Secreted peptide ligands are known to play key roles in the regulation of plant growth, development, and environmental responses. However, phenotypes for surprisingly few such genes have been identified via loss-of-function mutant screens. To begin to understand the processes regulated by the CLAVATA3 (CLV3)/ESR (CLE) ligand gene family, we took a systems approach to gene identification and gain-of-function phenotype screens in transgenic plants. We identified four new CLE family members in the Arabidopsis (Arabidopsis thaliana) genome sequence and determined their relative transcript levels in various organs. Overexpression of CLV3 and the 17 CLE genes we tested resulted in premature mortality and/or developmental timing delays in transgenic Arabidopsis plants. Overexpression of 10 CLE genes and the CLV3 positive control resulted in arrest of growth from the shoot apical meristem (SAM). Overexpression of nearly all the CLE genes and CLV3 resulted in either inhibition or stimulation of root growth. CLE4 expression reversed the SAM proliferation phenotype of a clv3 mutant to one of SAM arrest. Dwarf plants resulted from overexpression of five CLE genes. Overexpression of new family members CLE42 and CLE44 resulted in distinctive shrub-like dwarf plants lacking apical dominance. Our results indicate the capacity for functional redundancy of many of the CLE ligands. Additionally, overexpression phenotypes of various CLE family members suggest roles in organ size regulation, apical dominance, and root growth. Similarities among overexpression phenotypes of many CLE genes correlate with similarities in their CLE domain sequences, suggesting that the CLE domain is responsible for interaction with cognate receptors.
In self-incompatible (SI) plants, the S locus acts to prevent growth of self-pollen and thus promotes outcrossing within the species. lnterspecific crosses between SI and self-compatible (SC) species often show unilateral lncompatibility that follows the S I x SC rule: S I specles reject pollen from SC specles, but the reciproca1 crosses are usually compatible. The general validity of the S I x SC rule suggests a link between S I and interspeclflc pollen rejectlon; however, this link has been questioned because of a number of exceptlons to the rule. To clarlfy the role of the S locus in interspecific pollen rejection, we transformed severa1 Nicotlana species and hybrids with genes encodlng S A~ or SC,O RNase from S I N. alata. Compatibillty phenotypes in the transgenlc plants were tested using pollen from three SC specles showing unilateral incompatibility with N. alata. S RNase was lmpllcated ln rejecting pollen from all three specles. Rejection of N. plumbaginifolia pollen was similar to S allele-speclfic pollen rejection, showing a requirement for both S RNase and other genetic factors from N. alata. In contrast, S RNase-dependent rejectlon of N. glutinosa and N. tabacum pollen proceeded without these additional factors. N. alata also rejects pollen fmm the latter two specles through an S RNaseindependent mechanism. Our results lmplicate the S locus in all three systems, but lt 1s clear that multlple mechanisms contribute to interspecific pollen rejectlon. INTRODUCTIONMany plants have evolved genetically controlled selfincompatibility (SI) systems that promote outcrossing by restricting pollination between closely related individuals of the same species (de Nettancourt, 1977). Mechanisms also exist to restrict pollination between different species, but comparatively little is known about the control of interspecific pollination.As in other solanaceous plants, SI in the genus Nicotiana is controlled by a single multiallelic locus, the S locus (Newbigin et al., 1993). These plants employ a gametophytic SI system in which pollen is rejected if the S allele in the haploid pollen is the same as either S allele in the diploid pistil. S allele-specific pollen rejection occurs as pollen tubes grow through the extracellular matrix of the stylar transmitting tract (Newbigin et al., 1993). The products of the S locus in the style are the To whom correspondence should be addressed.S RNases (McClure et al., 1989). These glycoproteins are very abundant in the extracellular matrix of the transmitting tract and are also expressed in the stigma and in the epidermis of the placenta (Cornish et Anderson et al., 1989;McClure et al., 1993). S RNases are essential for S allele-specific pollen rejection Murfett et al., 1994), and their ribonuclease activity is required for this function . Following incompatible pollinations, RNA in selfpollen tubes is degraded, and this degradation is consistent with a cytotoxic model for pollen rejection (McClure et al., 1990;Gray et al., 1991; Dickinson, 1994). SI is therefore an active process i...
