Abstract
Mangrove forests in the Arabian Gulf are under continuous threat. To increase plantations of gray mangrove (Avicennia marina) in the United Arab Emirates (UAE), 27 endophytic actinobacterial isolates obtained from mangrove roots were in vitro pre-screened to detect the polyamines (PAs) [putrescine (Put), spermidine (Spd), and spermine (Spm)]. We also determined the abilities of the endophytic PA-producing actinobacterial isolates in enhancing the growth of A. marina under greenhouse conditions. Although three highly PA-producing isolates were recovered from inside mangrove root tissues, Streptomyces mutabilis UAE1 constantly colonized root and stem inner tissues for 12 weeks, suggesting an endophytic association between this actinobacterial isolate and mangrove seedlings. When roots were inoculated with S. mutabilis, mangrove growth was remarkably enhanced under gnotobiotic and greenhouse conditions. This was evident from the significant (P < 0.05) increases in dry weight and length of root (66.7 and 65.5%, respectively) and shoot (64.8 and 58.0%, respectively), number of branches (64.3%), total leaf area (40.2%), and photosynthetic pigments (54.5% chlorophyll a; 40.0% chlorophyll b; and 53.1% carotenoids) of mangrove compared to the PA-non-producing Streptomyces sp. or control treatment. Growth promotion in plants treated with S. mutabilis was also supported by significant (P < 0.05) elevations in the contents of mangrove in planta PAs, auxins, and cytokinins, accompanied by a decrease in abscisic acid levels. No difference was, however, detected in growth and amounts of PAs or any plant growth regulators (PGRs) in plants treated with or without the PA-non-producing isolate. Our findings indicate that plant growth parameters can be enhanced as a consequence of secretion of Put, Spd, and Spm by S. mutabilis resulting in regulation of endogenous PAs and other PGRs in mangrove tissues. This study is the first record that aims to improve sustainable agricultural management practices using plant growth promoting (PGP) actinobacteria, endophytic in mangrove tissues to promote mangrove growth under greenhouse conditions. Such research may allow this region to be a model to study the synergistic S. mutabilis–mangrove interactions and the future impacts on mangrove reforestation in the Arabian Gulf and elsewhere where mangrove forests are in threat.
Highlights
Mangroves are assemblages of salt-tolerant trees and shrubs that grow in intertidal regions of tropical and subtropical coastlines (Giri et al, 2011; Tregarot et al, 2021)
A total of 27 endophytic actinobacteria were isolated from the surface-sterilized root samples of which 23 (85.2%) isolates grew and heavily sporulated on inorganic salt starch agar (ISSA) plates containing 8% NaCl (Figure 1A), as a relevant indicator of salinity tolerance of certain isolates (Table 1)
Four isolates were not chosen for further experiments because they did not grow on ISSA containing 8% NaCl (Figure 1A)
Summary
Mangroves are assemblages of salt-tolerant trees and shrubs that grow in intertidal regions of tropical and subtropical coastlines (Giri et al, 2011; Tregarot et al, 2021). In addition to acting as intense carbon (C) sinks (Donato et al, 2011), mangrove forests stabilize shorelines, protect lands, and prevent erosions (Ahmed et al, 2021). Despite their importance, mangrove forests are globally under threat (Alongi, 2002); Iran and the Arabian Gulf countries including Kuwait, Bahrain, Qatar, Saudi Arabia, and the United Arab Emirates (UAE; Almahasheer, 2018; Elmahdy et al, 2020). This is the best “fruit” of a successful plantation program undertaken by the Emirate of Abu Dhabi-UAE that was initiated 20–30 years ago
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