Entry and development of plant disease from biology disscussion
Answers
The Roles of Physical and Chemical Signals in the Germination of Propagules of Plant Pathogens:
Many fungi, on encountering their host or some other solid substrate germinate, producing germ tubes which may differentiate into infection structures. These vary from being simple appressoria to complex structures such as ‘cushions’. Similarly, parasitic angiosperms such as Striga elaborate haustoria.
The stimuli provided by the host for germination, growth and the differentiation of infection structures are as follows:
1. Hydrophobicity:
Lee and Dean (1994) found a correlation between hydrophobicity of the contact surface and the formation of appressoria by the fungus, Magnaporthe grisea but with Phytophthora palmivora the situation was more complex.
Germlings of this organism that were free floating or were in contact with smooth substrates under high nutrient conditions (20 per cent pea broth) did not form appressoria regardless of the hydrophobicity of the contact surface.
In contrast, under low nutrient conditions (5 per cent pea broth), appressoria were formed on smooth substrates that were hydrophobic but not hydrophilic substrates. However, if these same surfaces were scratched, appressoria formed on the scratches, irrespective of the levels of nutrients or substrate hydrophobicity.
2. Hardness:
Xiao and co-workers (1994) reported that although conidia of M. grisea started to germinate whether they contacted a liquid or solid surface, appressoria only formed on solid surfaces and not on liquid or agar surfaces.
On freshly prepared agar surfaces, germ tubes of the fungus penetrated directly without the differentiation of appressoria but if the agar were allowed to dry partially appressoria were formed.
3. Chemical Signals:
Several chemical components of host plants have been implicated in the germination of propagules of plant pathogens and the differentiation of infection structures. In particular, the wax on the surface of aerial parts of the plant is a rich source of diverse compounds, which may play these roles.
For example – wax from rice leaves relieved the self-inhibition of conidia of the rice blast pathogen, Magnaporthe grisea, and stimulated appressorium production. In isolates of Colletotrichum gloeosporioides that infect avocado, specific components of waxes were involved since the surface wax of this host but not of other plants triggered both conidial germination and the development of infection structures. The fatty alcohol fraction of the wax, which comprised 5 per cent of the total, was the most effective and synthetic aliphatic n-fatty alcohols with 24 or more carbons were also active.
Reciprocally, wax of avocado was ineffective in inducing differentiation of other species of Colletotrichum which are pathogenic to other plants. Species of Colletotrichum which infect ripe climacteric fruit, such as tomato, banana and avocado are responsive to the ripening hormone ethylene forming multiple appressoria on glass surfaces in the presence of micromolar concentrations of the gas. Confirmation of the role of ethylene in these processes was obtained in experiments with transgenic tomatoes that did not produce ethylene.
On these the fungus was unable to germinate or form appressoria unless exogenous ethylene was supplied. Thus, these fungal pathogens of ripe climacteric fruit recognize the hosts’ ripening signal and this allows them to form the structures necessary for the launch of a successful attack at the appropriate time.
Flavonoids exuded from the roots of legumes stimulate the spore germination of several soil-borne fungi. For example Bagga and Straney (2000) found that naringenin was a powerful stimulant of the germination of macroconidia of Nectria haematococca, stimulation occurring in the low mM range.