Nitrification, a key process in the global nitrogen cycle that generates nitrate through microbial activity, may enhance losses of fertilizer nitrogen by leaching and denitrification. Certain plants can suppress soil-nitrification by releasing inhibitors from roots, a phenomenon termed biological nitrification inhibition (BNI). Here, we report the discovery of an effective nitrification inhibitor in the root-exudates of the tropical forage grass Brachiaria humidicola (Rendle) Schweick. Named ''brachialactone,'' this inhibitor is a recently discovered cyclic diterpene with a unique 5-8-5-membered ring system and a ␥-lactone ring. It contributed 60 -90% of the inhibitory activity released from the roots of this tropical grass. Unlike nitrapyrin (a synthetic nitrification inhibitor), which affects only the ammonia monooxygenase (AMO) pathway, brachialactone appears to block both AMO and hydroxylamine oxidoreductase enzymatic pathways in Nitrosomonas. global warming ͉ nitrogen pollution ͉ nitrous oxide emissions ͉ root exudation ͉ climate change M ost modern agricultural systems are based on large inputs of inorganic nitrogen (N), with ammonium (NH 4 ϩ ) being the primary N source (1, 2). Also, current crop management practices result in the development of highly nitrifying soil environments (3, 4). Nitrification results in the transformation of the relatively immobile NH 4 ϩ to highly mobile nitrate (NO 3 Ϫ ), making inorganic N susceptible to losses through leaching of NO 3 Ϫ and/or gaseous N emissions, potentially initiating a cascade of environmental and health problems (1, 2, 5, 6). Nitrous oxide (N 2 O) is one of the three major biogenic greenhouse gases contributing to global warming, produced primarily from denitrification processes in agricultural systems (5, 7). Also, assimilation of NO 3 Ϫ by plants can result in further N 2 O emissions directly from plant canopies (8). The low agronomic N-use efficiency (NUE) found in many agricultural systems is largely the result of N losses associated with nitrification (i.e., N losses from NO 3 Ϫ leaching and denitrification) (9-11). Most plants have the ability to assimilate both NH 4 ϩ and NO 3 Ϫ (12); therefore, nitrification does not need to be a dominant process in the N cycle for efficient N use.Nitrification is low in some forest and grassland soils (13-17). Since the early 1960s, some tropical grasses have been suspected of having the capacity to inhibit nitrification (18-21). However, this concept remained controversial due to the lack of direct evidence showing such inhibitory effects or the identification of specific inhibitors (22).We adopted a very sensitive bioassay using a recombinant luminescent Nitrosomonas europaea to detect biological nitrification inhibition (BNI) in plant-soil systems with the inhibitory activity of roots expressed in allylthiourea units (ATU) (23). Using this methodology, we were able to show that certain plants release nitrification inhibitors from their roots (23-26). Such BNI capacity appears to be relatively widespread among...

Wheat stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a devastating disease that can cause severe yield losses. A previously uncharacterized Pgt race, designated Ug99, has overcome most of the widely used resistance genes and is threatening major wheat production areas. Here, we demonstrate that the Sr35 gene from Triticum monococcum is a coiled-coil, nucleotide-binding, leucine-rich repeat gene that confers near immunity to Ug99 and related races. This gene is absent in the A-genome diploid donor and in polyploid wheat but is effective when transferred from T. monococcum to polyploid wheat. The cloning of Sr35 opens the door to the use of biotechnological approaches to control this devastating disease and to analyses of the molecular interactions that define the wheat-rust pathosystem.

f. sp. () causes wheat stem rust, a devastating fungal disease. The resistance gene confers immunity against this pathogen's most virulent races, including Ug99. We used comparative whole-genome sequencing of chemically mutagenized and natural isolates to identify a fungal gene named that is required for avirulence. The gene encodes a secreted protein capable of interacting with Sr35 and triggering the immune response. We show that the origin of isolates virulent on is associated with the nonfunctionalization of the gene by the insertion of a mobile element. The discovery of provides a new tool for surveillance, identification of host susceptibility targets, and characterization of the molecular determinants of immunity in wheat.

Background: Cassava, an allotetraploid known for its remarkable tolerance to abiotic stresses is an important source of energy for humans and animals and a raw material for many industrial processes. A full-length cDNA library of cassava plants under normal, heat, drought, aluminum and post harvest physiological deterioration conditions was built; 19968 clones were sequencecharacterized using expressed sequence tags (ESTs).

Due to its favorable agronomic traits, tolerance to abiotic stresses and adverse environments, cassava is the most important source of dietary carbohydrates for 750 million people around the world, and is produced mainly by subsistence farmers in marginally agricultural land. Physiological postharvest deterioration (PPD) of cassava roots is an endogenous and complex process that restricts their storage potential to only a few days after harvest. This physiological phenomenon is one of the main constraints in cassava agriculture with an enormous impact on the cassava market chain. It is estimated that losses due to PPD in cassava production in Latin America and the Caribbean and in Asia reach 10% and 8%, respectively, whereas in Africa they reach 29%. Several years of research have been accumulating evidence to consider PPD as a wounding stress deficient process involving changes in enzymatic activity and oxidative stress. The primary symptoms, the development of dark bluish or brownish radial veins or streaks near xylem vessels of the root pith tissue, appear within 2-3 days after harvest and spread to the neighboring parenchyma tissues producing a more general browning discoloration throughout the root. Secondary post-harvest deterioration, often appears when the roots suffer moderate to severe damage at harvest and is mediated by a wide range of pathogenic microorganisms, Several strategies have been proposed to overcome the problem, but each alternative has its limitations due to the variable results, lack of objective and systematic methodology for PPD evaluation, applications not conducive for use at farmer-level, limited genetic variability or absence of genetic and biochemical information. The present review examines the socioeconomic impact of PPD, the physiological, biochemical and molecular processes occurring in the root during PPD, as well as the current and future alternatives to overcome the problem.