Co2

FIGURE 14.7 Simplified model of carbon and nitrogen metabolism in the bacteroid and the infected plant cell to illustrate the supply of C and energy from the plant and the assimilation of NH4 produced by BNF.

aerobes and absolutely need O2 to generate ATP, the infected cortical cells of the plant produce an O2-carrying protein called leghemoglobin, which facilitates transfer of O2 to the bacteroid. In addition, the bacteroid produces a specialized terminal cytochrome oxidase that enables it to respire under low-O2 conditions.

As the rhizobia transform from free-living bacteria into bacteroids, their C metabolism is modified so that C is directed into the bacteroid to provide reductant and energy for BNF, and N assimilation is modified so that the plant benefits primarily from the BNF and not the rhizobia (Fig. 14.7). Malate and/or succinate are the carbon sources transported into the bacteroid and used in energy production and as C skeletons for bacteroid biosynthetic processes. Expression of plant genes involved with sucrose metabolism is enhanced in the rhizobia-infected plant cells. There is a severalfold increase in expression of sucrose synthase, which converts the photosynthate (sucrose) into precursors for the glycolytic pathway. Glycolysis breaks down the photosynthate to phosphoenolpyruvate, which can be processed to oxaloacetate and to malate by high levels of phosphoenolpyruvate carboxylase and malic dehydrogenase in the nodular plant cells.

The bacteroids catabolize malate and use the energy and reductant to produce ammonia via BNF. Until recently it was assumed that ammonia was directly transported from the bacteroid for assimilation into amino acids by plant nodule cells using GS, GOGAT, aspartate aminotransferase, and asparagine synthase. Waters et al. (1998) showed that B. japonicum bacteroids make alanine via reductive amination of pyruvate and transport it to the nodule cells. High bacteroid activities of malic enzyme (which converts malate to pyruvate) are consistent with malate having a second role of generating pyruvate for assimilation of NH3 into alanine. Hence, ammonia and amino acids (alanine) may be the source(s) of N transferred

TABLE 14.10 Families and Genera of N2-Fixing Plant-Frankia Actinorhizal Associationsa

Family

N2-fixing genera

Betulaceae

Alnus

Casuarinaceae

Casuarina

Coriariaceae

Coriaria

Datiscaceae

Datisca

Elaeagnaceae

Elaeagnus and Hippophae

Myricaceae

Myrica and Comptonia

Rhamnaceae

Ceanothus

Rosaceae

Cercocarpus and Purshia

aReprinted from Paul and Clark (1996) by permission of the publisher.

aReprinted from Paul and Clark (1996) by permission of the publisher.

to the plant cell for further processing. In legumes that form indeterminate nodules, N is transferred to growth sinks of the plant in the form of basic amino acids, asparagine and glutamine. In contrast, legumes that produce determinate nodules further convert fixed N from basic amino acids into compounds called ureides (allantoic acid, allantoin) that are transported to the growth sinks. Both types of molecules (basic amino acids and ureides) have extremely low C:N ratios and represent efficient N-transporting molecules.

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