O-Methylated Isoflavones Induce nod Genes of Mesorhizobium ciceri and Pratensein Promotes Nodulation in Chickpea.

Legume plants form symbiotic relationships with rhizobia, which allow plants to utilize atmospheric nitrogen as a nutrient. This symbiosis is initiated by secretion of specific signaling metabolites from the roots, which induce the expression of nod genes in rhizobia. These metabolites are called nod gene inducers (NGIs), and various flavonoids have been found to act as NGIs. However, NGIs of chickpea, the second major pulse crop, remain elusive. We conducted untargeted metabolome analysis of chickpea root exudates to explore metabolites with increased secretion under nitrogen deficiency. Principal component (PC) analysis showed a clear difference between nitrogen deficiency and control, with PC1 alone accounting for 37.5% of the variance. The intensity of two features with the highest PC1 loading values significantly increased under nitrogen deficiency; two prominent peaks were identified as O-methylated isoflavones, pratensein and biochanin A. RNA-seq analysis showed that they induce nodABC gene expression in the Mesorhizobium ciceri symbiont, suggesting that pratensein and biochanin A are chickpea NGIs. Pratensein applied concurrently with M. ciceri at sowing promoted chickpea nodulation. These results demonstrate that pratensein and biochanin A are chickpea NGIs, and pratensein can be useful for increasing nodulation efficiency in chickpea production.

Glycosylphosphatidylinositol-anchoring is required for the proper transport and extensive glycosylation of a classical arabinogalactan protein precursor in tobacco BY-2 cells.

Many precursors of plant arabinogalactan proteins (AGPs) contain a C-terminal glycosylphosphatidylinositol (GPI)-anchoring signal. Using NtAGP1, a classical tobacco AGP, as a model, and green fluorescent protein (GFP) and sweet potato sporamin (SPO) as tags, we analyzed the localization and modification of AGP and its mutant without GPI-anchoring signal (AGPΔC) in tobacco BY-2 cells. The NtAGP1 fusion proteins migrated as large smear on SDS-polyacrylamide gel, and these proteins also localized preferentially to the plasma membrane. In contrast, fusions of AGPΔC with GFP and SPO yielded several forms: The largest were secreted, whereas others were recovered in the endomembrane organelles, including vacuoles. Comparison of the glycan structures of the microsomal SPO-AGP and the secreted SPO-AGPΔC using antibodies against the glycan epitopes of AGP indicated that the glycan structures of these proteins are different. These observations indicate that GPI-anchoring is required for the proper transport and glycosylation of the AGP precursor.

Soyasaponins: A New Class of Root Exudates in Soybean (Glycine max).

Root exudates are plant metabolites secreted from the roots into the soil. These exudates are involved in many important biological processes, including acquisition of nutrients, defense and signaling to rhizosphere bacteria, such as isoflavones of soybean crucial for the symbiosis with rhizobium. Less is known, however, about other types of root exudates. This study shows that soybean roots secrete large amounts of soyasaponins (triterpenoid glycosides) as root exudates. The soyasaponins are classified into four groups, with group A being the most secreted of these compounds, whereas DDMP (2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one) soyasaponins is the group showing greatest accumulation in root tissues, suggesting a selection system for secreted compounds. Time-course experiments showed that the soyasaponin secretion peaked during early vegetative stages. In particular, soyasaponin Ah was the major compound secreted by soybean roots, whereas the deacetylated derivative Af was the major compound secreted specifically during the VE stage. The secretion of soyasaponins containing glycosyl moieties is an apparent loss of photosynthates. This phenomenon has been also observed in other legume species, although the composition of secreted soyasaponins is plant species dependent. The identification of triterpenoid saponins as major metabolites in legume root exudates will provide novel insights into chemical signaling in the rhizosphere between plants and other organisms.