Impaired auxin biosynthesis in the defective endosperm18 is due to a mutation at the ZmYuc1 gene encoding YUCCA1 protein in maize.

The phytohormone auxin (IAA) plays a fundamental role in vegetative and reproductive plant development., We characterized here a seed-specific viable maize mutant, defective endosperm18 (de18) that is impaired in IAA biosynthesis. de18 endosperm accumulates 10- to 15- fold less IAA and is known to have ~40% less dry mass compared to 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, ZmYuc1 and two Trp-aminotransferase co-orthologs, were done 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 the ZmYuc1 correlated with the reduced IAA levels in the mutant throughout endosperm development. Further, sequence analyses of ZmYuc1 cDNA and genomic clones revealed many changes specific to the mutant, including a 2-bp insertion that generated a premature stop codon and a truncated YUCCA1 protein of 212 amino acids, compared to the 400 amino acids in the De18. Consistent with these results are the recombinant protein data from E. coli where full length cDNA clones of ZmYuc1 of De18 and de18 endosperm showed normal and greatly reduced sizes of fusion proteins, ~70 kD and ~50 kD, respectively (each included a GST tag of 26.0 kD). The putative ~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 that encoded the YUCCA1 protein is the causal basis of impairment in a critical step in IAA biosynthesis, essential to endosperm development in maize.