Brown midrib2 (Bmr2) encodes the major 4-coumarate:coenzyme A ligase involved in lignin biosynthesis in sorghum (Sorghum bicolor (L.) Moench)

Successful modification of plant cell-wall composition without compromising plant integrity is dependent on being able to modify the expression of specific genes, but this can be very challenging when the target genes are members of multigene families. 4-coumarate:CoA ligase (4CL) catalyzes the formation of 4-coumaroyl CoA, a precursor of both flavonoids and monolignols, and is an attractive target for transgenic down-regulation aimed at improving agro-industrial properties. Inconsistent phenotypes of transgenic plants have been attributed to variable levels of down-regulation of multiple 4CL genes. Phylogenetic analysis of the sorghum genome revealed 24 4CL(-like) proteins, five of which cluster with bona fide 4CLs from other species. Using a map-based cloning approach and analysis of two independent mutant alleles, the sorghum brown midrib2 (bmr2) locus was shown to encode 4CL. In vitro enzyme assays indicated that its preferred substrate is 4-coumarate. Missense mutations in the two bmr2 alleles result in loss of 4CL activity, probably as a result of improper folding as indicated by molecular modeling. Bmr2 is the most highly expressed 4CL in sorghum stems, leaves and roots, both at the seedling stage and in pre-flowering plants, but the products of several paralogs also display 4CL activity and compensate for some of the lost activity. The contribution of the paralogs varies between developmental stages and tissues. Gene expression assays indicated that Bmr2 is under auto-regulatory control, as reduced 4CL activity results in over-expression of the defective gene. Several 4CL paralogs are also up-regulated in response to the mutation.     

Litter fall and litter decomposition in Pinus sylvestris forests of the eastern Pyrenees

Abstract. Litter fall and litter decomposition were studied in four mature stands of Pinus sylvestris (Scots pine) in the eastern Pyrenees. The stands were located in environments differing in bedrock type and exposition and were studied for two years. Mass-loss during the first year of decomposition was compared with other European P. sylvestris forests and regressed with environmental variables (temperature and rainfall) and latitude. The results suggested that the mean amount of needle fall (1760 kg-ha^-1·yr^-1) was within the range reported for northern European stands. There were more differences in the amount of litter fall between the four stands than between the two years studied. However, the needle fall pattern over the year showed significant differences between years in three of the four stands. Litter decomposition was similar in all the stands and only the one in drier conditions showed a lower decomposition rate. On the European scale the decomposition rate was positively related to mean annual temperature and annual rainfall. However, regression analysis suggests that there are other factors, not taken into account in this study, that are important for predicting the decomposition rate.     

Effect of nitrogen addition on decomposition of Calamagrostis angustifolia litters from freshwater marshes of Northeast China

Wetland ecosystems store a large amount of organic carbon (C) in soils, due to the slow decomposition rates of plant litter and soil organic matter. Increased nitrogen (N) availability induced by human activities and global warming may accelerate litter decomposition and affect soil organic C dynamics in wetlands. In the present study, we investigated the effect of N addition on decomposition of Calamagrostis angustifolia litters from freshwater marshes in the Sanjiang Plain of Northeast China under field and laboratory conditions. First, we assessed the changes in initial litter chemical composition and subsequent decomposition following three years of N addition at the rate of 24 g N m⁻² year⁻¹ under field conditions. Our results showed that N addition increased litter N concentration and decreased C/N ratio, and thus stimulated litter decomposition. Secondly, we examined the effect of increased N availability (0, 25, 50 and 100 mg N g⁻¹ litter) on litter decomposition under laboratory conditions. Increased exterior N availability also enhanced microbial respiration and increased litter mass loss under both waterlogging and non-waterlogging conditions. In addition, waterlogging conditions inhibited microbial respiration and suppressed litter mass loss. These findings demonstrated that N addition increased litter decomposition rates through improved litter quality and enhanced microbial activity in freshwater marshes of Northeast China. This implies that increased N availability accelerates litter decomposition rates, and thus may cause substantial losses of soil C and diminish and even reverse the C sink function of wetlands in the Sanjiang Plain of Northeast China.     

