Cytokinins are hormones that play an essential role in plant growth and development. The irreversible degradation of cytokinins, catalyzed by cytokinin oxidase, is an important mechanism by which plants modulate their cytokinin levels. Cytokinin oxidase has been well characterized biochemically, but its regulation at the molecular level is not well understood. We isolated a cytokinin oxidase open reading frame from maize (Zea mays), called Ckx1, and we used it as a probe in northern and in situ hybridization experiments. We found that the gene is expressed in a developmental manner in the kernel, which correlates with cytokinin levels and cytokinin oxidase activity. In situ hybridization with Ckx1 and transgenic expression of a transcriptional fusion of the Ckx1 promoter to the Escherichia coli beta-glucuronidase reporter gene revealed that the gene is expressed in the vascular bundles of kernels, seedling roots, and coleoptiles. We show that Ckx1 gene expression is inducible in various organs by synthetic and natural cytokinins. Ckx1 is also induced by abscisic acid, which may control cytokinin oxidase expression in the kernel under abiotic stress. We hypothesize that under non-stress conditions, cytokinin oxidase in maize plays a role in controlling growth and development via regulation of cytokinin levels transiting in the xylem. In addition, we suggest that under environmental stress conditions, cytokinin oxidase gene induction by abscisic acid results in aberrant degradation of cytokinins therefore impairing normal development.

Fast neutron radiation has been used as a mutagen to develop extensive mutant collections. However, the genome-wide structural consequences of fast neutron radiation are not well understood. Here, we examine the genome-wide structural variants observed among 264 soybean [Glycine max (L.) Merrill] plants sampled from a large fast neutron-mutagenized population. While deletion rates were similar to previous reports, surprisingly high rates of segmental duplication were also found throughout the genome. Duplication coverage extended across entire chromosomes and often prevailed at chromosome ends. High-throughput resequencing analysis of selected mutants resolved specific chromosomal events, including the rearrangement junctions for a large deletion, a tandem duplication, and a translocation. Genetic mapping associated a large deletion on chromosome 10 with a quantitative change in seed composition for one mutant. A tandem duplication event, located on chromosome 17 in a second mutant, was found to cosegregate with a short petiole mutant phenotype, and thus may serve as an example of a morphological change attributable to a DNA copy number gain. Overall, this study provides insight into the resilience of the soybean genome, the patterns of structural variation resulting from fast neutron mutagenesis, and the utility of fast neutron-irradiated mutants as a source of novel genetic losses and gains.

Reductions in kernel mass are observed when corn (Zea mays L.) kernels are grown in vitro or when unfavorable temperature occurs during endosperm cell division. We investigated the possibility that a decreased number of endosperm cells or a decreased number of starch granules is responsible for the reduced kernel mass in such environments. Three‐day‐old kernels of the single cross hybrid A619 ✕ W64A were placed in culture on a denned medium at 15, 30, and 35 °C, and their growth was compared with kernels from ears developed in the field or greenhouse. Kernels cultured at 30°C attained a final mass of 164 mg compared with 274 mg for field‐grown controls. At 30°C, endosperm cell division ceased approximately 10 days earlier, and the final number of cells was reduced by 34%. Final kernel mass was reduced by 49 and 78% when kernels were cultured at 15 and 35°C, respectively, compared with those grown at 30°C. At 35°C, the rate and duration of cell division in the endosperm, and the number of endosperm cells were severely reduced. In contrast, the rate of cell division decreased in kernels cultured at 15°C, but the duration was prolonged, and the number of endosperm cells formed was not affected. However, the number of starch granules initiated at 15 and 35°C was reduced by 70 and 97%, respectively. Final kernel mass was highly correlated with the number of endosperm cells (r=0.85,p≤0.01) and starch granules formed (r=0.76, p≤0.01). These data suggest that thermal regulation of the number of endosperm cells, starch granules, or both are mechanisms by which final kernel mass may be mediated. The reduction in mass of in vitro compared with field‐ or greenhouse‐grown kernels appears to be due mainly to a decline in the number of endosperm cells formed.

Studies were conducted to determine the effects of selected plant growth regulators on red color development of Red Norland potato periderm, and the mechanism by which color changes occurred. Two derivatives of the auxin 2,4-dichlorophenoxyacetic acid (2,4-D ester and 2,4-D amine), S-abscisic acid, n-propyl dihydrojasmonate, and the ethylene synthesis inhibitor aminoethoxyvinylglycine were foliar applied at 4, 6, and 8 weeks after emergence. Only the 2,4-D ester and 2,4-D amine significantly improved tuber periderm red color in two seasons of evaluation. Red color faded during storage regardless of treatment. Anthocyanin content at vine kill was significantly increased by 2,4-D treatment in 2004 but not 2005. The 2,4-D treatments increased the ratio of peonidin derivatives to pelargonidin derivatives compared with controls. This change in ratio may explain the observed change in hue angle with 2,4-D treatment compared to controls.Resumen Se realizaron trabajos para determinar los efectos de reguladores de crecimiento selectos sobre el desarrollo del color rojo del peridermo de papa de Red Norland y el mecanismo por el cual ocurrieron los cambios de color. Dos derivados de la auxina 2,4-ácido diclorofenoxiacético (2,4-D éster y la amina 2,4-D), S-ácido abscísico, n-propil dihidrojasmonato y el inhibidor de la síntesis del etileno aminoetoxivinilglicina se aplicaron a las 4, 6 y 8 semanas después de la emergencia. Solo el éster 2,4-D y la amina 2,4-D mejoraron significativamente el color rojo del peridermo en dos temporadas de evaluación. El color rojo disminuyó durante el almacenamiento en todos los tratamientos. El contenido de antocianina en el momento de la destrucción del follaje aumentó significativamente con el tratamiento de 2,4-D en el 2004, pero no en el 2005. Los tratamientos con 2,4-D incrementaron la relación de derivados de peonidina a derivados de pelargodina comparado con los testigos. Este cambio en la relación explicaría los cambios observados en el ángulo de matiz con el tratamiento 2,4-D comparado con los testigos.