The Juvenile Phase of Maize Sees Upregulation of Stress-Response Genes and Is Extended by Exogenous Jasmonic Acid

Abstract

As maize (Zea mays) plants undergo vegetative phase change from juvenile to adult, they both exhibit heteroblasty, an abrupt change in patterns of leaf morphogenesis, and gain the ability to produce flowers. Both processes are under the control of microRNA156 (miR156), whose levels decline at the end of the juvenile phase. Gain of the ability to flower is conferred by the expression of miR156 targets that encode SQUAMOSA PROMOTER-BINDING transcription factors, which, when derepressed in the adult phase, induce the expression of MADS box transcription factors that promote maturation and flowering. How gene expression, including targets of those microRNAs, differs between the two phases remains an open question. Here, we compare transcript levels in primordia that will develop into juvenile or adult leaves to identify genes that define these two developmental states and may influence vegetative phase change. In comparisons among successive leaves at the same developmental stage, plastochron 6, three-fourths of approximately 1,100 differentially expressed genes were more highly expressed in primordia of juvenile leaves. This juvenile set was enriched in photosynthetic genes, particularly those associated with cyclic electron flow at photosystem I, and in genes involved in oxidative stress and retrograde redox signaling. Pathogen- and herbivory-responsive pathways including salicylic acid and jasmonic acid also were up-regulated in juvenile primordia; indeed, exogenous application of jasmonic acid delayed both the appearance of adult traits and the decline in the expression of miR156-encoding loci in maize seedlings. We hypothesize that the stresses associated with germination promote juvenile patterns of differentiation in maize.

Figure 1.

Genome-wide differences in gene expression among plastochron 6 leaf primordia. A, Pairwise comparisons of genome-wide 8-mm leaf primordia expression (Pearson r²), with red hue increasing with difference. B, Hierarchical clustering of leaf primordia 1 to 12 using 16,772 differentially expressed transcripts. C, Pearson hierarchical clustering in Arraystar of 16,772 variably expressed transcripts, normalized to leaf 1. Major juvenile and adult phase-specific clusters are indicated. Per row, the color scale excludes the highest and lowest 1% of values. Red color indicates highest expression, and blue color indicates lowest expression.

Figure 2.

Pearson hierarchical clustering in Arraystar of 3,385 juvenile cluster transcripts. EJ and FJ indicate constituents of early juvenile and full juvenile clusters, respectively. Transcripts of miR156 loci are located as indicated. Per row, the color scale excludes the highest and lowest 1% of values. Red color indicates highest expression, and blue color indicates lowest expression.

Figure 3.

Adult up-regulated transcripts include those for SPBs and are coexpressed with miR172c. Per row, the color scale excludes the highest and lowest 1% of values. Red color indicates highest expression, and blue color indicates lowest expression.

Figure 4.

Promoter motifs found in the upstream 400 bp of transcripts most closely coexpressed with miR156f/g/i (A) or miR172c (B). The 10 most enriched motifs identified by the SCOPE and DREME programs, queried against known binding sequences in TOMTOM and with hits at q ≤ 0.1 (indicating the minimum false discovery rate that would include the hit), are presented. All motifs except ACGTRS originate from DREME.

Figure 5.

Juvenile-specific coexpression of miR156 with selected JA- and salicylic acid (SA)-associated transcripts, normalized to leaf 1 expression. Per row, the color scale excludes the highest and lowest 1% of values. Red color indicates highest expression, and blue color indicates lowest expression.

Figure 6.

JA effects on phase-specific leaf traits. A to F. Phase-specific traits. A, No trichomes on juvenile leaves. B, Trichomes on adult leaves. C to F, Toluidine Blue O-stained epidermal peals. C, Juvenile. D, Transition. E, Adult. F, Bulliform cells (adult). G, A single treatment of increasing concentrations of JA at seedling emergence increasingly reduced the area with trichomes and the final length of leaf 5, the first transition leaf. n = 10. H, Increasing doses of 5 mm JA, 2 d apart, delayed the appearance of adult traits in leaf 6. n = 12. I, Leaf 6 of JA-deficient tasselseed1 (ts1) mutants develop adult-specific trichomes earlier than wild-type (w.t.) sibs. n = 8. Mutant leaf 6 stained entirely blue with Toluidine Blue O with adult-type crenulation (data not shown). J, Hierarchical clustering using combined juvenile- and adult-specific sets of genes. JA-treated leaf 5 (5JA) clusters with leaves 3 and 4; untreated leaf 5 clusters with adult leaves.

Figure 7.

Leaf primordium expression patterns for selected redox and stress transcripts from the early juvenile cluster (A) and selected redox and photosynthetic transcripts coexpressed with miR156f/g/i loci (B). Per row, the color scale excludes the highest and lowest 1% of values. Red color indicates highest expression, and blue color indicates lowest expression.