Journal of Bioscience and Bioengineering VOL. 128 No. 5, 551e557, 2019 www.elsevier.com/locate/jbiosc Glucose and arabinose dependent mineral phosphate solubilization and its succinate-mediated catabolite repression in Rhizobium sp. RM and RS Ekta Joshi, Bhagya Iyer, and Shalini Rajkumar* Institute of Science, Nirma University, Sarkhej-Gandhinagar Highway, Ahmedabad, Gujarat 382481, India Received 19 December 2018; accepted 29 April 2019 Available online 28 May 2019 Rhizobium sp. RM and RS are Vigna radiata root nodule isolates with the ability to solubilize tricalcium phosphate and rock phosphate under 50 mM Tris-Cl buffering conditions. Rhizobium sp. RM and RS were unique as they could produce two different organic acids, gluconic acid and oxalic acid using glucose and arabinose, respectively, which are two of the most prominent sugars present in the rhizospheric soil. However, P solubilization in these isolates was repressed in the presence of succinate resembling the phenomenon of catabolite repression. RM and RS produced 24 mM and 20 mM gluconic acid in presence of glucose which was repressed to 10 mM and 8 mM, respectively, in glucose D succinate conditions. Similarly, RM and RS produced 28 mM and 23 mM oxalic acid in arabinose containing media which was repressed to 9 mM and 8 mM, respectively, in the presence of arabinose D succinate. Results of enzyme activities indicated 66% repression of quinoprotein glucose dehydrogenase in glucose D succinate compared to glucose grown cells and 84% repression of glyoxylate oxidase in arabinose D succinate compared to arabinose grown cells. This is perhaps the first report on mechanism of P solubilization in rhizobia through utilization of two different sugars, glucose and arabinose and its repression by succinate. Succinate-mediated catabolite repression of arabinose is the unique aspect of this study. Ó 2019, The Society for Biotechnology, Japan. All rights reserved. [Key words: Arabinose; Mineral phosphate solubilization; Glucose dehydrogenase; Arabinose dehydrogenase; 2-Keto-3-deoxyarabonate aldolase; Glycolaldehyde dehydrogenase; Glyoxylate oxidase] Apart from the use of chemical fertilizers, microbial phosphate efficient P solubilization (8,9). Gluconic acid seems to be the most solubilization and mineralization is the only feasible way to frequent agent for MPS in several P solubilizing bacteria such as enhance plant available phosphate. Phosphorus, the second most Pseudomonas sp. (2), Erwinia herbicola (10) and Pseudomonas important plant growth limiting macronutrient after nitrogen is cepacia (11). abundantly available in soil in both organic and inorganic forms Majority of the rhizobacteria reported so far have been shown to (1,2). Majority of the soil phosphorus is in a form unavailable for solubilize P only when grown on glucose. Other rhizospheric sugars plant uptake but can be converted to plant available forms by have received little or no attention in this regard. L-Arabinose is a phosphate solubilizing microorganisms (3,4). Mineral phosphate carbon source widely used by microorganisms including diazo- solubilization (MPS) is one of the most important plant growth trophs such as Azospirillum sp. (12), Bradyrhizobium sp. (13) and promoting activity shown by plant growth promoting rhizobacteria Rhizobium sp. (14). Arabinose is also reported to be present in the and it contributes significantly to the plant growth and root exudates of many plants at physiologically relevant concen- development. trations making it a prominent sugar present in the rhizospheric Bacteria belonging to various genera like Pseudomonas, Rhizo- soil (15). Arabinose is a substrate that could be used by rhizobia in bium, Klebsiella, Enterobacter, Acinetobacter, Bacillus, Erwinia, Bur- the rhizosphere; is a component of the plant cell wall and is present kholderia and Micrococcus are well known for solubilizing insoluble in arabinogalactan proteins secreted by plant root cells (16). Role of inorganic phosphate (P) compounds such as tricalcium phosphate a pentose such as arabinose on plant growth promoting traits of (TCP), dicalcium phosphate and rock phosphate (RP) (5,6). P solu- bacteria especially P solubilization has not been explored. L-Arabi- bilization is achieved through the secretion of low molecular nose is a superior carbon source and is metabolized by distinct weight organic acids like gluconate, ketogluconate, malate, succi- pathways that distinguish rhizobia into slow and fast growers (17). nate, fumarate, oxalate, and citrate, which cause acidification and In the slow-growing rhizobia (Bradyrhizobium japonicum and release of free P in a form available for plant uptake (5,7). Gluconic Rhizobium sp. 32H1), L-arabinose is metabolized through non- acid produced by gram-negative bacteria through periplasmic phosphorylated intermediates leading to the production of pyru- glucose oxidation pathway via pyrroloquinoline quinone depen- vate and glycolaldehyde (13). In this pathway, L-arabinose dent glucose