Location: Chemistry ResearchTitle: Impaired auxin biosynthesis in the defective endosperm18 mutant is due to mutational loss of expression in the ZmYuc1 gene encoding endosperm-specific YUCCA1 protein in maize. Author
Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/7/2012
Publication Date: 11/1/2012
Citation: Bernardi, J., Lanubile, A., Li, Q., Kumar, D., Kladnick, A., Marocco, A., Chourey, P.S., Cook, S.D., Ross, J.J. 2012. Impaired auxin biosynthesis in the defective endosperm18 mutant is due to mutational loss of expression in the ZmYuc1 gene encoding endosperm-specific YUCCA1 protein in maize.. Plant Physiology. 160:1318-1328. Interpretive Summary: Seed is a unit of crop yields in all agronomic plants, and maize in particular which is grown for food, feed and biofuel. Plant hormone auxin controls numerous signaling functions throughout development; most importantly seed where it is synthesized in the largest amounts. Yet, very little to nothing is known on the genetics of its biosynthesis in developing seeds. This report is a major advancement in correcting the deficiency in our basic knowledge through a cooperative effort of an international interdisciplinary team of scientists from Italy, Slovenia, University of Florida and, most importantly, CMAVE, Gainesville, FL. The results of this study show that a previously described small seed mutant with greatly reduced seed weight is due to several drastic molecular changes in a gene that codes for a critical seed-specific enzyme in a pathway for auxin biosynthesis. Consequently, developing seeds of the mutant lose ~95% of the auxin, have smaller and fewer cells, lesser DNA and, ultimately, the reduced yields. This is the first auxin-deficient seed mutant in any plant; impact of this discovery is greatly increased because it’s in maize, a crop of huge economic significance. Overall, the study identifies a gene of significant agronomic importance; further studies on this gene and its network of genes during seed development will provide much new knowledge of both basic and applied importance to seed development and crop yields.
Technical Abstract: A seed-specific maize mutant, defective endosperm18 (de18), accumulates approximately 40% less dry mass and 10- to 15- fold less auxin (IAA) as compared to the De18; however, a causal basis of these changes is not known. Cellular analyses here showed that the de18 developing endosperm had lower total cell number, a smaller cell volume and a reduced level of endoreduplication relative to the De18. Gene expression analyses of seed-specific Trp-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 that were specific to the mutant, including a major change of 2-bp insertion causing a translational frame-shift that led to the predicted YUCCA1 protein of only 212 amino acids relative to the normal 400 amino acids in the De18. The putative ~1.5 kb Yuc1 promoter region also showed many changes, including a 151 bp deletion in the mutant. Importantly, our concurrent high density mapping and annotation studies of chromosome 10, contigs 393 -397, showed that the De18 locus mapped to the gene ZmYuc1. Collectively, the data suggest that the molecular changes in the De18 gene that encodes the YUCCA1 protein is the causal basis of a critical step in IAA biosynthesis, which is essential to normal seed mass determination and development in maize.