Question

what is the difference between a fungal infected leaf and a normal leaf?

Answer 1

The leaf surface of a very large number of plant species
are covered by trichomes. Non-glandular trichomes are
specialized unicellular or multicellular structures that
occur in many different plant species and function in
xenobiotic detoxification and protecting the plant
against pest attack. By analysing the susceptibility of
trichome mutants, evidence is provided that indicates
the influence of leaf trichomes on foliar fungal infections
in Arabidopsis thaliana, probably by facilitating the
adhesion of the fungal spores/hyphae to the leaf
surface. A decreased trichome number in the hairless
Arabidopsis mutant gl1 enhances tolerance against the
necrotrophic fungus Botrytis cinerea. By contrast, the
try mutant shows an increased susceptibility to both
fungal infection and accumulation. Trichome density
does not influence infection by the soil-borne patho-
gen Rhizoctonia solani. In addition, the influence of
trichomes on foliar infection is supported by targeting
the high-level expression of the Trichoderma harzianum
a-1,3-glucanase protein to the specialized cell struc-
tures. Trichome expression of this anti-fungal hydrolase
shows a significant resistance to infection by the foliar
pathogen Botrytis cinerea. Resistance to this fungus is
not dependent on the constitutive induction of the
salicylic or jasmonic defence signalling pathways, but
the presence of the a-1,3-glucanase protein in trichomes.
Key words: Antifungal activity, phyllosphere, plant defence,
Rhizoctonia solani, Trichoderma harzianum.
Introduction
Trichomes are specialized unicellular or multicellular
structures derived from the epidermal cell layer. Multicel-
lular trichomes occur in many different species and often
form glands that secrete various compounds including
organic acids, polysaccharides, terpenes, nectar, or salt
(Werker, 2000). In Arabidopsis thaliana, the unicellular
non-glandular trichomes originate from a single epidermal
cell that differentiates to form a characteristic, three-
branched structure with an unknown function. Leaf
trichomes form on stem and leaf surfaces to an extent that
depends on the ecotype (Hu¨lskamp and Kirik, 2000;
Werker, 2000). Multiple mutants have been isolated in
Arabidopsis with altered trichome number or development.
The earliest stages of trichome morphogenesis are blocked
by mutations in the GLABROUS1 (GL1) and TRANS-
PARENT TESTA GLABRA (TTG1) loci. Trichomes are not
found in the gl1 leaves, and the epidermal cells are uniform
in size and shape. The ttg1 mutants also show the same
hairless phenotype and both mutations are epistatic to all
the remaining trichome mutations, suggesting that GL1
and TTG1 are required for the specification of the trichome
cell (Hu¨lskamp et al., 1994). However, the ttg1 mutant has
several pleiotropic defects, one of which is the lack of
anthocyanin biosynthesis (Walker et al., 1999). Mutations
at the TRYPTYCHON (TRY) locus also affect the number
of trichomes and produce a unique phenotype, where nests
of up to four trichomes occur in the place of a single
trichome (Hu¨lskamp and Kirik, 2000).
Generally, the non-glandular trichomes, such as those
present in Arabidopsis, are believed to be physical defen-
sive structures, and it has been demonstrated that natural
enemies impose selection for increased trichome density

Answer 2

Leaves that are exposed to fungi are always with the growth of fungus as in place, the growth of these fungi may be structures such as small pores and  sporadic growth of the spores, however the normal leaf are healthy longer life than the fungal leaf