Prospect of pink pigmented facultative methylotrophs in mitigating abiotic stress and climate change

Methylobacterium spp. mitigation of UV stress in mung bean (Vigna radiata L.)

Methylotrophs are a diverse group of bacteria that abundantly colonize the phyllosphere and have great potential to withstand UV irradiation because of their pigmented nature and ability to promote plant growth through various mechanisms. The present study investigated the effects of UVB radiation on plant growth-promoting (PGP) properties of methylotrophic bacteria and the growth of Vigna radiata L. A total of 55 methylotrophic bacteria were isolated from desert plants, and 15 methylotrophs were resistant to UVB radiation for 4 h. All UVB-resistant methylotrophs possess a methyldehydrogenase gene. Identification based on 16S rRNA gene sequencing revealed that all 15 UVB-resistant methylotrophs belonged to the genera Methylorubrum (07), Methylobacterium (07), and Rhodococcus (01). Screening of methylotrophs for PGP activity in the presence and absence of UVB radiation revealed that all isolates showed ACC deaminase activity and growth on a nitrogen-free medium. Furthermore, the production of IAA-like substances ranged from 8.62 to 85.76 µg/mL, siderophore production increased from 3.47 to 65.75% compared to the control. Seed germination assay with V. radiata L. (mung bean) exposed to UVB radiation revealed that methylotrophs improved seed germination, root length, and shoot length compared to the control. The present findings revealed that the isolates SD3, SD2, KD1, KD5, UK1, and UK3 reduced the deleterious effects of UVB radiation on mung bean plants and can be used to protect seedlings from UVB radiation for sustainable agriculture.

Characterization of C30 carotenoid and identification of its biosynthetic gene cluster in Methylobacterium extorquens AM1

Methylobacterium species, the representative bacteria distributed in phyllosphere region of plants, often synthesize carotenoids to resist harmful UV radiations. Methylobacterium extorquens is known to produce a carotenoid pigment and recent research revealed that this carotenoid has a C30 backbone. However, its exact structure remains unknown. In the present study, the carotenoid produced by M. extorquens AM1 was isolated and its structure was determined as 4-[2-O-11Z-octadecenoyl-β-glucopyranosyl]-4,4'-diapolycopenedioc acid (1), a glycosylated C30 carotenoid. Furthermore, the genes related to the C30 carotenoid synthesis were investigated. Squalene, the precursor of the C30 carotenoid, is synthesized by the co-occurrence of META1p1815, META1p1816 and META1p1817. Further overexpression of the genes related to squalene synthesis improved the titer of carotenoid 1. By using gene deletion and gene complementation experiments, the glycosyltransferase META1p3663 and acyltransferase META1p3664 were firstly confirmed to catalyze the tailoring steps from 4,4'-diapolycopene-4,4'-dioic acid to carotenoid 1. In conclusion, the structure and biosynthetic genes of carotenoid 1 produced by M. extorquens AM1 were firstly characterized in this work, which shed lights on engineering M. extorquens AM1 for producing carotenoid 1 in high yield.

Affecting Factors of Plant Phyllosphere Microbial Community and Their Responses to Climatic Warming-A Review

Phyllosphere microorganisms are not only an important part of plants, but also an important part of microorganisms. In this review, the function of phyllosphere microorganisms, the assembly mechanism of phyllosphere microorganisms, the driving factors of phyllosphere microbial community structure, and the effects of climate warming on phyllosphere microbial community structure were reviewed. Generally, phyllosphere microorganisms have a variety of functions (e.g., fixing nitrogen, promoting plant growth). Although selection and dispersal processes together regulate the assembly of phyllospheric microbial communities, which one of the ecological processes is dominant and how external disturbances alter the relative contributions of each ecological process remains controversial. Abiotic factors (e.g., climatic conditions, geographical location and physical and chemical properties of soil) and biological factors (e.g., phyllosphere morphological structure, physiological and biochemical characteristics, and plant species and varieties) can affect phyllosphere microbial community structure. However, the predominant factors affecting phyllosphere microbial community structure are controversial. Moreover, how climate warming affects the phyllosphere microbial community structure and its driving mechanism have not been fully resolved, and further relevant studies are needed.