Nuclear, chloroplast, and mitochondrial transcript abundance along a maize leaf developmental gradient.

In maize, the chloroplast chromosome encodes 104 genes whose roles are primarily in photosynthesis and gene expression. The 2,000-3,000 nuclear gene products that localize to plastids are required both to encode and regulate plastid gene expression as well as to underpin each aspect of plastid physiology and development. We used a new "three-genome" maize biogenesis cDNA microarray to track abundance changes in nuclear, chloroplast and mitochondrial transcripts in stage 2 semi-emerged leaf blades of one month-old maize plants. We report the detection and quantification of 433 nuclear, 62 chloroplast, and 27 mitochondrial transcripts, with the majority of the nuclear transcripts predicted or known to encode plastid proteins. The data were analyzed as ratios of expression of individual transcripts in the green tip (mature chloroplasts) versus the yellow base of the leaf (etioplasts). According to the microarray data at least 51 plastid genes and 121 nuclear genes are expressed at least two-fold higher in the tip of the leaf. Almost all (25) mitochondrial and 177 nuclear transcripts were expressed at least 2-fold higher in the leaf base. Independent quantification of a subset of each transcript population by RNA gel blot analysis and/or quantitative real time RT-PCR concurred with the transcript ratios determined by the array. Ontological distribution of the transcripts suggests that photosynthesis-related RNAs were most highly abundant in the leaf tip and that energy use genes were most highly expressed in the base. Transcripts whose products are used in plastid translation constituted the largest single ontological group with relatively equal numbers of genes in the three expression categories, defined as higher in tip, higher in base, or equally expressed in tip and base.