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Plants defend themselves against their enemies with an impressive arsenal of physical barriers, surveillance and defense proteins, enzymes, and toxic chemicals. Many different molecules are involved in the detection of invaders, suggesting that pathogen pressure selects for a broad array of defense strategies and a high diversity of recognition specificities in host species. Recent results in plants, however, show that immune system diversification can also have negative consequences; epistatic interactions among divergent immune system components can cause hybrid necrosis, a form of genetic incompatibility. This type of hybrid failure is frequently lethal, and characterized by the widespread induction of programmed cell death leading to tissue necrosis. In characterized examples, this is caused by hyperactivation of defense responses. Both the prevalence of hybrid necrosis in diverse plant taxa, and the growing indication that it may arise as a by-product of adaptation to the biotic environment, emphasize that it is likely a general factor in plant evolution. Since hybrid necrosis negatively impacts the progeny of certain crosses, divergence of the plant immune system may indirectly affect gene flow among populations, and perhaps contribute to the establishment or maintenance of species barriers. Key words: hybrid necrosis, incompatibility, plant autoimmunity, programmed cell death, defense, speciation necrosis as a general phenomenon. Les plantes se defendent elles-memes contre leurs ennemis grace a un impressionnant arsenal de barrieres physiques, de proteines de surveillance et de defense, d'enzymes et de toxines chimiques. Un grand nombre de molecules interviennent dans la detection des envahisseurs, suggerant que la pression de ces organismes pathogenes selectionne un large ensemble de strategies de defense et une forte diversite de specificites de reconnaissance chez les especes hotes. On a cependant recemment observe chez les plantes que la diversification du systeme immunitaire peut egalement avoir des consequences negatives; les interactions epistatiques entre composantes de systemes immunitaires divergents peuvent entraener la necrose d'hybrides, une forme d'incompatibilite genetique. Ce type d'echec des hybrides s'avere frequemment letal et s'accompagne par une large induction de mort cellulaire programmee conduisant a la necrose. Dans des exemples precis, ceci est cause par l'hyperactivation des reactions de defense. La prevalence de la necrose d'hybride dans divers taxons vegetaux aussi bien que l'indication croissante qu'elle decoule comme sous-produit de l'adaptation a l'environnement biotique, souligne qu'il s'agit vraisemblablement d'un facteur general d'evolution des plantes. Puisque la necrose des hybrides exerce des impacts negatifs sur la progeniture de certains croisements, la divergence du systeme immunitaire vegetal peut indirectement affecter le flux des genes dans les populations, et possiblement contribuer a l'arrivee ou au maintien de barrieres specifiques. Mots-cles : necrose des hybrides, incompatibilite, auto-immunite vegetale, mort cellulaire programmee, defense, speciation. [Traduit par la Redaction], Introduction Hybrid necrosis is a post-mating genetic incompatibility characterized by dwarfism, tissue necrosis, and sometimes lethality (see Bomblies and Weigel 2007). That particular combinations of normal parents can give rise [...]

Phenotypic variation on which selection can act during evolution may be caused by variation in activity level of developmental regulatory genes. In many cases, however, such genes affect multiple traits. This situation can lead to co-evolution of traits, or evolutionary constraint if some pleiotropic effects are detrimental. Here, we present an analysis of quantitative traits associated with gene copy number of two important maize regulatory genes, the duplicate FLORICAULA/LEAFY orthologs zfl1 and zfl2. We found statistically significant associations between several quantitative traits and copy number of both zf1 genes in several maize genetic backgrounds. Despite overlap in traits associated with these duplicate genes, zfl1 showed stronger associations with flowering time, while zfl2 associated more strongly with branching and inflorescence structure traits, suggesting some divergence of function. Since zfl2 associates with quantitative variation for ear rank and also maps near a quantitative trait locus (QTL) on chromosome 2 controlling ear rank differences between maize and teosinte, we tested whether zfl2 might have been involved in the evolution of this trait using a QTL complementation test. The results suggest that zfl2 activity is important for the QTL effect, supporting zfl2 as a candidate gene for a role in morphological evolution of maize.

