Defective endosperm 18 (De18) encodes a seed-specific YUCCA 1 protein essential for IAA biosynthesis, normal endosperm development and seed mass in maize.

Auxin (IAA) plays a fundamental role in vegetative and reproductive plant development. Developing seeds synthesize and accumulate the highest levels of IAA of all tissues in a plant; however, nothing is known about IAA biosynthesis genes / enzymes critical to endosperm development, nor is there any seed mutant known to be caused by IAA-deficiency. We analyzed a previously described (Theor. Appl. Genet., 1986, 72: 602) seed-specific viable mutant de18 that accumulates 10- to 15- fold less IAA and ~40% less dry mass compared to De18. Gene expression analyses of seed-specific Tryptophan-dependent IAA pathway genes, ZmYucca1 and two Trp-aminotransferases (Tar) co-orthologs (Molecular Plant, 2010, 3: 1026), were done using q-PCR to understand the molecular basis of 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. 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. The putative ~1.5 kb ZmYuc1 promoter region also showed many rearrangements, including a 151 bp deletion in the mutant. Further, our molecular mapping and annotation studies of chromosome 10, contigs 393 - 397, showed that the De18 locus mapped to the gene ZmYuc1. In conclusion: (1) The loss of De18-encoded YUCCA1 protein is the causal basis of IAA-deficient de18 mutant. (2) The lack of compensation of the mutant yuc1 function despite the high transcript abundance of the two ZmTar genes suggests that YUCCA1 constituted a rate-limiting step in a single pathway of multiple steps of IAA biosynthesis, consistent with the very recently reported studies (in the last two months) in Arabidopsis.