This passage is adapted from Diana Gitig, “Evolving Towards Mutualism.” ©2010 by Diana Gitig. Horizontal gene transfer is the passing of genetic material within a species or between species through asexual reproduction.
Plants, and all other living things, require nitrogen for
growth; it is an essential component of nucleic acids and
proteins. Although air is mostly nitrogen, this gaseous form is
inaccessible to plants and must be fixed into ammonium to
render it biologically relevant. Soil bacteria called rhizobia fix
nitrogen, but to do this they must first take up residence inside
the roots of legumes like pea, alfalfa, clover, and soybean.
Soon after a legume begins to grow, rhizobia invade its root
hairs and multiply, causing the plant to form specialized organs
—nodules—that contain the proliferating bacteria. This
symbiotic arrangement benefits both parties: legumes can
thrive without nitrogen fertilizers only if they have functional
nitrogen-fixing nodules, while the bacteria receive the energy
needed to multiply and fix nitrogen from the plant. When the
plant dies, the fixed nitrogen is released into the soil so other
plants can use it. This process has significant implications for
agriculture, as nitrogen is the most common nutrient deficient
in the earth’s soil and, thus, the one most commonly supplied
by chemical fertilizers.
Rhizobia are a diverse group taxonomically, genetically, and
metabolically. They can be found in distant genera. Their
symbiotic trait appears to have arisen independently multiple
times by horizontal transfer of genes. However, it is not
thought that this horizontal gene transfer is sufficient to confer
symbiosis, or to explain the biodiversity of rhizobia. There
must be selective pressures preventing or permitting the
expression of the acquired symbiosis trait and adaptive
mechanisms to deal with these pressures. But neither the
pressures nor the measures taken to circumvent them are
known.
Plants, and all other living things, require nitrogen for
growth; it is an essential component of nucleic acids and
proteins. Although air is mostly nitrogen, this gaseous form is
inaccessible to plants and must be fixed into ammonium to
render it biologically relevant. Soil bacteria called rhizobia fix
nitrogen, but to do this they must first take up residence inside
the roots of legumes like pea, alfalfa, clover, and soybean.
Soon after a legume begins to grow, rhizobia invade its root
hairs and multiply, causing the plant to form specialized organs
—nodules—that contain the proliferating bacteria. This
symbiotic arrangement benefits both parties: legumes can
thrive without nitrogen fertilizers only if they have functional
nitrogen-fixing nodules, while the bacteria receive the energy
needed to multiply and fix nitrogen from the plant. When the
plant dies, the fixed nitrogen is released into the soil so other
plants can use it. This process has significant implications for
agriculture, as nitrogen is the most common nutrient deficient
in the earth’s soil and, thus, the one most commonly supplied
by chemical fertilizers.
Rhizobia are a diverse group taxonomically, genetically, and
metabolically. They can be found in distant genera. Their
symbiotic trait appears to have arisen independently multiple
times by horizontal transfer of genes. However, it is not
thought that this horizontal gene transfer is sufficient to confer
symbiosis, or to explain the biodiversity of rhizobia. There
must be selective pressures preventing or permitting the
expression of the acquired symbiosis trait and adaptive
mechanisms to deal with these pressures. But neither the
pressures nor the measures taken to circumvent them are
known.