Author(s): Huai Wang [1, 5]; Tina Nussbaum-Wagler [1, 5]; Bailin Li [2]; Qiong Zhao [1]; Yves Vigouroux [1, 4]; Marianna Faller [2]; Kirsten Bomblies [1]; Lewis Lukens [3]; John F. [...]

Abstract Background Recent studies have shown that plant genomes have potentially undergone rampant horizontal gene transfer (HGT). In plant parasitic systems HGT appears to be facilitated by the intimate physical association between the parasite and its host. HGT in these systems has been invoked when a DNA sequence obtained from a parasite is placed phylogenetically very near to its host rather than with its closest relatives. Studies of HGT in parasitic plants have relied largely on the fortuitous discovery of gene phylogenies that indicate HGT, and no broad systematic search for HGT has been undertaken in parasitic systems where it is most expected to occur. Results We analyzed the transcriptomes of the holoparasite Rafflesia cantleyi Solms-Laubach and its obligate host Tetrastigma rafflesiae Miq. using phylogenomic approaches. Our analyses show that several dozen actively transcribed genes, most of which appear to be encoded in the nuclear genome, are likely of host origin. We also find that hundreds of vertically inherited genes (VGT) in this parasitic plant exhibit codon usage properties that are more similar to its host than to its closest relatives. Conclusions Our results establish for the first time a substantive number of HGTs in a plant host-parasite system. The elevated rate of unidirectional host-to- parasite gene transfer raises the possibility that HGTs may provide a fitness benefit to Rafflesia for maintaining these genes. Finally, a similar convergence in codon usage of VGTs has been shown in microbes with high HGT rates, which may help to explain the increase of HGTs in these parasitic plants.

Summary: S‐Acylation is a reversible post‐translational lipid modification in which a long chain fatty acid covalently attaches to specific cysteine(s) of proteins via a thioester bond. It enhances the hydrophobicity of proteins, contributes to their membrane association and plays roles in protein trafficking, stability and signalling. A family of Protein S‐Acyl Transferases (PATs) is responsible for this reaction. PATs are multi‐pass transmembrane proteins that possess a catalytic Asp−His−His−Cys cysteine‐rich domain (DHHC‐CRD). In Arabidopsis, there are currently 24 such PATs, five having been characterized, revealing their important roles in growth, development, senescence and stress responses. Here, we report the functional characterization of another PAT, AtPAT21, demonstrating the roles it plays in Arabidopsis sexual reproduction. Loss‐of‐function mutation by T‐DNA insertion in AtPAT21 results in the complete failure of seed production. Detailed studies revealed that the sterility of the mutant is caused by defects in both male and female sporogenesis and gametogenesis. To determine if the sterility observed in atpat21‐1 was caused by upstream defects in meiosis, we assessed meiotic progression in pollen mother cells and found massive chromosome fragmentation and the absence of synapsis in the initial stages of meiosis. Interestingly, the fragmentation phenotype was substantially reduced in atpat21‐1 spo11‐1 double mutants, indicating that AtPAT21 is required for repair, but not for the formation, of SPO11‐induced meiotic DNA double‐stranded breaks (DSBs) in Arabidopsis. Our data highlight the importance of protein S‐acylation in the early meiotic stages that lead to the development of male and female sporophytic reproductive structures and associated gametophytes in Arabidopsis. Significance Statement: This study highlights the importance of protein S‐acylation in the early meiotic stages that lead to the development of male and female sporophytic reproductive structures and associated gametophytes in Arabidopsis. [ABSTRACT FROM AUTHOR]

The floral homeotic gene AGAMOUS specifies stamen and carpel fate in the central whorls of Arabidopsis flowers. Transcription of AGAMOUS RNA is restricted to the center of developing flowers by several, partially redundant negative regulators, one of which is the homeotic gene APETALA2. We have identified regulatory elements that mediate transcriptional repression of AGAMOUS by APETALA2 and found that several redundant elements respond independently to loss of APETALA2 activity. Thus, redundancy at the level of cis-regulatory sequences is independent of redundancy at the level of trans-regulators. We have also found that only the early, but not the late, effects of APETALA2 on AGAMOUS require the meristem-identity protein LEAFY, a positive regulator of AGAMOUS. [C] 1999 Academic Press Key Words: Arabidopsis; flower development; homeotic genes; AGAMOUS; APETALA2; LEAFY.