Agriculture Reference
In-Depth Information
not to be fully effective in fixing nitrogen. Terpolilli et al. (2008) compared a number
of species of
Medicago
and endosymbionts and concluded that
Sinorhizobium medicae
strain WSM 419, in use in Australia as a commercial inoculant for barrel medic, is
fully effective with this host. As this strain has also now been sequenced, it is hoped
that it will be used in future in preference to
S. meliloti
. To date there is no reason to
question the effectiveness of
L. japonicus
with
Mesorhizobium loti
MAFF 303099, but like
Sinorhizobium meliloti
and
Medicago
, it is known to infect several
Lotus
species.
Another legume that has been promoted as a model is
Phaseolus vulgaris
. Its genome
is not much larger than that of the two model legumes described above and it has
the great advantage of being a very significant food source for many of the world's
poorer peoples, especially in South America and Africa. Its disadvantages are its large
seed size and the fact that it nodulates with a wide range of rhizobia. This may be one
reason why nodulation is often poor. In spite of these disadvantages, Broughton et al.
(2003) make a very good case for the study of this species as a model food legume,
and to assist this they set up an international consortium named 'Phaseomics'. The
consortium is looking at all of the many problems of this crop, including pests and
diseases, as well as nodulation. Finally,
Pisum sativum
, the plant that helped launch
the study of genetics, is still the subject of much research, including the most detailed
description of the exchange of amino acids that is used to ensure a high degree of
nodule effectiveness (Lodwig et al., 2003; Chapter 5).
The four species discussed so far, although from different tribes of legumes, all have
a root hair infection pathway. Two important grain legumes with non-hair infection are
lupins and peanuts (groundnuts). There are good arguments for a more detailed study
of these, not only as crops in their own right, but also because a better understanding of
their nodulation processes might be more relevant to the aim of extending nodulation
ability to non-legumes. Gepts et al. (2005) reported on a meeting that drew up a plan
for studying a wider range of legumes 'for food and feed'. This programme includes
both peanut and
Phaseolus
and (of course) soybean. Is there a case for including non-
papilionoid legumes? If the remit is extended beyond feeding humans, there certainly
is, as genera such as
Acacia
are vital browse plants for both domestic and wild animals
in much of Africa. Also the caesalpinioid species
Chamaecrista rotundifolia
is widely
used in some parts of the tropics as a forage plant.
Since beginning this topic, a major effort has been started to look at nodulation and
other characteristics of
Chamaecrista fasciculata
, as a model for caesalpinioid legumes.
This species, known in the United States as partridge pea, is a fairly small herb and eas-
ily grown. At a special session before the Fourth International Conference on Legume
Genetics and Genomics in Mexico, December 2008, several groups outlined their plans
for studying this species and significant new data may be expected soon. One in-
triguing paper has already emerged, giving the structure of a nodule haemoglobin.
Gopalasubramaniam et al. (2008) found that this was intermediate in form between
non-symbiotic haemoglobin and leghaemoglobin. This intermediate structure was pre-
dicted also to have intermediate oxygen-carrying properties, which could mean that
its role in facilitating oxygen diffusion (see Section 5.7) is less important than for the
papilionoid legumes that have so far been studied (Minchin et al., 2008). This raises
several interesting questions, such as the efficiency with which
Ch. fasciculata
nodules
fix nitrogen, There are no data making direct comparisons between nitrogen fixation
by caesalpinioid legumes and by members of the other sub-families.
