David W. Still | California State Polytechnic University at Pomona (original) (raw)

Papers by David W. Still

Plant and Soil, 2021

microbes within and upon them. We analyzed the microbiomes of these underground organs from both ... more microbes within and upon them. We analyzed the microbiomes of these underground organs from both an important crop plant (Medicago sativa) and a related legume (M. polymorpha) using metagenomic and culture-based techniques to identify the main cultivatable contributors to plant growth enhancement. Methods Using high-throughput sequencing, culturing, and in planta techniques, we identified and analyzed a broad population of the bacterial taxa within Medicago nodules and the surrounding soil. Results Fifty-one distinct bacterial strains were isolated and characterized from nodules of both Medicago

Theoretical and Applied Genetics, Jul 1, 2021

Bolting, de ned as stem elongation, marks the plant life cycle transition from vegetative to repr... more Bolting, de ned as stem elongation, marks the plant life cycle transition from vegetative to reproductive stage. Lettuce is grown for its leaf rosettes, and premature bolting may reduce crop quality resulting in economic losses. The transition to reproductive stage is a complex process that involves many genetic and environmental factors. In this study the effects of photoperiod and ambient temperature on bolting and owering regulation was studied by utilizing a lettuce mapping population to identify quantitative trait loci (QTL) and by gene expression analyses of genotypes with contrasting phenotypes. A recombinant inbred line (RIL) population, derived from a cross between PI251246 (early bolting) and cv. Salinas (late bolting), was grown in four combinations of short (8 hours) and long (16 hours) days and low (20°C) and high (35°C) temperature. QTL models revealed both genetic (G) and environmental (E) effects, and GxE interactions. A major QTL for bolting and owering time was found on chromosome 7 (qFLT7.2) and two candidate genes were identi ed by ne mapping, homology and gene expression studies. In short days and high temperature conditions qFLT7.2 had no effect on plant development, while several small-effect loci on chromosomes 2, 3, 6, 8 and 9 were associated with bolting and owering. Of these, the QTL on chromosome 2, qBFr2.1, co-located with the Flowering Locus T (LsFT) gene. Polymorphisms between parent genotypes in the promotor region may explain identi ed gene expression differences, and were used to design a genetic marker which may be used to identify the late bolting trait. Key Message Photoperiod and temperature conditions elicit different genetic regulation over lettuce bolting and owering. This study identi es environment-speci c QTLs and putative genes and provides information for genetic marker assay.

Plant Physiology, 1997

Endo-P-mannanase is hypothesized to be a rate-limiting enzyme in endosperm weakening, which is a ... more Endo-P-mannanase is hypothesized to be a rate-limiting enzyme in endosperm weakening, which is a prerequisite for radicle emergente from tomato (Lycopersicon esculentum Mill.) seeds. Using a sensitive, single-seed assay, we have measured mannanase activity diffusing from excised tomato endosperm caps following treatments that alter the rate or percentage of radicle emergence. Most striking was the 100-to more than 10,000-fold range of mannanase activity detected among individual seeds of highly inbred tomato lines, which would not be detected in pooled samples. In some cases a threshold-type relationship between mannanase activity and radicle emergence was observed. However, when radicle emergence was delayed or prevented by osmoticum or abscisic acid, the initial increase in mannanase activity was unaffected or even enhanced. Partially dormant seed lots displayed a bimodal distribution of activity, with low activity apparently associated with dormant seeds in the population. Gibberellin-and abscisic acid-deficient mutant seeds exhibited a wide range of mannanase activity, consistent with their variation in hormonal sensitivity. Although the presence of mannanase activity in the endosperm cap is consistently associated with radicle emergence, it is not the sole or limiting factor under all conditions. Even though tomato (Lycopersicon esculentum Mill.) is self-pollinating and cultivars are highly homozygous genetically, germination behavior varies considerably among individual seeds. We have developed a population-based hydrotime model that accurately describes germination time courses in response to environmental and hormonal factors (

