Chapter 21
The Hologenome Hypothesis and Its
Application to Plant-Microbe Interactions
on an Evolutionary Scale
S. Kouas, N. Khan, and A. M. Hirsch
Abstract When roots are pulled from the ground, one cannot ignore the soil
particles with their rhizosphere microbes still attached to the root surface and root
hairs. The rhizosphere is home to a vast variety of microorganisms that associate
with plants and either positively or negatively influences plant growth, while the
rhizosheath is defined as the weight of soil strongly attached to the plant root
surfaces. Root hairs, epidermal extensions that increase surface area, are a critical
component of rhizosheaths. Within the root and in continuation with the aerial
portion of the plant is the endosphere, providing both intra- and extracellular
locations for microbial habitation. Endophytic microbes often enter plant roots
through cracks in the epidermis or in areas where lateral roots emerge, or through
stomata, openings for gas exchange, in the aerial parts of the plant. Microbial
pathogens as well as symbionts and commensals enter into roots, stems, or leaves
employing these “doorways” to enter plant tissues—sources of nutrients and carbon
for microbes.
S. Kouas
Department of Molecular, Cell and Developmental Biology, Los Angeles, CA, USA
Faculty of Sciences of Gabes, Gabes, Tunisia
Plant Physiology and Functional Genomics Research Unit, Institute of Biotechnology/
University of Sfax, BP, Sfax, Tunisia
N. Khan
Department of Molecular, Cell and Developmental Biology, Los Angeles, CA, USA
A. M. Hirsch (*)
Department of Molecular, Cell and Developmental Biology, Los Angeles, CA, USA
Molecular Biology Institute, University of California-Los Angeles, Los Angeles, CA, USA
e-mail: ahirsch@ucla.edu
© Springer Nature Switzerland AG 2021
C. J. Hurst (ed.), Microbes: The Foundation Stone of the Biosphere, Advances in
Environmental Microbiology 8, https://doi.org/10.1007/978-3-030-63512-1_21
425
The Hologenome Hypothesis and Its
Application to Plant-Microbe Interactions
on an Evolutionary Scale
S. Kouas, N. Khan, and A. M. Hirsch
Abstract When roots are pulled from the ground, one cannot ignore the soil
particles with their rhizosphere microbes still attached to the root surface and root
hairs. The rhizosphere is home to a vast variety of microorganisms that associate
with plants and either positively or negatively influences plant growth, while the
rhizosheath is defined as the weight of soil strongly attached to the plant root
surfaces. Root hairs, epidermal extensions that increase surface area, are a critical
component of rhizosheaths. Within the root and in continuation with the aerial
portion of the plant is the endosphere, providing both intra- and extracellular
locations for microbial habitation. Endophytic microbes often enter plant roots
through cracks in the epidermis or in areas where lateral roots emerge, or through
stomata, openings for gas exchange, in the aerial parts of the plant. Microbial
pathogens as well as symbionts and commensals enter into roots, stems, or leaves
employing these “doorways” to enter plant tissues—sources of nutrients and carbon
for microbes.
S. Kouas
Department of Molecular, Cell and Developmental Biology, Los Angeles, CA, USA
Faculty of Sciences of Gabes, Gabes, Tunisia
Plant Physiology and Functional Genomics Research Unit, Institute of Biotechnology/
University of Sfax, BP, Sfax, Tunisia
N. Khan
Department of Molecular, Cell and Developmental Biology, Los Angeles, CA, USA
A. M. Hirsch (*)
Department of Molecular, Cell and Developmental Biology, Los Angeles, CA, USA
Molecular Biology Institute, University of California-Los Angeles, Los Angeles, CA, USA
e-mail: ahirsch@ucla.edu
© Springer Nature Switzerland AG 2021
C. J. Hurst (ed.), Microbes: The Foundation Stone of the Biosphere, Advances in
Environmental Microbiology 8, https://doi.org/10.1007/978-3-030-63512-1_21
425