Info

by 64% relative to that in the absence of B. chitinosporus. However, Bacillus pabuli, which enhanced spore germination and hyphal growth in the in vitro assay, had no effect on G. luteum colonization of pea roots. "Helper" bacteria that have stimulatory effects on hyphal growth and EM formation have been isolated from mantle hyphae, while a range of stimulatory, neutral, and beneficial effects of soil bacteria on EM colonization of plant roots has also been observed (Azcon-Aguilar and Barea, 1992).

symbioses involving n-fixing organisms

Leguminous plants have evolved morphologically and functionally specialized symbioses with a group of bacteria (rhizobia) in which the bacteria are rewarded with photosynthetic C in exchange for fixed N from atmospheric N2. The process occurs in root nodules that form when rhizobia infect the host via root hairs and trigger changes to root morphology (Fig. 10.6). Three new genera (Allorhizobium, Mesorhizobium, and Sinorhizobium) have been proposed, in addition to the genera (Azorhizobium, Bradyrhizobium, and Rhizobium) that existed when the previous edition of this book was published, in 1996. As a result of this symbiosis, leguminous plants tend to be early successional species and good colonizers of N-poor environments. They also facilitate the establishment and growth of non-N2-fixing plant species through the accumulation of fixed N in the soil environment. Several legume species are among the most widespread and abundant agricultural crops, including alfalfa, clover, beans, and soybeans. These agricultural legumes are responsible for a large portion of global biological N2 fixation.

The actinorhizal symbiosis between primarily woody plants and an actinomycete, Frankia, is less common than the rhizobium_legume association but is important for plant species that typically colonize stressed, N-poor sites in temperate regions. Familiar host species include alder (Alnus), bayberry (Myrica), sea buckthorn (Hippophae), and autumn olive (Elaeagnus) in temperate regions and beefwoods (Casuarina) in the tropics. The Frankia_actinorhizal symbiosis differs from the legume_rhizobium symbiosis in a number of ways. For example, Frankia exist within nodules in a vegetative, filamentous form, while rhizobia occur as bacteroids, resembling bacteria (Fig. 10.6). Also, while the genes involved in nitrogen fixation show some sequence homology, little similarity has been found among genes involved in nodule formation (Verghese and Misra, 2002).

Like in mycorrhizal symbioses, nodulation involves the recognition and response of both partners (Chap. 14). Briefly, bacteria adhere to the surface of root hairs, which then curl around the bacteria. Bacteria then initiate an infection thread that penetrates into the cortical region of the root, stimulating meristematic growth and nodule formation. Symbioses between plants and N2-fixing organisms, while morphologically and functionally less diverse than mycorrhizal associations, can be categorized by the types of nodules that are formed: determinate and indeterminate (Fig. 10.7). Rhizobia are found associated with both nodule types, but Frankia

998812 18KV XI! S&K* 2&! 8u» I 898813 18KV Xiei 8K" 3i Sua

FIGURE 10.6 (A) Soybean root system with nodules of Bradyrhizobiumjaponicum. (Image courtesy of Pedro Antunes.) (B, C) Electron micrographs of alfalfa nodule interior, showing rhizobial bacteroids. (Images courtesy of Joe Muldoon.)

998812 18KV XI! S&K* 2&! 8u» I 898813 18KV Xiei 8K" 3i Sua

FIGURE 10.6 (A) Soybean root system with nodules of Bradyrhizobiumjaponicum. (Image courtesy of Pedro Antunes.) (B, C) Electron micrographs of alfalfa nodule interior, showing rhizobial bacteroids. (Images courtesy of Joe Muldoon.)

infections mainly result in determinate nodules. Determinate nodules are spherical in shape and ephemeral, lasting only a few days or weeks. Found, for instance, on soybean roots, these nodules contain vascular bundles that aid in the transport of C into the nodule and N out of the nodule. Indeterminate nodules are elongate and long-lived, possessing an apical meristem that continuously produces new cells. Indeterminate nodules have been studied on clover and have a more extensive vascular system than determinate nodules.

As with mycorrhizal symbioses, rhizobial associations are not always beneficial for the plant. Non-N2-fixing strains of rhizobia are commonly found associated

Soybean: determinate nodule, root hair infection thread, non-persistent meristem.

Worm Farming

Worm Farming

Do You Want To Learn More About Green Living That Can Save You Money? Discover How To Create A Worm Farm From Scratch! Recycling has caught on with a more people as the years go by. Well, now theres another way to recycle that may seem unconventional at first, but it can save you money down the road.

Get My Free Ebook


Post a comment