Sinorhizobium melilotiMutants Deficient in Phosphatidylserine Decarboxylase Accumulate Phosphatidylserine and Are Strongly Affected during Symbiosis with Alfalfa

Abstract

Sinorhizobium meliloti contains phosphatidylglycerol, cardiolipin, phosphatidylcholine, and phosphatidylethanolamine (PE) as major membrane lipids. PE is formed in two steps. In the first step, phosphatidylserine synthase (Pss) condenses serine with CDP-diglyceride to form phosphatidylserine (PS), and in the second step, PS is decarboxylated by phosphatidylserine decarboxylase (Psd) to form PE. In this study we identified the sinorhizobial psd gene coding for Psd. A sinorhizobial mutant deficient in psd is unable to form PE but accumulates the anionic phospholipid PS. Properties of PE-deficient mutants lacking either Pss or Psd were compared with those of the S. meliloti wild type. Whereas both PE-deficient mutants grew in a wild-type-like manner on many complex media, they were unable to grow on minimal medium containing high phosphate concentrations. Surprisingly, the psd-deficient mutant could grow on minimal medium containing low concentrations of inorganic phosphate, while the pss-deficient mutant could not. Addition of choline to the minimal medium rescued growth of the pss-deficient mutant, CS111, to some extent but inhibited growth of the psd-deficient mutant, MAV01. When the two distinct PE-deficient mutants were analyzed for their ability to form a nitrogen-fixing root nodule symbiosis with their alfalfa host plant, they behaved strikingly differently. The Pss-deficient mutant, CS111, initiated nodule formation at about the same time point as the wild type but did form about 30% fewer nodules than the wild type. In contrast, the PS-accumulating mutant, MAV01, initiated nodule formation much later than the wild type and formed 90% fewer nodules than the wild type. The few nodules formed by MAV01 seemed to be almost devoid of bacteria and were unable to fix nitrogen. Leaves of alfalfa plants inoculated with the mutant MAV01 were yellowish, indicating that the plants were starved for nitrogen. Therefore, changes in lipid composition, including the accumulation of bacterial PS, prevent the establishment of a nitrogen-fixing root nodule symbiosis.Rhizobia are soil bacteria able to form a symbiosis with legume plants, which leads to the formation of nitrogen-fixing root nodules. The establishment and functioning of this symbiosis are based on the recognition of signal molecules, which are produced by both the bacterial and plant partners. Known recognition factors of the bacterial endosymbiont include nodulation (Nod) factors, extracellular polysaccharides, lipopolysaccharides, K antigens, and cyclic glucans (24, 53). These factors are required for nodule formation, the infection process, and the colonization of the root nodule. Recently it was demonstrated that adequate levels of phosphatidylcholine (PC) are also required in order to allow the formation of a fully functional symbiosis between Bradyrhizobium japonicum and its soybean host plant (35). Under conditions of phosphate limitation, Sinorhizobium meliloti replaces the majority of its phospholipids with phosphorus-free membrane lipids, such as sulfol...

Members of the Sinorhizobium meliloti ChvI regulon identified by a DNA binding screen

“…Proteins encoded by psd (phosphatidylcholine decarboxylase) and pssA (phosphatidylserine synthase) are responsible for the biosynthesis of phosphatidylethanolamine and phosphatidylserine respectively, and both of these phospholipids are also intermediates for phosphatidylcholine biosynthesis [27]. Mutants of these genes exhibit deficiencies in alfalfa symbiosis [27]. Aside from phospholipids synthesis, another link was found between SMc00550 and msbA2 using STRING 8.1.…”

Discovery of a bifunctional cardiolipin/phosphatidylethanolamine synthase in bacteria

“…A) suggesting a crucial role of PE in Xanthomonas . Similarly, a S. meliloti PE deficient mutant showed a drastic growth defect when grown in minimal medium (Sohlenkamp et al ., ) and was impaired in symbiosis with Alfalfa (Vences‐Guzman et al ., ). In contrast, E. coli lacking PE was blocked in cell division resulting in filamentous growth (Mileykovskaya et al ., ).…”

Identification and characterization of SHORTENED UPPERMOST INTERNODE 1, a gene negatively regulating uppermost internode elongation in rice

“…In contrast, in mammalian cells, the serine base-exchange is catalyzed by at least two different enzymes that differ in their substrate specificity. Several PSS enzymes have been found and their encoding PSS genes cloned from prokaryotes, lower eukaryotes, and plants (Delhaize et al 1999;Vences-Guzmán et al 2008;Smith and Bütikofer 2010;Yamaoka et al 2011). A wheat cDNA (TaPSS1) encoding PSS was cloned by Delhaize and co-workers.…”