the ability of plants to actively shape their
microbiomes through insight into the molec-
ular mechanisms of microbial-induced plant-
specialized metabolism ( 57 , 58 ). In field situations,
the microbiota present in the soil acts as the
main pool from which plants can select their
rhizosphere or root microbiome. Hence, soil
management and its impact on the soil micro-
biome remain of critical importance for engi-
neering root microbiota. On the other hand,
the agricultural environment and the geno-
types of the cultivated plants have an impact
on individual microbial genotypes ( 59 ). This
suggests that manipulation of plants and crop-
ping systems is the key to sustainable biocontrol.
When the biology underpinning plant-microbe
interactions is well understood, inoculation
with individual bacterial isolates can also be
effective ( 60 ). Further protection for plants
is provided by the initial soil microbiome act-
ing against pathogen invasion or by promot-
ing plant resistance and crop productivity
( 61 ). This provides a basis for engineering root
microbiota and for greater understanding of
the functions of root and soil microbiota in
plant growth and health.
Choice of microorganisms for dryland restoration
Many studies have focused on the resilience
of microbial communities in dry soils and on
the effects of soil degradation on a microbial
community structure ( 62 , 63 ). These studies
can help in selecting the most promising can-
didate species for degraded land restoration.
Several species of algae have been shown
to promote soil hydrological properties by
breaking water repellency and improving wa-
ter retention ( 64 , 65 ). In studies of the response
to desiccation, fungi showed higher resistance
than bacteria to changes in water availability
( 63 , 66 ), enabled by hyphae that may traverse
air-filled soil pores to access nutrients and wa-
ter. In bacterial communities, survival strategies
for dry conditions have long been examined
by testing physiological responses ( 67 , 68 ).
More recently, next-generation sequencing
techniques have helped to further explore
trends in abundances of major taxonomic
groups as a function of soil moisture condi-
tions ( 62 , 63 , 69 ). In the context of differences
between arid and non-arid environments, or
differential responses to desiccation stress,
bacterial abundances vary in response to dif-
ferent moisture levels in three main ways:
- With decreasing soil humidity, there is
an increase in abundances of Cyanobacteria
( 69 – 71 ) and monoderm bacteria (i.e., pro-
karyotic cells surrounded by a single mem-
brane) ( 68 , 72 ); this latter group includes
Actinobacteria ( 69 , 73 ), Chloroflexi ( 62 , 73 ),
and Firmicutes ( 70 , 73 , 74 ). All of these bacte-
rial taxa possess multiple mechanisms that help
them withstand harsh environments includ-
ing desiccation for extended periods, namely
a thick peptidoglycan layer ( 75 ) and an ability
to produce resting stages (e.g., endospores) ( 76 ).
2) Some of the terrestrial diderm bacteria
[phyla such as Acidobacteria ( 62 , 69 ) and Ver-
rucomicrobia ( 62 , 69 )] follow the opposite pat-
tern: Their abundances decline during dry-out
and increase with rewetting. An exception is
the class Chloracidobacteria of Acidobacteria
that is distinct from this group ( 69 , 77 ).
3) Other diderm phyla (e.g., Proteobacteria,
Planctomycetes, Bacteroidetes) do not show
a consistent response to aridity. Their abun-
dances in dry soil have been reported to in-
crease in some studies and to decrease in others
( 62 , 69 , 70 , 74 ).
Each of the three possible responses (in-
crease, decline, no change) was also found in
ribosomal synthesis of these phyla ( 63 , 74 ), a
response that usually serves as a proxy for
potential activity, although rRNA concentra-
tion and growth rate are not always simply
related ( 78 ).
Different bacterial species can produce
either hydrophilizing or hydrophobizing sub-
stances as a part of extracellular polymeric
substances (EPSs) ( 79 ) and thereby affect soil
hydrophobicity and water infiltration. Hydro-
philizing properties were found forBacillus
sphaericusof the Firmicutes phylum ( 79 ). Fur-
thermore, some Actinobacteria and Proteo-
bacteria are able to degrade waxes and thereby
reduce water repellency ( 80 , 81 ). This indicates
a promising potential use of these bacteria
to improve soil wettability and break down
hydrophobicity.
