Layer-specific programs of development in neocortical projection neurons.

How are long-range axonal projections from the cerebral cortex orchestrated during development? By using both passively and actively transported axonal tracers in fetal and postnatal ferrets, we have analyzed the development of projections from the cortex to a number of thalamic nuclei. We report that the projections of a cortical area to its corresponding thalamic nuclei follow highly cell-specific programs of development. Axons from cells in the deepest layers of the cerebral cortex (layer 6 and superficial subplate neurons) appear to grow very slowly and be delayed for several weeks in the cerebral white matter, reaching the thalamus over a protracted period. Neurons of layer 5, on the other hand, develop their projections much faster; despite being born after the neurons of deeper layers, layer 5 neurons are the first to extend their axons out of the cortical hemisphere and innervate the thalamus. Layer 5 projections are massive in the first postnatal weeks but may become partly eliminated later in development, being overtaken in number by layer 6 cells that constitute the major corticothalamic projection by adulthood. Layer 5 projections are area-specific from the outset and arise as collateral branches of axons directed to the brainstem and spinal cord. Our findings show that the early development of corticofugal connections is determined not by the sequence of cortical neurogenesis but by developmental programs specific for each type of projection neuron. In addition, they demonstrate that in most thalamic nuclei, layer 5 neurons (and not subplate or layer 6 neurons) establish the first descending projections from the cerebral cortex.

Isolation of Ethylene-Insensitive Soybean Mutants That Are Altered in Pathogen Susceptibility and Gene-for-Gene Disease Resistance1

Plants commonly respond to pathogen infection by increasing ethylene production, but it is not clear if this ethylene does more to promote disease susceptibility or disease resistance. Ethylene production and/or responsiveness can be altered by genetic manipulation. The present study used mutagenesis to identify soybean (Glycine max L. Merr.) lines with reduced sensitivity to ethylene. Two new genetic loci were identified, Etr1 and Etr2. Mutants were compared with isogenic wild-type parents for their response to different soybean pathogens. Plant lines with reduced ethylene sensitivity developed similar or less-severe disease symptoms in response to virulent Pseudomonas syringae pv glycinea and Phytophthora sojae, but some of the mutants developed similar or more-severe symptoms in response to Septoria glycines and Rhizoctonia solani. Gene-for-gene resistance against P. syringae expressing avrRpt2 remained effective, but Rps1-k-mediated resistance against P. sojae races 4 and 7 was disrupted in the strong ethylene-insensitive etr1-1 mutant. Rps1-k-mediated resistance against P. sojae race 1 remained effective, suggesting that the Rps1-k locus may encode more than one gene for disease resistance. Overall, our results suggest that reduced ethylene sensitivity can be beneficial against some pathogens but deleterious to resistance against other pathogens.

Manipulation of the phenolic chemistry of willows by gall-inducing sawflies

The ability to induce galls on plants has evolved independently in many insect orders, but the adaptive significance and evolutionary consequences of gall induction are still largely unknown. We studied these questions by analyzing the concentrations of various plant defense compounds in willow leaves and sawfly galls. We found that the galls are probably nutritionally beneficial for the sawfly larvae, because the concentrations of most defensive phenolics are substantially lower in gall interiors than in leaves. More importantly, changes in chemistry occur in a similar coordinated pattern in all studied willow species, which suggests that the insects control the phenolic biosynthesis in their hosts. The resulting convergence of the chemical properties of the galls both within and between host species indicates that the role of plant chemistry in the evolution of host shifts may be fundamentally less significant in gallers than in other phytophagous insects.