Within the resulting organelle, called the symbiosome, the bacteria multiply and ultimately differentiate into their nitrogen-fixing ‘bacteroid’ state. Bacteria are then released from infection threads into the cytoplasm of cortical cells via endocytosis, which leaves the bacteria surrounded by a host membrane called the symbiosome membrane ( Udvardi and Day, 1997). Concomitantly, dividing cortical cells form a nodule primordium inside which a new meristem is initiated that drives nodule organogenesis.
Rhizobia gain entry into cortical cells of developing nodules via an infection thread, which traverses the epidermal root hair cell that makes first contact with the bacteria, and subsequently ramifies throughout the cortical tissue. Symbiotic nitrogen fixation (SNF) in legumes takes place in specialized organs called nodules which develop from root cortical cells that start dividing following signal exchanges between the plant roots and rhizobia in the soil ( Brewin, 1991 Long, 2001 Oldroyd and Downie, 2004). It is estimated that 40–60 million tonnes of N are fixed annually by cultivated legumes ( Smil, 1999), saving about US$10 billion on nitrogen fertilizer ( Graham and Vance, 2003).
#Civ 5 utils atlas fonts free
Legumes account for one-third of the world’s primary crop production and are key to sustainable agriculture because they can ‘fix’ nitrogen (reduce N 2 to NH 3) in a symbiotic association with bacteria called rhizobia, providing crops with a free and renewable source of nitrogen. Legumes (family Fabaceae) are second only to grasses (Gramineae) in importance to humans as a source of food, feed for livestock, and raw materials for industry ( Graham and Vance, 2003). The data presented here represent an unparalleled resource for legume functional genomics, which will accelerate discoveries in legume biology. Comparative transcriptome analysis of Medicago versus Arabidopsis revealed significant divergence in developmental expression profiles of orthologous genes, which indicates that phylogenetic analysis alone is insufficient to predict the function of orthologs in different species. Interestingly, many legume-specific genes are preferentially expressed in nitrogen-fixing nodules, indicating that evolution endowed them with special roles in this unique and important organ. The atlas reveals massive differences in gene expression between organs that are accompanied by changes in the expression of key regulatory genes, such as transcription factor genes, which presumably orchestrate genetic reprogramming during development and differentiation. To enhance its value as a model, we have generated a gene expression atlas that provides a global view of gene expression in all major organ systems of this species, with special emphasis on nodule and seed development. Therefore, Medicago truncatula, which has a relatively small diploid genome, has been adopted as a model species for legume genomics. Unfortunately, most cultivated legumes are poor model systems for genomic research. Legumes played central roles in the development of agriculture and civilization, and today account for approximately one-third of the world’s primary crop production.
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