Intergeneric transfer and functional expression of the tomato disease resistance gene Pto.

Plant disease resistance loci have been used successfully in breeding programs to transfer traits from resistant germplasm to susceptible plant cultivars. The molecular cloning of plant disease resistance genes now permits the transfer of such traits across species boundaries by genetic transformation of recipient hosts. The tomato disease resistance gene Pto confers resistance to strains of the bacterial pathogen Pseudomonas syringae pv tomato expressing the avirulence gene avrPto. Transformation of Nicotiana benthamiana with Pto results in specific resistance to P. s. pv tabaci strains carrying avrPto. The resistant phenotype is manifested by a strong inhibition of bacterial growth and the ability to exhibit a hypersensitive response. Resistance cosegregates with the Pto gene in transgene selfings and testcrosses. Our results demonstrate the conservation of disease resistance functions across genus boundaries and suggest that the utility of host-specific resistance genes can be extended by intergeneric transfer.     

Calcium/Calmodulin-Dependent Protein Kinase Is Negatively and Positively Regulated by Calcium,Providing a Mechanism for Decoding Calcium Responses during Symbiosis Signaling

The establishment of symbiotic associations in plants requires calcium oscillations that must be decoded to invoke downstream developmental programs. In animal systems, comparable calcium oscillations are decoded by calmodulin (CaM)–dependent protein kinases, but symbiotic signaling involves a calcium/CaM–dependent protein kinase (CCaMK) that is unique to plants. CCaMK differs from the animal CaM kinases by its dual ability to bind free calcium, via calcium binding EF-hand domains on the protein, or to bind calcium complexed with CaM, via a CaM binding domain. In this study, we dissect this dual regulation of CCaMK by calcium. We find that calcium binding to the EF-hand domains promotes autophosphorylation, which negatively regulates CCaMK by stabilizing the inactive state of the protein. By contrast, calcium-dependent CaM binding overrides the effects of autophosphorylation and activates the protein. The differential calcium binding affinities of the EF-hand domains compared with those of CaM suggest that CCaMK is maintained in the inactive state at basal calcium concentrations and is activated via CaM binding during calcium oscillations. This work provides a model for decoding calcium oscillations that uses differential calcium binding affinities to create a robust molecular switch that is responsive to calcium concentrations associated with both the basal state and with oscillations.     

Strikingly high levels of heterozygosity despite 20 years of inbreeding in a clonal honey bee

Inbreeding (the mating between closely related individuals) often has detrimental effects that are associated with loss of heterozygosity at overdominant loci, and the expression of deleterious recessive alleles. However, determining which loci are detrimental when homozygous, and the extent of their phenotypic effects, remains poorly understood. Here, we utilize a unique inbred population of clonal (thelytokous) honey bees, Apis mellifera capensis, to determine which loci reduce individual fitness when homozygous. This asexual population arose from a single worker ancestor approximately 20 years ago and has persisted for at least 100 generations. Thelytokous parthenogenesis results in a 1/3 of loss of heterozygosity with each generation. Yet, this population retains heterozygosity throughout its genome due to selection against homozygotes. Deep sequencing of one bee from each of the three known sub‐lineages of the population revealed that 3,766 of 10,884 genes (34%) have retained heterozygosity across all sub‐lineages, suggesting that these genes have heterozygote advantage. The maintenance of heterozygosity in the same genes and genomic regions in all three sub‐lineages suggests that nearly every chromosome carries genes that show sufficient heterozygote advantage to be selectively detrimental when homozygous.     