Decomposition of spruce litter needles of different quality by Setulipes androsaceus and Thysanophora penicillioides

Different components of functional biodiversity, such as functional type richness and composition, have been reported to affect the decomposition of litter mixtures. In spite of the numerous reports of these effects, mechanisms underlying patterns of decomposition in litter mixtures are still unclear. We analyzed whether mixture decomposition was affected by: (a) the number of species in the mixture (mixture richness); and (b) the mixture’s functional composition (% of fast- vs. slow-decomposing species included in the mixture). We then tested if variation between observed and expected values of decomposition in mixtures was associated to: (c) the initial litter characteristics of the component species (initial nitrogen, lignin, cellulose and hemicellulose content of litters); and (d) the chemical heterogeneity of the mixtures (variation in the same chemical compounds between the components in each mixture). When up to 5 species representing different functional types were included, both species richness and functional composition showed statistically significant non-additive, and in general positive, effects on litter mixture decomposition. The positive effect of mixture richness on decomposition did not disappear, but was much less marked, when considering mixture with slow-decomposing species only. Although the main driver of decomposition in a mixture is still the average decomposability of the component species (itself largely determined by litter quality), the species interactions in a mixture add a consistent source of variability that is worth considering when predicting the decomposability of a given mixture. We showed that a greater positive difference between observed decomposition rates and that expected from its component species alone was found in mixtures with higher mean nitrogen content and a higher heterogeneity in non-labile compounds. Our results offer quantitative proof that litter chemical heterogeneity, as well as the mean quality of the mixture, can affect the decomposability in litter mixtures.     

Foliar decomposition in a broadleaf-mixed Korean pine (Pinus koraiensis Sieb. Et Zucc) plantation forest: the impact of initial litter quality and the decomposition of three kinds of organic matter fraction on mass loss and nutrient release rates

Foliar litter decomposition of nine species in broadleaf-mixed Korean pine plantation forests, northeast China was assessed over a 34-month field experiment using litterbag method. Litter mass loss generally followed a sequential decomposition of water-soluble fraction (WSF), acid-soluble fraction (ASF), and acid-insoluble fraction (AIF). WSF decomposition contributed most of litter mass loss in the first 6 months, while ASF accounted for most of litter mass loss thereafter. There existed significant autocorrelations among the initial litter quality indices. Initial N, K, Ca, AIF, AIF/N, ASF/N, and WSF/N were significantly related to the percent remaining of litter mass, N, P, Ca, and Mg in both month 12 and month 34. No litter quality can significantly predict the percent remaining of AIF and K. N and P were immobilized by all litters, but Ca, Mg, and K exhibited minor or no immobilization phase. N was the most limiting element in this forest based on the results of correlation analysis and nutrient elements release dynamics. The relationships between WSF, ASF, and AIF loss and N or P release rate fitted the polynomial regression. The decomposition of WSF and ASF were faster than N and P were mineralized during the study. AIF loss rate relative to N and P loss varied greatly among species, with high-N litter showing slower AIF decomposition rates than N and P. The loss rates of WSF and ASF were in proportion to that of K, Ca, and Mg, while AIF decomposed slower than K, Ca, and Mg. This suggested that the decomposition of WSF and ASF caused the net release of K, Ca, and Mg. Responsible Editor: David E. Crowley.     

Leaf litter quality and decomposition rates of yellow birch and sugar maple seedlings grown in mono-culture and mixed-culture pots at three soil fertility levels

Seedlings of yellow birch (Betula alleghaniensis Britton) and sugar maple (Acer saccharum Marsh.) were grown for 2 years in mono-culture and mixed-culture and at three fertility levels. Following the second growing season, senescent leaves were analysed for N concentration, acid hydrolysable substances (AHS), and nonhydrolysable remains (NHR). A litter sub-sample was then inoculated with indigenous soil microflora, incubated 14 weeks, and mass loss was measured. Litter-N was significantly higher at medium than at poor fertility, as well as in yellow birch than in sugar maple litter. The species effect on litter-N increased with increasing fertility. At medium fertility, litter-N of sugar maple litter was lower in mixed-culture than in mono-culture. AHS, NHR as well the NHR/N ratio were significantly higher in yellow birch than in sugar maple litter. At medium fertility, the NHR/N ratio of sugar maple litter was significantly lower in mono-culture than in mixed-culture. Mass loss was significantly greater at medium and rich fertility than at poor fertility, and in yellow birch than in sugar maple litter. At poor fertility, mixed-litter decomposed at a rate comparable to yellow birch, whereas at medium and rich fertility, mixed-litter decomposed at a rate comparable to sugar maple. There was a significant positive relationship between litter-N and mass loss. A similar positive relationship between NHR and mass loss was presumed to be a species effect on decomposition. Results support the hypothesis that species × fertility and species × mixture interactions can be important determinants of litter quality and, by implication, of site nutrient cycling.