# Cysteine-rich receptor-like secreted protein 1 promotes intercellular infection and enhances nodulation in Aeschynomene indica

## TL;DR

This study identifies a protein that helps bacteria infect plants through cracks in roots, improving plant growth and nodulation.

## Contribution

The study reveals a novel role for CRRSP1 in promoting intercellular infection and nodulation in Aeschynomene indica.

## Key findings

- Overexpression of CRRSP1 increases nodule number and plant growth in Aeschynomene indica.
- Exogenous CRRSP1 promotes rhizobial attachment and biofilm formation at root cracks.
- CRRSP1 enhances rhizobial motility and lateral root crack colonization.

## Abstract

Nitrogen-fixing bacteria establish symbiotic relationships with their host plants via two different entry systems: root hair-mediated (intracellular) entry and intercellular entry. However, the molecular mechanisms underlying the intercellular entry system have received relatively little research attention. In this study, we compared the transcriptomes of the nodules and roots of Myrica rubra, which forms an ancient type of symbiosis with Frankia via intercellular entry. We found that cysteine-rich receptor-like secreted protein 1 (CRRSP1) was highly upregulated in M. rubra nodules. We then investigated the function of MrCRRSP1 in Aeschynomene indica, which establishes symbiosis with Bradyrhizobium sp. ORS285 through an intercellular entry system. The overexpression of MrCRRSP1 and AiCRRSP1 in A. indica enhanced the nodule number and plant growth. Exogenous application of glutathione S-transferase (GST)-tagged MrCRRSP1 and AiCRRSP1 in A. indica promoted rhizobial attachment at cracks in the lateral root base, as well as rhizobial motility and biofilm formation. These results suggest that CRRSP1 promotes nodulation by enhancing rhizobial attachment to lateral root cracks. In addition to providing new insights into the molecular mechanisms underlying nodule formation through intercellular entry, this research enhances our understanding of actinorhizal plant–Frankia symbiosis.