The Arabidopsis thaliana Glutamate-like Receptor Family (AtGLR) - Communication in Plants: Neuronal Aspects of Plant Life - page 199

that excess of one subunit could disrupt the formation of normal het-
eromers and may have relatively widespread effects, which might be dif-
ficult to interpret but would give crude information about the processes
in which At GLRs are involved. At GLRs have been suggested to have wide-
ranging functions so phenotyping requires testing of plant responses to
a range of different ligands (informed by Sect. 13.4.2), including gluta-
mate, glycine, GABA, ABA, and other amino acids, as well as agonists and
antagonists of animal iGluRs. Plant responses may involve germination,
root growth, circadian-asssociated phenotypes such as aberrant hypocotyl
angle or length, responses to light, nutrients and toxic ions, solute accu-
mulation, and electrical and calcium signalling. Phenotyping needs to be
informed to some extent by the reported physiological effects of amino
acids on plant behaviour, but conversely physiological characterisation
will depend partly on mutant characterisation to predict plant responses
to ligands.
13.4.4
Heterologous Expression
Heterologous expression is desirable to demonstrate ion channel function
of At GLRs. However expression in Xenopus oocytes has been uninfor-
mative (Sect. 13.3.3). Chimaeric iGluRs constructed by transplanting the
pore domain of apparently non-functional iGluRs (such as KA-binding
proteins and C. elegans iGluR) into rat iGluRs have been used to demon-
strate functionality of the pore (Villmann et al. 1997; Strutz-Seebohm et
al. 2003). Although not proof of function in vivo this approach could indi-
cate whether the pore region is capable of ion conduction and its potential
selectivity. Use of homologous or heterologous plant systems such as cul-
tured mesophyll cells may result in correct processing. Alternatively the
possible toxicity of some At GLRs in yeasts and E. coli ,whichsuggests
functional expression (Davenport 2002), could be used to advantage by in-
ducible expression of toxic At GLRs, which would permit at least ion uptake
studies. In some cases coexpression of subunits in heterologous systems
may be required to produce functional ion channels. iGluRs function as
heteromers and some subunits show no channel activity when expressed
as homomers (Dingledine et al. 1999). The large size of the At GLR family
and the evidence for simultaneous expression of many different At GLRs
in single cells (Sect. 13.3.1) makes it difficult to predict likely heteromeric
combinations, and it may be desirable to coexpress large pools of sub-
units simultaneously, or to use a yeast two-hybrid approach to identify
interacting subunits.