Plant basal resistance - Universiteit Utrecht

Chapter 1
constitutively produced in blackcurrant leaves, but is pathogen-inducible in rice leaves after
infection (Kodama et al., 1988).
Glucosinolates are N- and S- containing indolic phytoanticipins that are
exclusively found in Brassicaceae. Upon tissue damage, glucosinolates are hydrolysed by
endogenous β-thioglucoside glucohydrolases, also known as myrosinases, which results in
the accumulation of toxic metabolites, such as isothiocyanates, thiocyanates and nitriles
(Halkier and Gershenzon, 2006). Glucosinolates can be directly toxic, but their activity is
mostly based on deterrence of plant attackers, including mammals, birds, insects, mollusks,
nematodes, bacteria and fungi (Halkier and Gershenzon, 2006).
The most abundant class of phytoanticipins in Poaceae are benzoxazinones
(BXs). These phenolic compounds have broad-spectrum defence activity against insects,
nematodes, bacteria and insects (Niemeyer, 1988, 2009). Their biosynthesis originates from
indole and is mostly under developmental control, which leads to accumulation of less
inactive BX-glucosides in the vacuole (Frey et al., 2009). BX-glucosides are hydrolysed by
β-glucosidases upon tissue disruption, which leads to the release of biocidal aglycone BXs
(Nikus and Jonsson, 1999).
Semiochemicals
Semiochemicals are naturally produced low molecular weight compounds used as signals
in communication between organisms. Based on their effects, they can be divided into:
pheromones (chemical cues used for intra-species communication), kairomones (chemcials
used for host identification and location), allomones (defence secretions which only
serve the producing organism itself) and allelochemicals (signalling chemicals used for
communication between individuals of different species). Semiochemicals can be volatile
or non-volatile. Volatile semiochemicals can act over long distances, while non-volatile
semiochemicals more likely act over shorter ranges (Romeis and Zebitz, 1997). Plant-
derived semiochemicals can originate from wide range of biosynthetic pathways, but
predominantly come from the lipoxygenase and isoprenoid pathways. Well know examples
of above-ground semiochemicals are monoterpenes, such as (E)-ocimene, sesquiterpenes,
such as germacrene D, (E)-β-farnesene, and the aromatic compounds methyl salicylate. The
emission of volatile semio-chemicals signals is not restricted to above-ground plant parts,
the sesquiterpene (E)-β-caryophyllene was found to be released from maize root upon
feeding by the Western Corn Rootworm, which can attract entomapathogenic nematodes
(Rasmann et al., 2005).
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