Roots of young soybean (Glycine max (L.) Merr.) seedlings inoculated with Rhizobium japonicum Kirchner USDA 110 ARS were examined in serial sections by light microscopy to ascertain the extent of infection. The location of each infection site was established in relation to the zones of root and root hair development at the time of inoculation. Each infection locus was classified as to its relative state of differentiation using a developmental scale encompassing the first 10 days of nodule development. Both the initiation and maturation of Rhizobium infections were found to be governed by the acropetal development of host root hairs. Regions of the root where mature root hairs were present at the time of inoculation were not susceptible to Rhizobium infection. Infections developed most frequently in root hairs which emerged shortly after inoculation. Many infections formed on the root but relatively few developed into nodules. Most infection loci which formed infection threads stopped developing at stages prior to meristem formation. A high proportion of the infection loci were pseudoinfections, i.e., localized areas of cortical cell division without infection thread formation. The maturation of infections in younger regions of the root was suppressed by prior exposure of older regions of the root to rhizobia. Development was suppressed at stages after meristem formation but before nodule emergence.

Approximately 185,000 Gossypium EST sequences comprising >94,800,000 nucleotides were amassed from 30 cDNA libraries constructed from a variety of tissues and organs under a range of conditions, including drought stress and pathogen challenges. These libraries were derived from allopolyploid cotton (Gossypium hirsutum; AT and DT genomes) as well as its two diploid progenitors, Gossypium arboreum (A genome) and Gossypium raimondii (D genome). ESTs were assembled using the Program for Assembling and Viewing ESTs (PAVE), resulting in 22,030 contigs and 29,077 singletons (51,107 unigenes). Further comparisons among the singletons and contigs led to recognition of 33,665 exemplar sequences that represent a nonredundant set of putative Gossypium genes containing partial or full-length coding regions and usually one or two UTRs. The assembly, along with their UniProt BLASTX hits, GO annotation, and Pfam analysis results, are freely accessible as a public resource for cotton genomics. Because ESTs from diploid and allotetraploid Gossypium were combined in a single assembly, we were in many cases able to bioinformatically distinguish duplicated genes in allotetraploid cotton and assign them to either the A or D genome. The assembly and associated information provide a framework for future investigation of cotton functional and evolutionary genomics.

Mature leaves of Phaseolus vulgaris L. (red kidney bean), Xanthium strumarium L. (cocklebur), and Gossypium hirsutum L. (cotton) were used to study accumulation of abscisic acid (ABA) during water stress. The water status of individual, detached leaves was monitored while the leaves slowly wilted, and samples were cut from the leaves as they lost water. The leaf sections were incubated at their respecitive water contents to allow ABA to build up or not. At least 8 h were required for a new steady-state level of ABA to be established. The samples from any one leaf covered a range of known water potentials (ψ), osmotic pressures (π), and turgor pressures (p). The π and p values were calculated from "pressure-volume curves", using a pressure bomb to measure the water potentials. Decreasing water potential had little effect on ABA levels in leaves at high turgor. Sensitivity of the production of ABA to changes in ψ progressively increased as turgor approached zero. At p=1 bar, ABA content averaged 4 times the level found in fully turgid samples. Below p=1 bar, ABA content increased sharply to as much as 40 times the level found in unstressed samples. ABA levels rose steeply at different water potentials for different leaves, according to the ψ at which turgor became zero. These differences were caused by the different osmotic pressures of the leaves that were used; ψ must cqual -π for turgor to be zero. Leaves vary in π, not only among species, but also between plants of one and the same species depending on the growing conditions. A difference of 6 bars (calculated at ψ=0) was found between the osmotic pressures of leaves from two groups of G. hirsutum plants; one group had previously experienced periodic water stress, and the other group had never been stressed. When individual leaves were subsequently wilted, the leaves from stress-conditioned plants required a lower water potential in order to accumulate ABA than did leaves from previously unstressed plants. On the basis of these results we suggest that turgor is the critical parameter of plant water relations which controls ABA production in water-stressed leaves.

The number of nodules which develop on the primary root of soybean seedlings (Glycine max L. Meff) after inoculation with Rhizobium japonicum is substantially diminished in the region of the root developmentally 10 to 15 hours younger than the region maximally susceptible to nodulation at the time of inoculation. This rapid inhibition ofnodulation has been investipted by inoculating soybean seedlings with rhizobia at two different times, 15 (4). Inocula were prepared from mid-exponential phase cultures by centrifugation, washing with sterile phosphate buffered saline, resuspension in one-tenth-strength phosphate-buffered saline, and dilution with half-strength N-free Jensen's plant growth medium to the desired cell density (4). Approximate cell densities were determined from the absorbance of diluted suspensions at 620 nm (0.03 A620 nm = 1 x 108 cells/ml). To test the effects of dead R. japonicum cells, suspensions of bacteria were diluted to 1 x I05 cells/ml and exposed in a Petri dish to sufficient UV irradiation to kill more than 99.9% of the cells. Aliquots from such suspensions and from unexposed suspensions of the same bacteria were used to inoculate test and control seedlings, respectively.Cultures of the heterologous rhizobia R. leguminosarum (strain 603)

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