522 Produce Degradation: Reaction Pathways and their Prevention
conducive to mycotoxin production. Methods for controlling mycotoxins are largely
preventive. They^ include good agricultural practice and sufficient drying of^ crops
after harvest. The construction of fungi-resistant plant breeds and introduction of
biocontrol agents are future preventative strategies.
17.4 REGULATION OF GENE EXPRESSION IN
MICROBES ASSOCIATED WITH PLANTS17.4.1 QUORUM SENSING
Quorum sensing, also equivalent to autoinduction or cell-to-cell communication,
allows both Gram-positive and Gram-negative bacteria to sense each other in the
local environment (reviewed by Whitehead et al.,^ 2001). In Gram-negative bacteria
the quorum-sensing system was first described as the autoinducer of the lux regulon,
responsible for bioluminescence of the marine symbiotic bacterium Vibrio fisheri.
In this system a lux I gene encodes for an autoinducer synthase and lux R encodes
a transcriptional activator protein (R-protein). The autoinducer synthase produces
an autoinducer molecule that freely passes the cytoplasmic membrane and accumu-
lates in the microenvironment surrounding the cell. The levels of autoinducer
increase with cell density and the effecter molecule begins to accumulate within
cells due to the decreasing concentration gradient. At a critical concentration the
autoinducer molecule associates with the R-protein and the complex subsequently
binds to specific DNA sequences upstream of target genes, thereby enhancing tran-
scription. Therefore, the quorum-sensing system enables a population of bacterial
cells to coordinate gene function as opposed to acting individually. This allows for
more effective and efficient invasion of the plant.
The most-studied autoinducers in Gram-negative bacteria are N-acylhomoserine
lactones (AHLs), which vary in length, saturation state, and C3 substitution of the
N-acyl side chain. AHLs control the production of many secondary metabolites and
virulence determinants in a range of Gram-negative bacteria. In most cases, quorum
sensing^ is related to pathogenicity traits such as conjugal transfer^ of the Ti plasmids
from Agrobacterium tumefaciens (see later), the production of extracellular^ cell-wall-
degrading enzymes in E. carotovora, the induction of virulence^ factors in P. aerug-
inosa, and antibiotic production in P. aurefaciens. In^ addition, there is growing
evidence that the ability to form^ surface-associated, structured, and cooperative
consortia (referred^ to as biofilms) is also controlled by quorum sensing.
Quorum sensing plays a significant role in plant–bacteria interactions. Plants
(e.g., pea seedlings) release AHL molecules in root exudates that can induce or
suppress quorum-controlled genes in rhizobacteria (Bauer and Teplitski, 2001). The
main question yet to be answered is what physiological role plant-produced AHLs
have. The inhibitory AHLs would logically prevent expression of virulent genes in
pathogens, thereby acting as an effective means of defense. The role of stimulatory
AHLs is less clear but possibly leads to an uncoordinated attack by pathogens and
hence places less stress on the plant’s defenses.
In Gram-positive bacteria quorum sensing has been shown for genetic compe-
tence in Bacillus subtilis and Streptococcus aureus. Significantly, the production of