In self-incompatible (SI) plants, the S locus acts to prevent growth of self-pollen and thus promotes outcrossing within the species. Interspecific crosses between SI and self-compatible (SC) species often show unilateral incompatibility that follows the SI x SC rule: SI species reject pollen from SC species, but the reciprocal crosses are usually compatible. The general validity of the SI x SC rule suggests a link between SI and interspecific pollen rejection; however, this link has been questioned because of a number of exceptions to the rule. To clarify the role of the S locus in interspecific pollen rejection, we transformed several Nicotiana species and hybrids with genes encoding SA2 or SC10 RNase from SI N. alata. Compatibility phenotypes in the transgenic plants were tested using pollen from three SC species showing unilateral incompatibility with N. alata. S RNase was implicated in rejecting pollen from all three species. Rejection of N. plumbaginifolia pollen was similar to S allele-specific pollen rejection, showing a requirement for both S RNase and other genetic factors from N. alata. In contrast, S RNase-dependent rejection of N. glutinosa and N. tabacum pollen proceeded without these additional factors. N. alata also rejects pollen from the latter two species through an S RNase-independent mechanism. Our results implicate the S locus in all three systems, but it is clear that multiple mechanisms contribute to interspecific pollen rejection.
Biomass is a prime target for genetic engineering in forestry because increased biomass yield will benefit most downstream applications such as timber, fiber, pulp, paper, and bioenergy production. Transgenesis can increase biomass by improving resource acquisition and product utilization and by enhancing competitive ability for solar energy, water, and mineral nutrients. Transgenes that affect juvenility, winter dormancy, and flowering have been shown to influence biomass as well. Transgenic approaches have increased yield potential by mitigating the adverse effects of prevailing stress factors in the environment. Simultaneous introduction of multiple genes for resistance to various stress factors into trees may help forest trees cope with multiple or changing environments. We propose multi-trait engineering for tree crops, simultaneously deploying multiple independent genes to address a set of genetically uncorrelated traits that are important for crop improvement. This strategy increases the probability of unpredictable (synergistic or detrimental) interactions that may substantially affect the overall phenotype and its long-term performance. The very limited ability to predict the physiological processes that may be impacted by such a strategy requires vigilance and care during implementation. Hence, we recommend close monitoring of the resultant transgenic genotypes in multi-year, multi-location field trials.
We have cloned and sequenced a number of auxin-responsive cDNAs and their corresponding genes from soybean and Arabidopsis. Each of these genes, with the exception of GH2/4, is transcriptionally regulated specifically by auxins within minutes after hormone application. The auxin-responsive mRNAs are induced some 3-60-fold depending on the type of mRNA analysed, the tissue examined, the dose and duration of auxin application, and the manipulation of the organ tested. Some of the mRNAs show rapid turnover kinetics. The mRNAs show distinct patterns of organ-specific, tissue-specific, and developmental-specific expression. The promoters of the auxin-responsive genes have been fused to the E. coli uidA gene which encodes β-glucuronidase (GUS) and transferred into tobacco and/or Arabidopsis via Agrobacterium T-DNA. These promoters and parts of these promoters have been used to follow the expression patterns and auxin-inducibility of the reporter genes in transgenic plants. We are attempting to identify minimal auxin-responsive elements and gravity-responsive elements within these promoters. We have also fused the auxin-inducible promoters to bacterial genes that encode cytokinin and auxin biosynthetic or conjugating enzymes to study the effects of organ, tissue, and developmental-specific expression of cytokinins and auxins on plant growth, development, and physiology.
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