Symbiotic effectiveness of Bradyrhizobium strains isolated from Parasponia and tropical legumes on Parasponia host species

The symbiotic effectiveness of Bradyrhizobium strains isolated from three species of Parasponia and from legumes were compared on Parasponia grown in Leonard-jars. Effectiveness of each symbiotic association was estimated from dry weight and total nitrogen of shoots and nodules of plants grown on medium free of combined nitrogen. Twenty strains isolated from three species of Parasponia were found to vary in their effectiveness on P. andersonii, the least effective fixing one fifth of the nitrogen of the most effective strains. The outcome of the symbiosis was not associated with the host source of the test strain. P. andersonii, P. rugosa and P. rigida responded differently to a selection of seven strains of Parasponia Bradyrhizobium; some strains were either ineffective or fully effective on each host, while others varied in their symbiotic performance. P. andersonii fixed significantly (P < 0.001) larger quantities of nitrogen than either P. rugosa or P. rigida with p. rigida being the least effective. In contrast to Bradyrhizobium strains from Parasponia spp. which formed nodules rapidly (within 11–20 days), nine strains isolated from legumes required between 25 and 74 days to form partially effective nodules. The thre Parasponia species formed relatively large quantities of nodule tissue relative to the amount of nitrogen fixed and shoot dry matter produced. The Bradyrhizobium isolated from Parasponia plants growing in Papua New Guinea soils could be grouped together on the basis of their infection characteristics on Parasponia and legumes.     

Nitrogen fixation byRubus ellipticus J. E. Smith

Summary Nitrogenase activity as assayed by acetylene reduction was observed in detachedRubus ellipticus J. E. Smith root nodules collected in the field and tested under ambient conditions. The nitrogenase activity was 8.4 moles C₂H₄.gfr. wt nodule^–1.h^–1 or 24.0 moles C₂H₄.g dry wt nodule^–1.h^–1 being at a rate comparable with that measured in some other non-legumes assayed in Java at the same time under similar conditions. Nodule morphology bore little resemblance to the root nodules of other non-leguminous plants and nodule structure was different from the other rosaceous examples.The endophyte inhabiting the root nodules was actinomycetal.     

The flavonoid naringenin stimulates the intercellular colonization of wheat roots by Azorhizobium caulinodans

We have studied intercellular colonization of wheat roots by Azorhizobium caulinodans and other diazotrophic bacteria, using strains marked with the lacZ reporter gene to facilitate their detection and identification. A. caulinodans was observed by light and electron microscopy to enter the roots of wheat at high frequency at the points of emergence of lateral roots (lateral root cracks). After lateral root crack colonization, bacteria moved into intercellular spaces within the cortical cell layer of roots. The flavonoid naringenin at 10 and 100 mmol m^–3 significantly stimulated root colonization. The roles of the structural nodABC genes and the regulatory nodD gene were also studied; lateral root crack colonization of wheat was shown to be Nod factor- and NodD-independent. Similar frequencies of lateral root crack colonization were observed following inoculation of wheat with Azospirillum brasilense. Colonization by A. brasilense was stimulated by naringenin and also by other flavonoid molecules.     

塔克拉玛干沙漠南缘豆科与非豆科植物的氮分配

在豆科与非豆科植物光合特性的研究中发现,非豆科植物具有更高的光合速率,与其低的叶氮含量相矛盾。在沙漠中氮素是限制植物生长的关键因子之一,考虑到豆科植物的生物固氮作用和叶氮大部分分配于光合系统,我们假设:(1)非豆科植物具有更低的叶氮含量;(2)分配更少的叶氮于光合系统;(3)具有更高的最大净光合速率(Pmax)和光合氮素利用效率(PNUE)。为了验证这些假设,以塔克拉玛干沙漠南缘的豆科植物骆驼刺(Alhagi sparsifolia)和非豆科植物柽柳(Tamarix ramosissima)、花花柴(Karelinia caspica)为研究对象,比较了它们的叶氮含量、氮分配、Pmax和PNUE等。结果表明:(1)非豆科植物比豆科植物确实有更低的叶氮含量,且差异达到显著水平;(2)非豆科植物分配更少的叶氮于光合系统,但在光合系统内部具有更高效的氮分配机制;(3)非豆科植物具有更高的Pmax和PNUE。在光合系统内部,非豆科植物分配更多的叶氮于羧化系统,而豆科植物分配更多的叶氮于捕光系统。对于非豆科植物而言,其更高的Pmax、PNUE、水分利用效率和表观量子产量,取决于将更多的叶氮投入到羧化和电子传递系统中。这些生理优势决定了塔克拉玛干沙漠南缘非豆科植物高效的资源捕捉和利用能力。     

