plant surface microbiology.pdf

5 Diversity and Functions of Soil Microflora in Development of Plants 89
gibberellin-like substances (Kumar and Lonsane 1989; Steenhoudt and Vanderleyden
2000). The common bacterial genera are Arthrobacter, Azotobacter,
Azospirillum, Pseudomonas, Rhizobium, Bacillus, Brevibacterium and Flavobacterium.
Actinomyces and Nocardia are the important actinomycetes and
Fusarium, Gibberella, Aletrnaria, Aspergillus, Penicillium and Rhizopus are
known fungi.
Cytokinins, N 6 -substituted aminopurines, regulate cell division and differentiation
in certain plant tissues. Cytokinins play an important role in nodule
development and formation. Along with auxins, cytokinins stimulate mature
root cells to undergo polyploid mitosis. Symbiotic N 2-fixing bacteria, Rhizobium,
free-living N 2-fixing bacteria Azospirillum and Azotobacter,and mycorrhizal
fungus, Rhizopogon roseolus are known to produce cytokinins in the
rhizosphere along with other growth-promoting substances (Nieto and
Frankenberger 1989). Other bacteria that produce cytokinins or cytokininlike
substances include Agrobacterium, Bacillus, Paenibacillus and Pseudomonas
(Timmusk et al. 1999).
Ethylene (C 2H 4) is the only phytohormone that is a gas under physiological
temperature and pressure. Ethylene is considered to be a promoter of senescence
and an inhibitor of growth and elongation. It can also promote flowering,
fruit ripening and stimulate cell elongation in certain plants (Elsgaard
2001). Bacterial species of Aeromonas, Citrobacter, Arthrobacter, Erwinia, Serratia,
Klebsiela and Streptomyces, and fungal species of Acremonium,
Alternaria, Mucor, Fusarium, Pythium, Neurospora and Candida are capable
of producing ethylene (Subba Rao 1999).
Abscisic acid (ABA) is generally involved in deceleration or cessation of
plant growth.ABA is active in regulating abscission of young leaves and fruits,
dormancy of buds and seeds, and ripening of fruit. ABA production in two
bacterial species, Azospirillum brasilense and Rhizobium spp. and several phytopathogenic
fungi such as Cercospora, Fusarium, Cladsporium, Monilia, Pestatoria
and Verticillium has been demonstrated (Frankenberger and Arshad
1995; Paul and Clark 1998).
Siderophores are low molecular weight (