dehydrogenase (Gcd) is shown to result in most metabolism begins with oxidation to L-arabino-g-lactone, followed by hydration forming L-arabonate which undergoes dehydration to L-2-keto-3-deoxyarabonate. It is split by an aldolase reaction to * Corresponding author. Tel.: þ91 79 30642757; fax: þ91 2717 241916. E-mail address: [email protected] (S. Rajkumar). pyruvate and glycolaldehyde (13). Glycolaldehyde is subsequently 1389-1723/$ e see front matter Ó 2019, The Society for Biotechnology, Japan. All rights reserved. https://doi.org/10.1016/j.jbiosc.2019.04.020 552 JOSHI ET AL. J. BIOSCI.BIOENG., metabolized through an oxalate degradation pathway (18).In 20 mM glucose, 20 mM arabinose, 20 mM succinate, 10 mM glucose þ 10 mM succinate and 10 mM arabinose þ 10 mM succinate. Cells were harvested by contrast, L-arabinose metabolism produces L-2-keto-3- centrifugation at 11,270 Âg for 5 min at 4C (31) and were washed with 80 mM deoxyarabonate in fast growing rhizobia (Sinorhizobium meliloti phosphate buffer (pH 7.5). Cell pellet was resuspended in small volume of the and Rhizobium leguminosarum) which is oxidized by a-ketogluta- same buffer containing 20% glycerol and 1 mM DTT and sonicated (Syclon rate semialdehyde dehydrogenase to a-ketoglutarate that enters Ultrasonic Cell Crusher, Ningbo Haishu Sklon Electronic Instrument Co., Ltd., TCA cycle (19). Ningbo, China) for 2e3 min at a pulse rate of 30 s at 500 Hz on ice. Cell debris  Succinate on the other hand is one of the major organic acid was removed by centrifugation at 11,270 g at 4 C for 30 min and the supernatant was directly used for enzyme assays. For Gcd assay, whole cells were present in plant root exudates which is known to repress glucose washed thrice with N saline, resuspended in N saline and used directly (32). All utilization by a mechanism termed as succinate-mediated catabo- the enzyme assays were performed at 28C. Protein quantification was done by lite repression (20). Organic acid mediated MPS phenotype has Lowry method (33). One unit of enzyme activity was defined as the amount of been shown to be repressed by succinate in Pseudomonas (21), protein required to convert 1 mmol of substrate to product per minute. Klebsiella pneumoniae (22) and Rhizobium sp. Td3 (23). Even though Glucose dehydrogenase (Gcd) (EC 1.1.5.2) Gcd (EC 1.1.5.2) assay was per- formed by determining the spectrophotometric reduction in the absorbance of 2,6- phosphate solubilization by bacteria has been reported since long, dichlorophenolindophenol (DCIP) at 600 nm as described by Rajput et al. (22). The fi À failure of these PSBs in eld conditions due to succinate-mediated specific activity of enzyme was defined as IU (mg protein) 1. catabolite repression may restrict their use as biofertilizers. The Arabinose dehydrogenase (EC 1.1.1.46) Arabinose dehydrogenase (EC present study is aimed at determining the exact biochemical basis 1.1.1.46) assay was carried out by measuring the formation of NADPH continuously at of MPS phenotype in rhizobia using arabinose and glucose as car- 340 nm in a spectrophotometer. The assay mixture (1 ml) contained 10 mM L- bon source and its succinate-mediated repression. arabinose, 100 mM Tris-Cl (pH 9.0), 10 mM NADP and sonicated cells. Enzyme activity was measured every 30 s for 6 min (14). L-2-Keto-3-deoxyarabonate aldolase (L-KDA aldolase) (EC 4.1.2.18) L-2- Keto-3-deoxyarabonate aldolase (EC 4.1.2.18) assay was carried out as described MATERIALS AND METHODS by Pedrosa and Zancan (13) with some modifications. The reaction mixture contained 40 mM HEPES buffer (pH 8.0), 10 mM MgCl2,7.5mM sodium pyruvate, 10 mM KDA and sonicated cells. The L-KDA aldolase activity was estimated by Bacterial species: characteristics and growth condition Rhizobium sp. RM thiobarbituric acid method (34). Enzyme activity was measured every 30 s for 8 min. and RS used for this study are Vigna radiata root nodule isolates which were maintained routinely on yeast extract mannitol agar (YEMA) (HiMedia Laboratories Glycolaldehyde dehydrogenase (EC 1.2.1.21) Glycolaldehyde dehydroge- Private Limited, Mumbai, India) and preserved at À20C as glycerol stocks. For nase (EC 1.2.1.21) assay was carried out as described by Duncan (14) with some characterization and identification, bacterial isolates were subjected to a series of modifications. The reaction mixture contained 1 mM glycolaldehyde, 2.5 mM standard morphological and biochemical tests as per the Bergey’s manual of sys- NAD, 100 mM sodium glycine buffer (pH 9.5) and sonicated cells. Enzyme activity temic bacteriology (24). was measured every 30 s for 10 min. Molecular characterization based on 16S rRNA gene sequence Molecular Glyoxylate oxidase (EC 1.2.3.5) Glyoxylate oxidase (EC 1.2.3.5) assay was identification was carried out through sequencing of 16S rRNA gene following carried out using DCIP linked reaction system (35). The assay was performed as amplification using universal primers F27 (50-AGA GTT TGA TCA TGG CTC AG-30) (25) described by Rajput et al.