Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

362 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
synthesis of antifungal metabolites, the production of fungal cell wall–lysing enzymes or
competition for sites on the root (Glick et al., 1998).
Regarding the direct mechanism of facilitating plant nutrition, the means by which PGPR
enhance the nutrient status of host plants can be categorized into five areas: (1) biological
nitrogen fixation; (2) increasing the availability of nutrients in the rhizosphere; (3) inducing
increases in root surface area; (4) enhancing other beneficial symbioses of the host; and
(5) combination of modes of action (for a review, see Vessey, 2003). Examples of PGPR that
exert a positive effect on plant growth–facilitating nutrients uptake are nitrogen-fixing bacteria
(Azospirillium), siderophore-producing bacteria (Pseudomonas), sulfur-oxidizing bacteria
(Thiobacillus), and phosphate-mineral solubilizing bacteria (Bacillus, Pseudomonas)
(Vessey, 2003).
The most well-studied PGPR are the rhizobia (including the Allorhizobium, Azorhizobium,
Bradyrhizobium, Mesorhizobium, Rhizobium, and Sinorhizobium) for their ability to
fix nitrogen in their legume hosts. The role of biological nitrogen fixation as a mechanism for
growth promotion is still controversial. Many biofertilizing PGPR produce phytohormones,
which are believed to modify growth patterns in roots by changing assimilate-partitioning
patterns in plants. Those modifications might increase the absorptive surface of plant roots
for uptake of water and nutrients. There are species of Pseudomonas and Bacillus with the
ability to produce phytohormones (e.g., indole acetic acid, cytokinins, and gibberellins) and
growth regulators. However, only in the cases of Pseudomonas putida (Hall et al., 1996),
Bacillus subtilis (Jiménez-Delgadillo, 2004), and Azospirillum, direct evidences exist for
a role of plant growth regulators in PGPR-elicited growth promotion. In fact, the most
well-studied PGPR system in nonlegume hosts is the nitrogen-fixing genus Azospirrillum.
Inoculation of seeds with nearly all Azospirillum strains causes increases in root length,
number of root hairs, number of root branches, and root surface area (Bashan and Holguin,
1997, 1998). Different mechanisms, such as phytohormone production, nitrate reduction,
and nitrogen fixation, have been proposed to explain improved plant growth following inoculation
with Azospirillum. The production of indole-3-acetic acid (IAA) by Azospirillum
appears to be the most likely explanation for growth promotion. However, it is highly unlikely
that IAA alone causes yield increases that have been reported on a large number of
crops (reviewed by Bashan and de-Bashan, 2004).
The effect of PGPR-promoting root lengthening has been explained by Glick and
coworkers (Hall et al., 1996; Glick et al., 1998). The proposed model suggests that bacterial
production of IAA stimulates plant cell proliferation or elongation and results in plant
production of 1-aminocyclopropane-1-carboxylate (ACC), an ethylene precursor. The ACC
produced by the host plants is taken up by the PGPR P. putida strain and is cleaved by
ACC deaminase, resulting in a decrease of ethylene production in roots. The net biological
effects of this system are increased root elongation of the plant and the nitrogen source
for the PGPR. The more recent discoveries of the involvement of cytokinins (de Salamone
et al., 2001) and possibly gibberellins (Gutierrez-Manero et al., 2001) opens the possibility
that even more plant growth–regulating substances may be involved in the promotion
of plant growth by some PGPR. Undoubtedly, more plant growth–regulating
substances have yet to be discovered. It is likely that the mode of action of currently
identified and yet to be discovered PGPR will involve production of substances, which will
mimic or influence the action of these newer plant growth–regulating substances (Vessey,
2003).