All of these findings suggest the possible
application in arid environments of either
monoderm bacteria (Actinobacteria, Chloro-
flexi, Firmicutes) and Cyanobacteria or the gen-
eralists that expose drought-resistant patterns
(e.g., fungi, algae, Proteobacteria). Recently,
fungi have been shown to facilitate coloniza-
tion of dry soil by bacteria ( 82 ), and therefore
studies investigating co-inoculations of bacte-
ria and fungi are also of interest.Types of land degradation and the state
of the art of microbe use in land restoration
Land degradation is the loss of the intrinsic
physical, chemical, or biological soil charac-
teristics by natural or anthropogenic processes,
resulting in a reduction or eradication of vital
ecosystem functions ( 83 ). Land degradation
estimations at the global scale are complex
and vary between studies (Fig. 2). Because no
formal classification of land degradation ex-
ists, we propose three principal and intercon-
nected processes of degradation ( 84 ):
1) The physical loss or physical transforma-
tion of soils (e.g., associated with erosion, land-
slides, drought, severe fires, soil compaction,
and sealing);
2) The loss of soil chemical properties, such as
decline in fertility and organic carbon contentdue to acidification, salinization, or nutrient de-
ficiency; and
3) Soil contamination.
Microorganisms can promote soil restora-
tion in each of these cases. Here, we discuss
each of the three degradation types with spe-
cial emphasis on driving causes most com-
monly associated with global environmental
change, namely drought, fire, and soil salin-
ization (Fig. 3).Physical loss of soil
Physical land degradation can affect soil at all
latitudes and on all continents. One driving
cause is the increased frequency and severity
of drought. Drylands already cover an esti-
mated one-third of Earth’s terrestrial surface
( 85 ) and are expected to increase in size as a
result of climate change and human activities
( 86 ). In drylands, strong winds and ultraviolet
radiation lead to decreases in soil nutrients
and organic matter, deterioration of soil struc-
ture, and increases in salinity, water defici-
ency, and physical instability ( 87 ). Under such
stresses, plants suffer from a number of bio-
chemical, morphological, and physiological
modifications that suppress their growth and
productivity ( 88 ). PGPR found in dry environ-
ments are well adapted to extreme environ-
mental conditions such as drought, heat, and
high salinity, providing essential nutrients
from soil or through N fixation and enhancing
plant tolerance to abiotic and biotic stresses
( 89 ). PGPR in drylands possess unique traits,
including a stronger expression of genes re-
lated to dormancy and osmoregulation as well
as a weaker expression of genes regulating nu-
trient cycling and catabolism ( 89 ). They are also
capableofincreasingthesolublesugarscon-
tent and chlorophyll content in the leaves, which
enable drought stress tolerance in plants ( 90 ).
Highly desiccation-tolerant PGPR, such as Actino-
bacteria, produce the sugar trehalose, which
increases their abiotic stress tolerance and at
the same time symbiotically protects plants
against drought by induction of their stress-
response genes ( 91 ). Over the past decade,
there has been increased interest in the devel-
opment of biofertilizers based on desert PGPR
that can promote plant tolerance against abiotic
stress in drylands ( 88 , 89 ). Inoculating soil in
degraded drylands with biological crusts can
contribute to the recovery of ecosystem func-
tionsatthelandscapescale,suchaserosion
resistance by improving soil stability and nu-
trient cycling. Using living inoculum has shown
to be more effective in drylands than replacing
topsoil or organic carbon additions ( 92 ).
Low-severity fires, such as grassland fires or
prescribed fires for landscape management,
enhance soil properties by reducing surface
flow and erosion and increasing soil organic
matter and nutrient content ( 93 ). In contrast,
wildfires greatly change soil physical propertiesCobanet al.,Science 375 , eabe0725 (2022) 4 March 2022 3 of 10
RESEARCH | REVIEW