Hydrotime is a way to describe the relationship between water potential (\|/) and seed germinatio... more Hydrotime is a way to describe the relationship between water potential (\|/) and seed germination rates and percentages. The minimum \|/ that allows germination to be completed is called the base water potential (\|/j,). The ger mination rate (inverse of time to radicle emergence) increases linearly as the seed y increases above i|/i, to its maximum rate in water (\|/ = 0 MPa). As t|q, values vary among individual seeds, germination rates also vary, resulting in lack of uniformity that can be quantified by the standard deviation in t|q, val ues (<3yb)The hydrotime constant (0h) indicates the inherent speed of ger mination in a seed lot. Thus, hydrotime analysis quantifies the speed of ger mination (0h), the stress tolerance of germination (\|/i,) and the uniformity of germination for a seed lot (Cfyj,), which are all useful indicators of seed vigor. Hydrotime analysis of seed lots under diverse conditions allows them to be ranked according to their potential for successful emergence. It is also a valuable tool for developing and assessing seed enhancement treatments such as pelleting and priming. Hydrotime analysis can be simplified into an endpoint test that could be useful for ranking seed lots according to vigor and for diagnosing seed lot potential under stressful conditions. The Hydrotime Concept in Seed Germination The hydrotime concept is a unifying model to describe the patterns of germi nation that occur in response to the water potential (\|f) of the seed's environ ment. It is similar to thermal time, or degree-days, in which the degrees in excess of a base or threshold temperature (Ty), multiplied by the time to a develop mental event (for example, radicle emergence) is a constant. Gummerson (1986) proposed that in analogy with thermal time or degree-days responses in rela tion to temperature, the time to germination is related to the magnitude of the difference between the \p of the seed or environment and the physiological \|f threshold for radicle emergence (\|/&). Interestingly, Gummerson (1986) showed that in the case of germination responses to reduced t\f, the total hydrotime (MPa-hours or MPa-days) to radicle emergence was the same for all seeds in the population, but that individual seeds varied in their threshold \|/ at which radicle emergence would be prevented. The following equation describes the basis of the hydrotime model (Bradford, 1990; 1995): 0H=[V|f-yb(g)]tg (1) where 0h is the hydrotime constant (MPa h), \|/ is the actual seed water potential Kent J. Bradford* Department of Vegetable Crops, University of California, Davis, CA 95616-8631; David W. Still, Department of Plant Science, California State Polytechnic University, Pomona, CA 97168. Original results of D.W.S. supported in part by California Agriculture Research Initiative. ''Cor responding author. Received 19 September 2003.

Nutritional Genomics, 2011

Phytochemicals in Nutrition and Health, 2002

![Research paper thumbnail of PROTEIN PATTERNS DURING GERMINATION OF Lactuca sativa `Empire](https://attachments.academia-assets.com/87865836/thumbnails/1.jpg)

HortScience, 1990

Lettuce seeds were germinated at 20 C in the dark in water and sampled at various intervals durin... more Lettuce seeds were germinated at 20 C in the dark in water and sampled at various intervals during the first 18h of germination to determine quantitative and qualitative differences in proteins. The soluble protein fraction was partitioned into albumins and globulins by dialysis and the proteins of the globulin fraction were visualized by SDS-PAGE. Heat stable proteins were obtained by boiling the proteins, cooling on ice, and resuspending in buffer.The soluble protein content remained constant during the first 8h of germination. Thereafter protein content decreased and was 6% of the amount present in unimbibed seed in 21 day old seedlings. The ratio of heat stable to heat unstable proteins decreased during the germination process. No differences in banding patterns were observed when the soluble protein fraction were run on SDS-PAGE. However, on gels run with the globulin fraction a 57 kD protein appeared 4 and 8 h after imbibition and had disappeared by 12 h after imbibition. The ...

