Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

3.2 PGPR Grouped According to Action Mechanismsj47
Figure 3.1 Tryptophan-dependent auxin biosynthetic pathways in plants and microorganisms.
Thereasonsomanyrhizospherebacteriaareabletoproduceauxinsisstillunknown.
Some authors suggest that these bacteria have a tryptophan-related metabolism and that
auxinbiosynthesisisadetoxification alternative [6]. Other authors propose that auxins
have some cellular function because a clear relationship has been observed between
auxin and AMPc levels, which regulate many metabolic processes [34]. However, the
mutualistic view of this fact could be correct, that is, auxin synthesis improves plant
growth; hence, there is greater exudation and thus more nutrients for rhizobacteria. This
hypothesis explains a beneficial association between rhizospheric microorganisms and
the plant. The plant controls the energy flux in the system, since it is the primary
producer, and contributes most of the organic matter to the rhizosphere. Auxins principally
affect plant roots [50]. Those released by rhizobacteria mainly affect the root system,