Running Title: Plant Growth Promoting Halophilic Archaea
Introduction
Salinity is one of the most adverse environmental factors limiting the
productivity of crop plants. It covers over 6% of the land worldwide
[1 ̵̶ 3]. Salt stress affects the agricultural land in many ways which
include ion toxicity, osmotic stress, nutrient (N, Ca, K, P, Fe, Zn)
deficiency and oxidative stress on phosphorous uptake from soil
[2,4]. It reduces the crop production and has adverse effect on
germination due to toxicity and change in enzymatic activities
[5,6]. Halophytes are plants that grow well in saline soil and
water, such as para grass, Salsola stocksii, Kochia indica andAtriplex amnicola [7]. Therefore, these offer a better
alternative where conventional crops cannot be raised, and drainage is
too expensive. They may also contribute significantly to the developing
world’s supply of food, fiber, fuel and fodder [8].
Halophiles are microorganisms that grow in environments with high salt
concentrations. Halophytes rhizosphere harbors a great diversity of
halophilic bacterial, archaeal and fungal halophiles. The diverse
microbial communities associated with halophyte rhizospheres help these
plants to cope with high salinity and drought stress [9,10].
Microorganisms residing in the rhizosphere play a crucial role in plant
fitness and productivity, especially under extreme conditions
[6,11,12]. Studies of rhizosphere microbiomes have revealed their
influence on chemical exudates responsible for the production and
secretion of signaling molecules by both microbes and plants [13].
Halophilic archaea grow in environments with salt concentrations varying
from 15 to 30% NaCl [14,15]. They can survive at high salt
concentrations reaching up to 5M NaCl [16]. Halophilic archaea use
‘salt in’ for their survival in all high salt concentrations
[16,14]. Some halophiles including Halorhodospira andHalorubrum revealed the use of both strategies for salt
adaptation. Previous studies on solar salterns showed the occurrence of
a major new phylotype, called nanohaloarchaea with a small cell size of
approximately 0.5 µm [17].
Plant growth promoting (PGP) microbes living in the rhizosphere of
halophytes play an important role in plant health and soil fertility
under salinity stress conditions. They enhance plant growth and increase
grain yield of various crops such as wheat, corn, rice, sugarcane and
legumes by solubilization of minerals (P, K, Zn), nitrogen fixation,
production of compounds such as indole acetic acid (phytohormone),
siderophores, HCN, and breaking down of complex organic materials for
the easy uptake by plants [18,19]. Only a few studies reported the
isolation and characterization of halophilic archaeal genera includingHalobacterium, Natrinema, Haloferax, Natrococcus andHaloarcula from the rhizosphere of halophytes such asAbutilon, Dicanthium, Sporobolous and Sueada [20 ̵̶ 22].
Data on phosphate solubilization, siderophore and IAA (indole acetic
acid) production is available from methanogens, haloarchaea and
thermococci [23 ̵̶ 25]. Halophilic archaeal genera includingNatrinema, Halobacterium and Halococcus have been reported
for P-solubilization and production of organic acids such as citric,
succinic, oxalic, lactic, acetic and isovaleric acids [20,26].
Halophilic archaea isolated from hypersaline polluted environments also
have the ability to tolerate heavy metal resistance for nickel, cadmium,
uranium, chromium and zinc [27-29].
The present work endeavors to identify culturable halophilic archaeal
diversity isolated from the rhizosphere of halophytes (S.
stocksii and A. amnicola ) and non-rhizosphere soil samples
collected from Khewra Salt Mines, Pakistan.
This study is the first report on
the characterization of plant growth promoting halophilic archaeal
strains with their ability to solubilize phosphate, fix atmospheric
nitrogen, production of indole acetic acid, siderophores and
exopolysaccharides as well as identification of genes and operons
involved in plant growth promotion, secondary metabolism, environmental
adaptation, glycerol metabolism and membrane transportation from the
genomes of Halorubrum lacusprofundi HL1RP11 andHalobacterium noricense NRS2HaP9.