Robert E. Pruitt

Genetic Evidence for a Long-Range Activity That Directs Pollen Tube Guidance in Arabidopsis.

The fertilization process of plants is governed by different kinds of cell-cell interactions. In higher plants, these interactions are required both for recognition of the pollen grain by the female reproductive system and to direct the growth of the pollen tube inside the ovary. Despite many years of study, the signaling mechanisms that guide the pollen tube toward its target, the ovule, are largely unknown. Two distinct types of principles, mechanical and chemotropic, have been suggested to account for the directed growth of the pollen tube. The first of these two types of models implies that the guidance of the pollen tube depends on the architecture and chemical properties of the female reproductive tissues, whereas the latter suggests that the ovule provides a signal for the target-directed growth of the pollen tube. To examine such a role for the ovules, we analyzed the growth path of pollen tubes in mutants defective in ovule development in Arabidopsis. The results presented here provide unique in vivo evidence for an ovule-derived, long-range activity controlling pollen tube guidance. A morphological comparison of the ovule mutants used in this study indicates that within the ovule, the haploid embryo sac plays an important role in this long-range signaling process.

Genome-wide non-mendelian inheritance of extra-genomic information in Arabidopsis.

A fundamental tenet of classical mendelian genetics is that allelic information is stably inherited from one generation to the next, resulting in predictable segregation patterns of differing alleles. Although several exceptions to this principle are known, all represent specialized cases that are mechanistically restricted to either a limited set of specific genes (for example mating type conversion in yeast) or specific types of alleles (for example alleles containing transposons or repeated sequences). Here we show that Arabidopsis plants homozygous for recessive mutant alleles of the organ fusion gene HOTHEAD (HTH) can inherit allele-specific DNA sequence information that was not present in the chromosomal genome of their parents but was present in previous generations. This previously undescribed process is shown to occur at all DNA sequence polymorphisms examined and therefore seems to be a general mechanism for extra-genomic inheritance of DNA sequence information. We postulate that these genetic restoration events are the result of a template-directed process that makes use of an ancestral RNA-sequence cache.

Epidermal cell interactions: a case for local talk

Epidermal cell interactions play an important role in reproduction, creating variation in floral structure and modulating gametophyte selection and growth during pollination. Recent progress in understanding the processes by which epidermal cells become responsive to contact, indicate that the developmental quiescence of the epidermis is regulated by a set of genes, some of which modulate the permeability properties of the cell wall and cuticle. Research on mutants of Arabidopsis has revealed a mechanistic uniformity between two processes involving epidermis-mediated interactions that had hitherto been considered to be biologically distinct: postgenital organ fusion and pollen hydration.

Plant genetics: a decade of integration

The last decade provided the plant science community with the complete genome sequence of Arabidopsis thaliana and rice, tools to investigate the function of potentially every plant gene, methods to dissect virtually any aspect of the plant life cycle, and a wealth of information on gene expression and protein function. Focusing on Arabidopsis as a model system has led to an integration of the plant sciences that triggered the development of new technologies and concepts benefiting plant research in general. These enormous changes led to an unprecedented increase in our understanding of the genetic basis and molecular mechanisms of developmental, physiological and biochemical processes, some of which will be discussed in this article.

Identification of the Cellular Location of Internalized Escherichia coli O157:H7 in Mung Bean, Vigna radiata, by Immunocytochemical Techniques

Escherichia coli O157:H7 has been associated with numerous outbreaks involving fresh produce. Previous studies have shown that bacteria can be internalized within plant tissue and that this can be a source of protection from antimicrobial chemicals and environmental conditions. However, the types of tissue and cellular locations the bacteria occupy in the plant following internalization have not been addressed. In this study, immunocytochemical techniques were used to localize internalized E. coli O157:H7 expressing green fluorescent protein in germinated mung bean (Vigna radiata) hypocotyl tissue following contamination of intact seeds. An average of 13 bacteria per mm(3) were localized within the sampled tissue. The bacteria were found to be associated with every major tissue and corresponding cell type (cortex, phloem, xylem, epidermis, and pith). The cortical cells located on the outside of the vascular bundles contained the majority of the internalized bacteria (61%). In addition, the bacteria were localized primarily to the spaces between the cells (apoplast) and not within the cells. Growth experiments were also performed and demonstrated that mung bean plants could support the replication of bacteria to high levels (10(7) CFU per plant) following seed contamination and that these levels could be sustained over a 12-day period. Therefore, E. coli O157:H7 can be internalized in many different plant tissue types after a brief seed contamination event, and the bacteria are able to grow and persist within the plant.

