Comparative transcriptome analysis of differentially expressed genes related to the physiological changes of yellow-green leaf mutant of maize

Abstract

Chlorophylls, green pigments in chloroplasts, are essential for photosynthesis. Reduction in chlorophyll content may result in retarded growth, dwarfism, and sterility. In this study, a yellow-green leaf mutant of maize, indicative of abnormity in chlorophyll content, was identified. The physiological parameters of this mutant were measured. Next, global gene expression of this mutant was determined using transcriptome analysis and compared to that of wild-type maize plants. The yellow-green leaf mutant of maize was found to contain lower contents of chlorophyll a, chlorophyll b and carotenoid compounds. It contained fewer active PSII centers and displayed lower values of original chlorophyll fluorescence parameters than the wild-type plants. The real-time fluorescence yield, the electron transport rate, and the net photosynthetic rate of the mutant plants showed reduction as well. In contrast, the maximum photochemical quantum yield of PSII of the mutant plants was similar to that of the wild-type plants. Comparative transcriptome analysis of the mutant plants and wild-type plants led to the identification of differentially expressed 1,122 genes, of which 536 genes were up-regulated and 586 genes down-regulated in the mutant. Five genes in the chlorophyll metabolism pathway, nine genes in the tricarboxylic acid cycle and seven genes related to the conversion of sucrose to starch displayed down-regulated expression. In contrast, genes encoding a photosystem II reaction center PsbP family protein and the PGR5-like protein 1A (PGRL1A) exhibited increased transcript abundance.

Keywords: Chlorophyll biosynthesis; Photosynthesis; Secondary metabolism; Transcriptome analysis; Tricarboxylic acid cycle; Yellow-green leaf.

Figure 1. Phenotypic characteristics change of yellow-green leaf mutant plants.

(A) Indicated two yellow-green leaf mutant plants and a normal green leaf plant at the same age. Maize plants in this image were at the five-leaf stage. (B) Showed the contents of chlorophyll a and chlorophyll b in the normal green leaf inbred line and the yellow-green leaf mutant. (C) showed the contents of eight carotenoid compounds including neoxanthin, violaxanthin, capsanthin, zeaxanthin, β-cryptoxanthin, ɑ-carotene, β-carotene and lutein. Small letters a and b above the columns indicate differences between the yellow-green leaf mutant and the normal green leaf inbred line at P < 0.05, according to least significant difference (LSD) tests. FW is the abbreviation of the fresh weight.

Figure 2. Chlorophyll fluorescence and photosynthesis parameters in the yellow-green leaf mutant inbred line and the normal green leaf inbred line.

(A) Indicated the original chlorophyll fluorescence parameters. (B) Showed real-time fluorescence yield Ft in the normal green leaf inbred line and the yellow-green leaf mutant. The point pointed by the arrows indicated the turned on of the actinic light. (C) Indicated additional chlorophyll fluorescence parameters of the normal green leaf inbred line and the yellow-green leaf mutant. Small letters a and b indicate differences between the yellow-green leaf mutant and the normal green leaf inbred line at P < 0.05, according to least significant difference (LSD) tests. (D) Showed light response curves of net photosynthesis in the yellow-green leaf mutant and the normal green leaf inbred line. Pn is the net photosynthesis rate. The light response curves were measured at nine PAR levels (0, 50, 100, 200, 400, 800, 1,200, 1,600 and 2,000 μmolphotons·m⁻²·s⁻¹). The dark respiration rate is defined as the Pn value when the light response curve intersects the Y-axis. The measurement was performed for three times. The error bars represented the standard errors.

Figure 3. Differentially expressed genes and its enriched metabolic pathways between the yellow-green leaf mutant and the normal green leaf inbred line.

(A) Indicated differentially expressed genes between the yellow-green leaf mutant and the normal green leaf inbred line. Differentially expressed genes were selected by q-value < 0.005 & |log2 (fold change)| > 1. The X axis indicates gene expression changes in different samples, and the Y axis indicates the significant degree of gene expression changes. Scattered points represent each gene, the red dots represent differentially up-regulated genes, the green dots represent differentially down-regulated genes, and the blue dots represent no significant difference gene. YGL, yellow-green leaf mutant; NGL, normal green leaf inbred line; −log10 (padj), the corrected p-value (padj < 0.05). (B) Showed the pie chart of enriched metabolic pathways of genes differentially expressed in the yellow-green leaf mutant and the normal green leaf inbred line. The pie chart was generated by submission of the differentially expressed genes to the online Mercator sequence annotation tool (http://www.plabipd.de/portal/mercator-sequence-annotation).

Figure 4. Differentially expressed genes involved in chlorophyll metabolism between the yellow-green leaf mutant and the normal green leaf inbred line.

Figure 5. Differentially expressed genes involved in photosynthesis light reactions and carbon reactions between the yellow-green leaf mutant and the normal green leaf inbred line.

Figure 6. Differentially expressed genes in the tricarboxylic acid cycle between the yellow-green leaf mutant and the normal green leaf inbred line.

Figure 7. Differentially expressed genes involved in sucrose to starch conversion between the yellow-green leaf mutant and the normal green leaf inbred line.