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FUNGAL NUTRITION 119

In addition to these points, fungi accumulate and

store phosphates in excess of their immediate require-

ments, typically as polyphosphates in the vacuoles.

It is not surprising that plants have mycorrhizal asso-

ciations with fungi, because this is probably the most

efficient way for the plant to obtain its phosphorus

supply. The mycorrhizal fungal hyphae have a very

high surface area for uptake of phosphorus and other

mineral nutrients. The cost to the plant in terms of

supplying sugars to the fungus would be much less

than the cost of continuously producing new roots

(Chapter 13).

Iron

Iron is needed in relatively small amounts but is

essential as a donor and acceptor of electrons in

cellular processes, including the cytochrome system in

aerobic respiration (Chapter 7). Iron normally occurs

in the ferric (Fe^3 +) form, insolubilized as ferric oxides

or hydroxides at a pH above 5.5, and it is taken up by

a different process compared with other mineral nutri-

ents. Iron must be “captured” from the environment

by the release of iron-chelating compounds termed

siderophores. These compounds chelate a ferric ion

(Fe^3 +) then they are reabsorbed through a specific

membrane protein and Fe^3 +is reduced to Fe^2 +within

the cell, causing its release because the siderophore

has a lower affinity for Fe^2 +than for Fe^3 +. Finally, the

siderophore is exported again to capture a further

ferric ion. Siderophores and their specific membrane pro-

teins are produced only in response to iron-limiting

conditions.

All the fungal siderophores that have been charac-

terized to date are of the hydroxamatetype (Fig. 6.8).

Their structures, functions, and applications are

reviewed by Renshaw et al. (2002). Despite their

high affinity for Fe^3 +, these fungal siderophores have

much lower affinity than do the siderophores (e.g.

pseudobactinand pyoverdine) produced by fluores-

cent pseudomonads which are common on plant

roots. This raises the possibility that fluorescent

pseudomonads could be used for the control of

plant-pathogenic fungi in the root zone of crops.

For example, pseudobactin-producing pseudomonads

can suppress germination of the chlamydospores

of Fusarium oxysporumon low-iron media, whereas

mutant pseudomonads, deficient in siderophore pro-

duction, are ineffective. However, we shall see in

Chapter 12 that competition for iron is only one of

several ways in which Pseudomonasspp. can control

plant-pathogenic fungi.

Efficiency of substrate utilization

Industrial microbiologists are specially concerned with

the efficiency of substrate conversion into microbial

cells or cell products. Microbial ecologists have been

Fig. 6.8The hydroxamate siderophores of fungi, used to capture ferric iron. The simplest of these are the linear fusarinines,
widely found in species of Fusarium, Penicillium, and Gliocladium. They sequester iron at the position marked by the
asterisk. More complex siderophores such as coprogen have several fusarinine units that bind iron in a ring-like struc-
ture (one of these units is shown by the broken line).