Nest architecture and genetic differentiation in a species complex of Australian stingless bees

We investigated the taxonomic significance of nest shape and its putative role in speciation in Trigona (Heterotrigona) carbonaria and T. (H.) hockingsi, two sibling species of stingless bee species from eastern Australia. These species are primarily distinguished by their nest architecture, as in all other respects they are nearly identical. We genotyped 130 colonies from six locations in Queensland at 13 microsatellite loci together with 106 additional colonies from six other Indo-Pacific Trigona species. Whether they were present in allopatry or in sympatry, colonies that displayed the T. carbonaria or the T. hockingsi nest architecture could be unambiguously differentiated at the genetic level. However, T. hockingsi colonies were classifiable into two highly differentiated paraphyletic and geographically separate populations, one in northern and one in southern Queensland. These two populations probably belong to two distinct species, T. hockingsi and T. davenporti nov. sp. Our results suggest that nest architecture characters are relevant but not sufficient criteria to identify species in this group. Consequently, modifications of nest architecture are probably not of prime importance in the speciation process of Australian stingless bees, although nest architecture differences probably result from relatively simple mechanisms. The rare interspecific hybrid colonies detected did not display a nest with an intermediate form between T. hockingsi and T. carbonaria.     

Ethylene inhibits the Nod factor signal transduction pathway of Medicago truncatula

Legumes form a mutualistic symbiosis with bacteria collectively referred to as rhizobia. The bacteria induce the formation of nodules on the roots of the appropriate host plant, and this process requires the bacterial signaling molecule Nod factor. Although the interaction is beneficial to the plant, the number of nodules is tightly regulated. The gaseous plant hormone ethylene has been shown to be involved in the regulation of nodule number. The mechanism of the ethylene inhibition on nodulation is unclear, and the position at which ethylene acts in this complex developmental process is unknown. Here, we used direct and indirect ethylene application and inhibition of ethylene biosynthesis, together with comparison of wild-type plants and an ethylene-insensitive supernodulating mutant, to assess the effect of ethylene at multiple stages of this interaction in the model legume Medicago truncatula. We show that ethylene inhibited all of the early plant responses tested, including the initiation of calcium spiking. This finding suggests that ethylene acts upstream or at the point of calcium spiking in the Nod factor signal transduction pathway, either directly or through feedback from ethylene effects on downstream events. Furthermore, ethylene appears to regulate the frequency of calcium spiking, suggesting that it can modulate both the degree and the nature of Nod factor pathway activation.     

Queen mating frequency and maternity of males in the stingless bee Trigona carbonaria Smith

Summary. Aspects of the sociobiology of the native Australian stingless bee Trigona carbonaria (Hymenoptera, Meliponini) were investigated using highly variable microsatellite markers. The queen mating frequency was estimated by examining genotypes of samples of workers from five colonies across three to five microsatellite loci. In each case, results were consistent with the workers being progeny of a queen mated with a single male. Microsatellite analysis of haploid males from two of the colonies suggested they predominantly arose from queen-laid eggs. As workers are more related to sons-of-workers than to sons-of-queens in monandrous colonies, this is somewhat surprising, and suggests that there may be colony-level costs associated with worker reproduction, or that queens are able to regulate worker reproduction via the ritualized cell provisioning and oviposition process. Males from another T. carbonaria colony were found to be diploid.     

The NFP locus of Medicago truncatula controls an early step of Nod factor signal transduction upstream of a rapid calcium flux and root hair deformation

Establishment of the Rhizobium-legume symbiosis depends on a molecular dialogue, in which rhizobial nodulation (Nod) factors act as symbiotic signals, playing a key role in the control of specificity of infection and nodule formation. Using nodulation-defective (Nod-) mutants of Medicago truncatula to study the mechanisms controlling Nod factor perception and signalling, we have previously identified five genes that control components of a Nod factor-activated signal transduction pathway. Characterisation of a new M. truncatula Nod- mutant led to the identification of the Nod Factor Perception (NFP) locus. The nfp mutant has a novel phenotype among Nod- mutants of M. truncatula, as it does not respond to Nod factors by any of the responses tested. The nfp mutant thus shows no rapid calcium flux, the earliest detectable Nod factor response of wild-type plants, and no root hair deformation. The nfp mutant is also deficient in Nod factor-induced calcium spiking and early nodulin gene expression. While certain genes controlling Nod factor signal transduction also control the establishment of an arbuscular mycorrhizal symbiosis, the nfp mutant shows a wild-type mycorrhizal phenotype. These data indicate that the NFP locus controls an early step of Nod factor signal transduction, upstream of previously identified genes and specific to nodulation.