Abstract
The phytohormone auxin (indole-3-acetic acid [IAA]) plays a fundamental role in vegetative and reproductive plant development. Here, we characterized a seed-specific viable maize (Zea mays) mutant, defective endosperm18 (de18) that is impaired in IAA biosynthesis. de18 endosperm showed large reductions of free IAA levels and is known to have approximately 40% less dry mass, compared with De18. Cellular analyses showed lower total cell number, smaller cell volume, and reduced level of endoreduplication in the mutant endosperm. Gene expression analyses of seed-specific tryptophan-dependent IAA pathway genes, maize Yucca1 (ZmYuc1), and two tryptophan-aminotransferase co-orthologs were performed to understand the molecular basis of the IAA deficiency in the mutant. Temporally, all three genes showed high expression coincident with high IAA levels; however, only ZmYuc1 correlated with the reduced IAA levels in the mutant throughout endosperm development. Furthermore, sequence analyses of ZmYuc1 complementary DNA and genomic clones revealed many changes specific to the mutant, including a 2-bp insertion that generated a premature stop codon and a truncated YUC1 protein of 212 amino acids, compared with the 400 amino acids in the De18. The putative, approximately 1.5-kb, Yuc1 promoter region also showed many rearrangements, including a 151-bp deletion in the mutant. Our concurrent high-density mapping and annotation studies of chromosome 10, contig 395, showed that the De18 locus was tightly linked to the gene ZmYuc1. Collectively, the data suggest that the molecular changes in the ZmYuc1 gene encoding the YUC1 protein are the causal basis of impairment in a critical step in IAA biosynthesis, essential for normal endosperm development in maize.
Highlights
The phytohormone auxin plays a fundamental role in vegetative and reproductive plant development
The significance of the large abundance of Indole-3-acetic acid (IAA) in developing endosperm remains to be understood, except that it may be used during the very early stages of seed germination because .90% of the total IAA is in biologically inactive conjugated storage form (Jensen and Bandurski, 1994; LeClere et al, 2008). Such a role in germination is consistent with the fact that there are very few viable seed mutants reported in maize that are linked to IAA deficiency, single-locus recessive mutants with various abnormalities in either embryo or endosperm development and with low IAA levels were reported by Lur and Setter (1993)
Torti et al (1986) reported dramatic reductions in total IAA across the entire developmental period of endosperm (12–40 d after pollination (DAP)), they reported that at 20 DAP, there was no reduction in free IAA; that result is not supported by our data
Summary
The phytohormone auxin (indole-3-acetic acid [IAA]) plays a fundamental role in vegetative and reproductive plant development. The significance of the large abundance of IAA in developing endosperm remains to be understood, except that it may be used during the very early stages of seed germination because .90% of the total IAA is in biologically inactive conjugated storage form (Jensen and Bandurski, 1994; LeClere et al, 2008) Such a role in germination is consistent with the fact that there are very few viable seed mutants reported in maize that are linked to IAA deficiency, single-locus recessive mutants (defective kernels [dek]) with various abnormalities in either embryo or endosperm development and with low IAA levels (measured by ELISA) were reported by Lur and Setter (1993). Our collective data, based on the cloning and sequencing of ZmYuc and on mapping studies, indicate that ZmYuc and De18 are tightly associated and that the aberrant YUC1 protein in de is the causal basis of IAA deficiency and the small seed phenotype in that mutant
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