plant surface microbiology.pdf

8 ACC Deaminase-Containing Plant Growth-Promoting Bacteria 139
epinasty, leaf chlorosis, necrosis, and reduced fruit yield. Two of the ACC
synthase genes, LE-ACS7 and LE-ACS2, are rapidly induced in the roots of
flooded tomato plants. Of these two genes, LE-ACS7 is expressed earliest
after flooding and LE-ACS2 is expressed approximately 8 h after flooding;
the gene, LE-ACS7 is also involved in the early wound response of tomato
leaves (Shiu et al. 1998). Since ACC oxidase-catalyzed ethylene synthesis cannot
occur in the anaerobic environment of flooded roots, ACC is transported
into the aerobic shoots where is converted to ethylene (Bradford and Yang
1980; Else and Jackson 1998). Treatment of tomato plants with ACC deaminase-containing
plant growth-promoting bacteria significantly decreases the
damage suffered by these plants – damage that is caused by the deleterious
effects of ethylene which normally occurs as a consequence of flooding
(Grichko and Glick 2001). These ACC deaminase-containing plant growthpromoting
bacteria can act as a sink for ACC, thereby lowering the level of
ethylene that can be formed in the shoots. The tomato plants are thus “protected”
against flooding.
Many of the symptoms of a diseased plant arise as a direct result of the
stress imposed by the infection. That is, much of the damage sustained by
plants infected with fungal phytopathogens occurs as a result of the response
of the plant to the increased levels of stress ethylene (Van Loon 1984). It has
also been observed that exogenous ethylene often increases the severity of a
fungal infection and, as well, ethylene synthesis inhibitors significantly
decrease the severity of a fungal infection. In a study with over 60 different
cultivars and breeding lines of wheat, ethylene production increased as a
result of infection with the fungal phytopathogen, Septoria nodorum, and
was correlated with increased plant disease susceptibility (Hyodo 1991). The
damage caused by the fungal phytopathogen, Alternaria, decreased in cotton
plants by treating them with chemical inhibitors of ethylene synthesis
(Bashan 1994). The levels of both ethylene and disease severity decreased in
melon plants infected by the fungal phytopathogen, Fusarium oxysporum,
following treatment of the plants with ethylene inhibitors (Cohen et al.
1986). Fungal disease development increased in both cucumber plants
infected with Colletotrichum lagenarium (Biles et al. 1990) and in tomato
plants infected with Verticillium dahliae (Cronshaw and Pegg 1976) when
the plants were pretreated with ethylene. Treatment with ethylene inhibitors
decreased disease severity in roses, carnations, tomato, pepper, French-bean
and cucumber infected with the fungus, Botrytis cinerea (Elad 1988 and
1990).
Several biocontrol strains were transformed with the Enterobacter cloacae
UW4 ACC deaminase gene and the effect of the transformation was assessed
by using the cucumber-Pythium ultimum system (Wang et al. 2000). The
results of the experiments indicated that ACC deaminase-containing biocontrol
bacterial strains were significantly more effective than biocontrol strains
that lacked this enzyme. Moreover, transgenic tomato plants that express ACC