Mechanisms of infection of plants by nitrogen fixing organisms

Heterotrophic nitrogen-fixing microorganisms can enter plants via wounds, root hairs or intact epidermises. All at some stage need the ability to digest primary cell walls and/or middle lamellas. None appears to digest secondary walls. The ability of any organism to infect a particular plant reflects (a) the enzymes produced by the microorganism (and possibly, as part of its reaction, the plant); (b) the exact nature of the primary wall; (c) the distribution of secondary walls. Plants may respond to infection by hypersensitive and other reactions which could be triggered by production of cell wall fragments. Infection threads of secondary wall material may be essential for root hair infection and where cell boundaries are crossed. Entry into host cells other than by infection threads involves a delicate balance between endophyte and host. This may only be achieved in one or a few cells, which may then divide repeatedly to produce a symbiotic structure.     

Contrasting responses of legume versus non-legume shrubs to soil water and nutrient shortages in the Mu Us Sandland

Aims Legumes and non-legumes usually differ in using soil water and nutrients. Both water and nutrients are scarce in the semi-arid Mu Us Sandland where legume and/or non-legume shrubs coexist/dominate. Here, we addressed the responses of legume versus non-legume shrubs to different soil water and nutrient conditions.Methods We conducted an experiment in which a legume (Hedysarum laeve) and a non-legume (Artemisia ordosica) were used, both of which are dominant species in the Mu Us Sandland. Seedlings of these two species were subjected to three water levels (45.0, 67.5 and 90.0 ml every 3 days) and three nutrient treatments (0, 0.1% and 0.2% nutrient solution every week) during the experiment.Important findings Interactions between water and nutrients on total biomass, root weight ratio and rain use efficiency (RUE) were detected in A. ordosica but not in H. laeve, suggesting that water effects on A. ordosica but not on H. laeve are dependent on soil nutrients. Nutrient addition alleviated drought stress and increased RUE in A. ordosica. The interspecific differences in response to soil water and nutrients may be linked to the ability of plants to fix nitrogen. In addition, under low-soil water or nutrient conditions, H. laeve produced more biomass than A. ordosica, and the opposite was the case under high-soil resources. The relationship between relative growth rate (RGR) and RUE [or nutrient use efficiency (NUE)] varied with two species. RGR of A. ordosica was positively correlated with both RUE and NUE while RGR of H. laeve was negatively correlated with NUE. The different responses may be linked to the trade-off between high-growth rate and low-resource use efficiency.     

Biology of theParasponia-Bradyrhizobium symbiosis

Parasponia remains the only non-legume known to nodulate withRhizobium/Bradyrhizobium. It is a pioneer plant that is capable of rapid growth and fixing large quantities of nitrogen. In addition to its high agronomic potential, the symbiosis offers the scientist the unique opportunity of studying differences at the molecular level of both partners, and to investigate any possible extension of the symbiosis to other non-legumes of importance. Haemoglobin has been found in the nodule tissue ofParasponia and other nodulated non-legumes and the gene for it has been found and expressed in non-nodulating plants such asTrema tomentosa andCeltis australis. Bradyrhizobium strains isolated from species ofParasponia growing in Papua New Guinea form a group that are more specific in their host requirements thanBradyrhizobium strains from tropical legumes from the same area. They do not effectively nodulate (except CP283) tropical legumes, andParasponia is not readily nodulated withRhizobium andBradyrhizobium strains from legumes. The effectiveness of the symbiosis is influenced by host species, theBradyrhizobium strain and the environment.Parasponia andersonii forms a more effective symbiosis than the other species tested. In competition studies with strains from legumes, isolates fromParasponia always dominate in nodules onParasponia.