Planta medica, Jan 9, 2015

Herbal medicines and botanicals have long been used as sole or additional medical aids worldwide.... more Herbal medicines and botanicals have long been used as sole or additional medical aids worldwide. Currently, billions of dollars are spent on botanicals and related products, but minimal regulation exists regarding their purity, integrity, and efficacy. Cases of adulteration and contamination have led to severe illness and even death in some cases. Identifying the plant material in botanicals and phytomedicines using organoleptic means or through microscopic observation of plant parts is not trivial, and plants are often misidentified. Recently, DNA-based methods have been applied to these products because DNA is not changed by growth conditions unlike the chemical constituents of many active pharmaceutical agents. In recent years, DNA barcoding methods, which are used to identify species diversity in the Tree of Life, have been also applied to botanicals and plant-derived dietary supplements. In this review, we recount the history of DNA-based methods for identification of botanica...

Agriculture is practiced on 38% of the landmass on Earth, and having replaced natural ecosystems,... more Agriculture is practiced on 38% of the landmass on Earth, and having replaced natural ecosystems, it is the largest terrestrial biome on Earth. Agricultural biomes are typically focused on annual crops that are produced as a succession of genetically uniform monocultures. Compared to the ecosystems they replaced, agroecosystems provide fewer ecosystem functions and contain much less biodiversity. The large-scale conversion from natural lands to agriculture occurred centuries ago in the Old World (Africa, China, Europe, and India), but in many areas during the latter 20th and early 21st centuries, especially tropical areas with rich biodiversity, agriculture is an emerging industry. Here, displacement of natural ecosystems is also a late 20th-century occurrence, and much of it is ongoing. Regardless of where or when agriculture was established, biodiversity declined and ecosystem services were eroded. Agricultural practices are the second largest contributor to biodiversity loss, due...

Bolting, defined as stem elongation, marks the plant life cycle transition from vegetative to rep... more Bolting, defined as stem elongation, marks the plant life cycle transition from vegetative to reproductive stage. Lettuce is grown for its leaf rosettes, and premature bolting may reduce crop quality resulting in economic losses. The transition to reproductive stage is a complex process that involves many genetic and environmental factors. In this study the effects of photoperiod and ambient temperature on bolting and flowering regulation was studied by utilizing a lettuce mapping population to identify quantitative trait loci (QTL) and by gene expression analyses of genotypes with contrasting phenotypes. A recombinant inbred line (RIL) population, derived from a cross between PI251246 (early bolting) and cv. Salinas (late bolting), was grown in four combinations of short (8 hours) and long (16 hours) days and low (20°C) and high (35°C) temperature. QTL models revealed both genetic (G) and environmental (E) effects, and GxE interactions. A major QTL for bolting and flowering time wa...

HortTechnology

Brassica crops have indeterminate growth and flower over an extended period of time. Harvested se... more Brassica crops have indeterminate growth and flower over an extended period of time. Harvested seed is therefore comprised of seed of varying degrees of physiological maturity and quality. Using population-based threshold models, broccoli (Brassica oleracea L. Group Italica), brussels sprouts (B. oleracea L. Group Gemmifera), red cabbage (B. oleracea L. Group Capitata), and rapeseed (B. napus L.) were characterized during seed development with respect to sensitivity to abiotic stress (reduced water potential) and shelf life. Using these models our data suggests that the physiological patterns of seed development are the same in all brassica crops we have tested to date. These population-based models can be used to provide a biological basis in which to evaluate cultural, postharvest and storage practices to ensure the production and maintenance of seed vigor.

HortScience

Brassica crops have an extended flowering period due to both progressive development within a giv... more Brassica crops have an extended flowering period due to both progressive development within a given raceme and variability among multiple racemes. Early harvest can result in poor seed quality due to immaturity, while delayed harvest may sacrifice yield due to shattering. To characterize the development of seed quality, we measured maturity indices and conducted vigor tests on hybrid red cabbage (Brassica oleracea var capitata) seed harvested weekly starting 33 days after full bloom (DAF). Viability and germination rate increased from the top to the bottom of the raceme, and were maximal by 40 and 48 DAF, respectively. After 48 DAF, there was little difference in seed quality due to position on the raceme. Seed dry weight also reached a plateau by 48 DAF, when rapid dehydration began. Sensitivity of germination to inhibition by reduced water potential or abscisic acid (ABA) was assessed using a threshold model based upon germination rates. Germination became less sensitive to both f...

