Convergent Evolution of Endosymbiont Differentiation in Dalbergioid and Inverted Repeat-Lacking Clade Legumes Mediated by Nodule-Specific Cysteine-Rich Peptides

Abstract

Nutritional symbiotic interactions require the housing of large numbers of microbial symbionts, which produce essential compounds for the growth of the host. In the legume-rhizobium nitrogen-fixing symbiosis, thousands of rhizobium microsymbionts, called bacteroids, are confined intracellularly within highly specialized symbiotic host cells. In Inverted Repeat-Lacking Clade (IRLC) legumes such as Medicago spp., the bacteroids are kept under control by an arsenal of nodulespecific cysteine-rich (NCR) peptides, which induce the bacteria in an irreversible, strongly elongated, and polyploid state. Here, we show that in Aeschynomene spp. legumes belonging to the more ancient Dalbergioid lineage, bacteroids are elongated or spherical depending on the Aeschynomene spp. and that these bacteroids are terminally differentiated and polyploid, similar to bacteroids in IRLC legumes. Transcriptome, in situ hybridization, and proteome analyses demonstrated that the symbiotic cells in the Aeschynomene spp. nodules produce a large diversity of NCR-like peptides, which are transported to the bacteroids. Blocking NCR transport by RNA interference-mediated inactivation of the secretory pathway inhibits bacteroid differentiation. Together, our results support the view that bacteroid differentiation in the Dalbergioid clade, which likely evolved independently from the bacteroid differentiation in the IRLC clade, is based on very similar mechanisms used by IRLC legumes.

Nodule‐specific PLAT domain proteins are expanded in the Medicago lineage and required for nodulation

“…S10; Laporte et al ., ; Trujillo et al ., ), Aeschynomene NCR‐like peptides (Fig. S11; Czernic et al ., ) and calmodulin‐like proteins (Fig. S12; Liu et al ., ), which have all been studied previously in terms of their nodule‐related function and are known to be specific to certain legume lineages.…”

“…Czernic et al . () concluded that NCR‐like peptides also induce bacteroid terminal differentiation in Aeschynomene legumes, which are closely related to Arachis . The Aeschynomene NCR‐like genes described by Czernic et al .…”

Host-secreted antimicrobial peptide enforces symbiotic selectivity in Medicago truncatula

“…These genes are predominantly expressed in the infected nodule cells. Targeting these defensin-like peptides to symbiosomes is essential for terminal bacteroid differentiation required for nitrogen fixation (19,27). It was speculated that the galegoid legumes such as Medicago have evolved NCR peptides to "enslave" rhizobial microsymbionts as terminally differentiated bacteroids with organelle-like properties optimized for nitrogen fixation (28)(29)(30)(31).…”

Hopanoid lipids: from membranes to plant–bacteria interactions

“…Some of this harshness is necessary for efficient nitrogen fixation, including the creation of a low pH environment to provide protons for nitrogenase 122 and concentrated carbon osmolytes for increased bacteroid metabolism 123 . Other stresses result from host control mechanisms, such as antimicrobial nodule-specific cysteine-rich peptides that change the morphology, proliferation and gene expression of bacteroids to maximize their productivity 124–126 . Bacteroid-containing host cells undergo many rounds of proliferation after the initial internalization event to form the mature root nodule.…”