Complex sexual signals for the male gametophyte

The pollen grain and the pollen tube that grows from it are complex entities which must respond to a diverse array of signals to carry out their roles in sexual reproduction. Research is beginning to reveal the nature both of the signals and of the signal transduction machinery that converts these signals into directional, polarized growth.

Increased outcrossing in hothead mutants (Reply)

Arising from: S. J. Lolle, J. L. Victor, J. M. Young & R. E. Pruitt 434, 505–509 (2005).The results obtained by Peng et al. are consistent with an increased amount of outcrossing in hth mutants of Arabidopsis thaliana. Some of the results, such as the acquisition of a novel transgene, would be difficult to explain by any other mechanism. Outcrossing was a possibility that we thoroughly explored early on in our investigation, but we discounted it as an explanation because it was inconsistent with many of our experimental results.

Movement of Salmonella serovar Typhimurium and E. coli O157:H7 to Ripe Tomato Fruit Following Various Routes of Contamination

Salmonella serovars have been associated with the majority of foodborne illness outbreaks involving tomatoes, and E. coli O157:H7 has caused outbreaks involving other fresh produce. Contamination by both pathogens has been thought to originate from all points of the growing and distribution process. To determine if Salmonella serovar Typhimurium and E. coli O157:H7 could move to the mature tomato fruit of different tomato cultivars following contamination, three different contamination scenarios (seed, leaf, and soil) were examined. Following contamination, each cultivar appeared to respond differently to the presence of the pathogens, with most producing few fruit and having overall poor health. The Micro-Tom cultivar, however, produced relatively more fruit and E. coli O157:H7 was detected in the ripe tomatoes for both the seed- and leaf- contaminated plants, but not following soil contamination. The Roma cultivar produced fewer fruit, but was the only cultivar in which E. coli O157:H7 was detected via all three routes of contamination. Only two of the five cultivars produced tomatoes following seed-, leaf-, and soil- contamination with Salmonella Typhimurium, and no Salmonella was found in any of the tomatoes. Together these results show that different tomato cultivars respond differently to the presence of a human pathogen, and for E. coli O157:H7, in particular, tomato plants that are either contaminated as seeds or have a natural opening or a wound, that allows bacteria to enter the leaves can result in plants that have the potential to produce tomatoes that harbor internalized pathogenic bacteria.

Plant genetics: Hothead healer and extragenomic information (reply)

Chaudhury and Ray propose alternative models to account for our observed pattern of non-mendelian inheritance in the hothead (hth) mutant of Arabidopsis.

18 From Pollination to Fertilization in Arabidopsis

The life cycle of plants comprises two alternating forms of the organism, one which is diploid (the sporophyte) and one which is haploid (the gametophyte). In higher plants, the gametophytic form of the organism has been greatly reduced. On the male side it includes the pollen grain, the pollen tube grown from it, and the two sperm cells that migrate down the pollen tube to participate in fertilization. The female gametophyte is usually a seven-celled structure, the embryo sac, embedded within the sporophytic tissue of the ovule. Although the size and developmental complexity of these gametophytes have been greatly reduced relative to the lower vascular plants, the function remains the same: to bring about the union of haploid gametes to form a sporophytic zygote and thus reinitiate the cycle. The term fertilization is usually defined as the final events of this process, that is, gamete and nuclear fusion. We define the term “fertilization process” to include all interactions between the male gametophyte and the sporophytic or gametophytic female tissues necessary to achieve the successful production of a zygote. During the fertilization process, the male gametophyte must grow and develop in a precise manner to deliver the sperm cells to the embryo sac. The process begins with the arrival of the pollen grain on the surface of the stigma (pollination), where it undergoes hydration and germinates to grow a pollen tube. This tube penetrates the surface of the stigmatic cell and grows basally toward, and ultimately into, the ovary. During the...