Agriculture Reference
In-Depth Information
Ferrous Fe is relatively soluble but it is easily oxidised to ferric Fe by atmo-
spheric oxygen. The solubility of Fe
3+
is highly influenced by the soil pH. Indeed in
alkaline conditions Fe
3+
hydrolyses water producing Fe(OH)
3
that polymerises and
precipitates together with inorganic anions. Free Fe
3+
is soluble up to 10
6
MatpH
3.3, but in aerated soil at neutral-basic pH, the concentration of free Fe
3+
and Fe
2+
is
estimated to be less than 10
15
M (Marschner
1995
). This is much lower than the
optimal concentration needed by plants that require between 10
4
and 10
8
MFe
3+
.
This solubility problem strongly impacts Fe availability that represents a severe
constraint for plant development and yield. In particular, Fe is considered the third
most limiting plant nutrient after nitrogen and phosphorus. Hence, even though Fe
is quite abundant in soil, Fe deficiency represents a major problem for worldwide
agriculture in calcareous-alkaline soil, which comprises 30 % of all arable land.
Upon Fe deficiency, plants display typical symptoms such as leaf chlorosis and
reduced growth. Therefore the quest for Fe use efficient crop plants is a goal of plant
breeding and biotechnology.
Plants are always dealing with the dual nature of Fe and have developed
sophisticated mechanisms to ensure adequate Fe acquisition from the soil and at
the same time to make it available for biological processes in the cell avoiding toxic
reactions.
Fe content in crop plants, which constitute a widely utilised staple food, signif-
icantly influences Fe assimilation by the human population. Fe deficiency is one of
the most diffuse nutritional problems in the world, with around 30 % of the world
population affected according to World Health Organization (
http://www.who.int/
nutrition/topics/ida/en/
). Therefore, understanding the mechanisms used by plants
to cope with changing Fe availability is a prerequisite not only for improving crop
yield but also for a positive impact on human nutrition.
In this chapter, the strategies adopted by plants for Fe uptake will be reviewed,
with focus on mechanisms of regulation. The partitioning of Fe in the cell and the
interaction with other nutrients such as sulfur will also be presented.
Fe Deficiency and Plant Responses
Higher plants use distinct strategies to ensure Fe solubilisation and uptake. In the
1980s, R¨mheld and Marschner (
1986
) divided plants into two groups according to
their Fe uptake mechanisms. Dicotyledonous and non-graminaceous monocotyle-
donous plants belong to the Strategy I or reduction strategy group, whereas Poaceae
to the Strategy II or chelation strategy group.
Strategy I is based on (1) soil acidification to increase Fe solubility, (2) reduction
of Fe
3+
to Fe
2+
in the rhizosphere and (3) uptake of Fe
2+
across the root plasma
membrane (see Fig.
5.1
). This strategy was first characterised in
Lycopersicum
esculentum
(tomato) and
Pisum sativum
(pea) as model crop plants. Recently, most
of the studies in this respect have focused on
Arabidopsis thaliana
, which repre-
sents a powerful tool for cell biology investigations. The genes responsible for the
