Land use change alters carbon and nitrogen dynamics mediated by fungal functional guilds within soil aggregates.

Land use change is one of the greatest threats to soil biodiversity and ecological functions; however, how such a transition affects soil carbon (C) and nitrogen (N) dynamics driven by fungal communities at the aggregate level remains unclear. Here, we explored the variation in soil C and N pools, specific enzyme activities and fungal communities and functional guilds within three aggregate sizes (megaaggregates, > 2 mm; macroaggregates, 0.25-2 mm; microaggregates, < 0.25 mm) in a natural forest, 12- and 24-year-old rubber monocultures and corresponding agroforestry systems in tropical China. Tropical forest conversion to rubber monocultures generally reduced C and N pools in all aggregates, while agroforestry systems decreased microbial biomass C and N. Carbon- and N-degrading enzyme activities responded differently to forest conversion and were enhanced in agroforestry systems. The levels of C and N pools and their related enzyme activities increased as the aggregate size decreased. Moreover, fungal compositional shifts in dominance from copiotrophic Ascomycota and Basidiomycota (r-strategists) into oligotrophic Zygomycota (K-strategists) were noted following forest conversion, resulting in more pathogenic fungi at the expense of saprotrophic and arbuscular mycorrhizal fungi. Pathogenic fungi were greatly inhibited due to abundant Mortierella after the establishment of 12-year-old agroforestry systems. The diversity of saprotrophic fungi was the highest in microaggregates. Regardless of land use type, aggregate-associated C and N pools, especially DOC, MBC, NO3--N and DON in microaggregates, were interactively mediated by functional guilds of fungi, which was primarily driven by soil pH. These results highlight the importance of fungal functional guilds in determining C and N dynamics at the aggregate level and provide insights into the sustainable management of cash tree plantations.

Functional Genomics in the Study of Metabolic Pathways in Medicago truncatula: An Overview.

In addition to its value as a model system for studies on symbiotic nitrogen fixation, Medicago truncatula has recently become an organism of choice for dissection of complex pathways of secondary metabolism. This work has been driven by two main reasons, both with practical implications. First Medicago species possess a wide range of flavonoid and terpenoid natural products, many of which, for example, the isoflavonoids and triterpene saponins, have important biological activities impacting both plant and animal (including human) health. Second, M. truncatula serves as an excellent model for alfalfa, the world's major forage legume, and forage quality is determined in large part by the concentrations of products of secondary metabolism, particularly lignin and condensed tannins. We here review recent progress in understanding the pathways leading to flavonoids, lignin, and triterpene saponins through utilization of genetic resources in M. truncatula.

A role for leucoanthocyanidin reductase in the extension of proanthocyanidins.

Proanthocyanidins (PAs) are the second most abundant plant polyphenolic compounds after lignin. PAs affect taste, mouth feel and astringency of many fruits, wines and beverages1,2, have been associated with reduced risks of cardiovascular disease, cancer and Alzheimer's disease3-5, can improve nutrition and prevent bloat in ruminant animals6 and enhance soil nitrogen retention7. PAs are oligomers and polymers of flavan-3-ols, primarily (-)-epicatechin and (+)-catechin, but the mechanism by which the monomers polymerize and become insoluble is currently unknown. Leucoanthocyanidin reductase (LAR) has been shown to convert leucocyanidin to (+)-catechin8,9. Here, we report that loss of function of LAR in the model legume Medicago truncatula leads unexpectedly to loss of soluble epicatechin-derived PAs, increased levels of insoluble PAs, and accumulation of 4ß-(S-cysteinyl)-epicatechin, which provides the 4→8 linked extension units during non-enzymatic PA polymerization. LAR converts 4ß-(S-cysteinyl)-epicatechin back to epicatechin, the starter unit in PAs, thereby regulating the relative proportions of starter and extension units and consequently the degree of PA oligomerization.