Gini provides a tonic for root hair analysis
Root hairs are single-cell protuberances that elongate by tip
growth, and which are formed from specialized precursor cells (trichoblasts) at
particular locations in the growing root. Under some conditions, individual
plants of the same species and genotype differ markedly in the density of root hairs,
as shown in the very interesting study by a joint Chinese–American team based
at Nanjing University
and Pennsylvania State University
(He et al., pp. 287–293). They have applied a mathematical function,
the Gini coefficient, to inequalities in root hair density in Arabidopsis
thaliana. As is very clearly explained by the authors, the Gini coefficient
is based on the Lorenz curve that was originally used to describe inequalities
in wealth distribution within human society, but it is equally applicable to
studies of other types of inequality, including root hair density. Both density
and inequality were greatly affected by the nutrient content of the growth
medium, especially with respect to phosphorus (P). Under conditions of P
sufficiency, root hair density was lowest and inter-plant inequality was
highest. When the amount of available P was decreased, the root hair density
increased. This is not, of course, a new observation, but what is new is the
clear demonstration that the degree of inequality decreased as the provision of
P declined. Indeed, at the lowest P concentrations used, the Gini coefficient
was reduced almost to zero. Ethylene clearly has a role in these responses
because inhibition of ethylene action at low P concentrations prevented the
increase in density and the decrease in inequality, while addition of the
ethylene precursor 1-methylcyclopropene at high P concentrations mimicked the
effects of low P on density and inequality. Of course, what remain to be
discovered are the internal control mechanisms by which these effects are
manifest – a challenging line of further research for the authors.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
