize proteins (34). The genes for many of these plastid proteins have been mapped to the plastid genome. In addition, the plastid genomes of liverwort (Marchantia polymorpha L.), tobacco (Nicotiana tabacum L.), and rice (Oryza sativa L.) have been shown to contain about 40 other ORFs4 (10, 23, 30), which suggests that plastid genomes could encode yet unidentified functions, some of which might be specific to nongreen plastids.This study was initiated to obtain information about the role of the plastid genome in the molecular events controlling differentiation of amyloplasts during the development of maize (Zea mays L.) endosperm tissue. Maize endosperm amyloplasts differentiate from proplastids rather than from another differentiated plastid type, such as conversion from chloroplasts, which can occur in other plant tissues and organs. Hence, the time course of amyloplast development in endosperm can be followed directly from proplastids. Visually detectable differentiation of maize endosperm amyloplasts begins about 8 to 10 DAP (17) and proceeds until the average endosperm cell contains approximately 200 to 400 amyloplasts, each with a single starch granule.To serve as the basis for further comparisons, our initial objective was to establish that the genome organization of plastids in developing maize endosperm was the same as for maize leaf chloroplasts. Based on restriction endonuclease digestion patterns, amyloplasts of wheat (Triticum aestivum L.) endosperm (2) and sycamore (Acer pseudoplatanus L.) suspension cells (22), as well as the pigmented chromoplasts of tomato (Lycopersicon esculentum L.) (11,15,20), pepper (Capsicum annuum L.) fruit (7), and daffodil (Narcissus pseudonarcissus) flowers (32) contain genomes indistinguishable from the respective chloroplast genomes.We also examined the steady-state accumulation of plastid transcripts in maize endosperm tissue versus leaves because such information was not available for cereal endosperm amyloplasts and we were interested in determining whether endosperm-specific transcripts or transcript accumulation patterns could be identified. In most studies of transcription

Complete RNA editing of unspliced and dicistronic transcripts of the intron-containing reading frame IRF170 from maize chloroplasts.

“…It is not clear how inhibitory effects that one might expect for the function (or maturation) ofthe serine tRNA are avoided, but similar situations have been reported for other polycistronic chloroplast transcripts containing antisense tRNA sequences (31,32). Our observation of the rps4-IRF170 linkage supports the supposition that the entire gene cluster constitutes a single transcriptional unit (18), although it still remains to be determined whether the flanking tRNA genes (trnT and trnR) have to be included in this unit.…”

“…As already outlined above, the almost uniform polarity of the rps4-rpsl4 gene cluster suggests that the entire cluster is transcribed into a single polycistronic primary transcript and then processed into monocistronic mRNAs as has been observed for several other chloroplast gene clusters (1,2). A transcriptional linkage of the entire rps4-rpsl4 gene cluster and cleavage to a monocistronic IRF170 transcript in maize appears evident from the observation of 9.5-and 0.95-kb bands in Northern blots detected with an IRF170 probe (18). Transcriptional start sites have, on the other hand, also been identified in the psaB-rpsl4 intergenic region of maize (28) and in the IRFI70-psaA intergenic regions of rice (29) and tobacco (30).…”

“…is positioned in the central part ofthe large single-copy region (LSC) of the maize plastome. Its constituent genes trnT, rps4, psaA, psaB, rpsl4, and trnR, which had been analyzed previously by several groups (18,(20)(21)(22)(23)(24) (the region comprising the rps4, trnS, IRF170, and psaA genes had also been analyzed in our laboratory by G. L. Igloi, unpublished results), show the same polarity as IRF170, which suggests that the entire cluster constitutes a single transcriptional unit even though the rps4-IRF170 intergenic region does contain the trnS gene (23) of opposite polarity.…”

“…1), whereas a homologous reading frame without introns could be identified in the cyanobacterium Synechocystis (16). Although the function of the encoded peptide, consisting in maize of 170 amino acids, is not known, the absence of an IRF170 homolog in the plastome of the nonphotosynthetic parasitic plant Epifagus (17) as well as differences in the processing of IRF170 transcripts between amyloplasts and chloroplasts from maize (18) appears to suggest a function as a component of the photosynthetic apparatus. This supposition is further supported by the codon usage of IRF170-encoded mRNAs (15) and by the existence of a cyanobacterial IRF170 homolog (16).…”

Multiple Checkpoints for the Expression of the Chloroplast-Encoded Splicing Factor MatK

“…no correlation between splicing rates and the amount of the final gene product has been established for chloroplast RNAs). In a few cases, tissue-specific changes in the abundance of spliced to unspliced mRNAs have been found (Barkan, 1989;McCullough et al, 1992), but whether this is caused by altered splicing rates or via differential stability of spliced RNA versus precursor RNA was not shown. For tRNAs with introns, changes in splicing would directly impact the amount of the final gene product.…”