Tissue Culture in Forestry and Agriculture, 1985

Annals of botany, 2005

Echinacea angustifolia is a widespread species distributed throughout the Great Plains region of ... more Echinacea angustifolia is a widespread species distributed throughout the Great Plains region of North America. Genetic differentiation among populations was investigated along a 1500 km north-south climatic gradient in North America, a region with no major geographical barriers. The objective of the study was to determine if genetic differentiation of populations could be explained by an isolation-by-distance model or by associations with climatic parameters known to affect plant growth and survival. Historical climatic data were used to define the nature of the climatic gradient and AFLP markers were used to establish patterns of population genetic differentiation among ten Echinacea populations collected from North Dakota to Oklahoma. A total of 1290 fragments were scored using six EcoRI/MseI and three PstI/MseI primer combinations. Assessment of the correlation between climatic, genetic and geographic distances was assessed by Mantel and partial Mantel tests. PstI/MseI combinati...

... Trickle irrigation of field-grown stock based on net evaporation. HortScience, 19: 304-306. S... more ... Trickle irrigation of field-grown stock based on net evaporation. HortScience, 19: 304-306. Still, DW and Davies, Jr., FT, 1992. ... Plant Soil, 120: 79-85. Zajicek, JM and Heilman, JL, 1991. Transpiration by crape myrtle cultivars surrounded by mulch, soil and turfgrass surfaces. ...

Multidrug resistance is a major problem in small-cell lung cancer (SCLC). Ganoderma lucidum is a ... more Multidrug resistance is a major problem in small-cell lung cancer (SCLC). Ganoderma lucidum is a widely used herb in traditional Chinese medicine. We tested the effects of Ganoderma on drug-sensitive (H69) and multi-drug resistant (VPA) human SCLC cells. Both cells showed equal cytotoxicity when incubated with extracts of mycelia of 9 species of Ganoderma, including G. lucidum. Cells treated with the IC(50) of cytotoxic Ganoderma and analyzed by flow cytometry-PI staining showed increases in S phase. When compared untreated controls or SCLC cells treated with extracts of non-cytotoxic Ganoderma species, cells treated with extracts of cytotoxic Ganoderma species responded with an induction of apoptosis similar to cells treated with the chemotherapeutic drugs etoposide and doxorubicin. This was shown by four criteria: increased DNA fragmentation within cells as measured by ELISA; increased TUNEL staining for DNA breaks; increased specific activities of caspases 3 and 9, but not caspase 8 by colorimetric assays, indicating the endogenous pathway; and similar patterns changes in the expressions of 9 genes involved in the cell cycle/apoptosis, as measured by RT-PCR and capillary electrophoresis. Pre-incubation of drug-resistant SCLC cells with cytotoxic Ganoderma reduced the IC(50) for etoposide (3.4-0.21 microM) and doxorubicin (0.19-0.04 microM). These results show that extracts of several species of Ganoderma are cytotoxic to both drug-sensitive and drug-resistant SCLC cells, are pro-apoptotic, induce gene expression patterns that are similar to SCLC cells treated with chemotherapeutic drugs, and can reverse resistance to chemotherapeutic drugs.

Over evolutionary time, plants adapt to changing environments through genetic variation. Short te... more Over evolutionary time, plants adapt to changing environments through genetic variation. Short term adaptation to environmental variability occurs through modulating gene expression leading to developmental and morphological changes. Although phenotypic plasticity may be adaptive to natural populations, maintaining a stable phenotype for many agronomic and horticultural traits across environments is a key, but elusive, breeding goal. To examine the genetic architecture and molecular mechanisms underlying trait stability, a lettuce recombinant inbred line (RIL) population was grown in multiple environments over several years and locations to observe genotype x environment interactions. The phenotypes of the RIL population were obtained for genetically complex traits including seed germination, antioxidant and vitamin content of leaves. Strikingly different germination capacity and nutritional content for a given RIL family across production years was observed. Assessment of genes enc...