SUMMARYLegume root architecture involves not only elaboration of the root system by the formation of lateral roots but also the formation of symbiotic root nodules in association with nitrogen-fixing soil rhizobia. The Medicago truncatula LATD/NIP gene plays an essential role in the development of both primary and lateral roots as well as nodule development. We have cloned the LATD/NIP gene and show that it encodes a member of the NRT1(PTR) transporter family. LATD/NIP is expressed throughout the plant. pLATD/NIP-GFP promoterreporter fusions in transgenic roots establish the spatial expression of LATD/NIP in primary root, lateral root and nodule meristems and the surrounding cells. Expression of LATD/NIP is regulated by hormones, in particular by abscisic acid which has been previously shown to rescue the primary and lateral root meristem arrest of latd mutants. latd mutants respond normally to ammonium but have defects in responses of the root architecture to nitrate. Taken together, these results suggest that LATD/NIP may encode a nitrate transporter or transporter of another compound.

Recent in planta studies have shown that strains Fny and LS of Cucumber mosaic virus (CMV) display differential genetic diversities, Fny and LS having higher and lower mutation frequencies, respectively (J. S. Pita and M. J. Roossinck, J Virol 87:790 -797, 2012 http://dx.doi.org/10.1128/JVI.01891-12). In this article, we show that these virus strains have differential recombination frequencies as well. However, the high-diversity Fny strain is a low-recombination virus, whereas the very-low-diversity LS strain is instead a high-recombination virus. Unlike the mutation frequency that was determined by both RNAs 1 and 2, the control elements of recombination frequency reside predominantly within RNA 2, specifically within the 2a gene. IMPORTANCERecombination is an important mechanism in virus evolution that can lead to increased or decreased variation and is a major player in virus speciation events that can lead to emerging viruses. Although viral genomes show very frequent evidence of recombination, details of the mechanism involved in these events are still poorly understood. We show here that the reciprocal effects of high mutation frequency and low recombination frequency (and vice versa) involve the RNA-dependent RNA polymerase of the virus, and we speculate that these evolutionary events are related to differences in processivity for two strains of the same virus. G enetic recombination is the formation of chimeric molecules from segments previously separated on the same molecule or present on different parental molecules (1). RNA recombination is one of the major forces increasing diversity of RNA viruses. It has been well studied for many plant RNA viruses under natural or experimental conditions (for a recent review, see reference 2). In spite of extensive use of Cucumber mosaic virus (CMV) as a model system for virus evolution studies, not much about the role of recombination in the life cycle of CMV is known, other than a propensity for exchanges in the 3= end in reassorted viruses (3,4). CMV (genus Cucumovirus; family Bromoviridae) is an extremely successful virus that infects plants all around the world. The variety of available strains of CMV and the divided genome of the virus provide a useful and convenient system to study many general principles of virus evolution and ecology. CMV RNA 1 encodes the methyltransferase and helicase domains (1a), and RNA 2 encodes the polymerase (2a), together forming the viral components of the replicase complex (5, 6). RNA 2 also encodes the 2b protein (7) involved in RNA silencing suppression, systemic spread, and symptom production, and RNA 3 encodes the 3a (movement) and coat proteins (8, 9). Previously, we used the CMV system to elucidate the forces behind RNA virus population diversity. We identified the genes that are related to the different population diversity levels in different strains of CMV, and we mapped the regions that are associated with high and low levels of diversity (10). We used the same system to understand the mechanisms underlying t...