The research paper by Nahoko Uchi, Mitsutaka Fukudome, Narumi Nozaki, Miyuzu Suzuki, Ken-ichi Osuki, Shuji Shigenobu, Toshiki Uchiumi titled
“Antimicrobial Activities of Cysteine-rich Peptides Specific to Bacteriocytes of the Pea Aphid Acyrthosiphon pisum” (Vol. 34, No. 2, p. 155-160) wins The Most Valuable Paper of The Year 2019 in Microbes and Environments.

Controlling cell growth and cell division of symbionts is essential for a host organism to maintain a successful endosymbiotic relationship. While some legumes are known to use Nodule-specific Cysteine-Rich (NCR) peptides to control the growth of endosymbiotic bacteria in their nodules, the molecular mechanisms remain uncertain. In any other host-symbionts system, we also do not know much about how the hosts regulate the symbionts.

Aphids and Buchnera is a well-known obligate endosymbiotic system between insects and bacteria. Buchnera, a gammaproteobacterial endosymbiont, has lost many genes from the genome, resulting in being incapable of living outside of the host bacteriocytes. In this study, the authors chemically synthesized six of seven Bacteriocyte-specific Cysteine-Rich (BCR) peptides that are known to express in bacteriocyte of a pea aphid Acyrthosiphon pisum, and examined the impacts on the growth of E. coli and Sinorhizobium meliloti. As a result, four BCR peptides exhibited antimicrobial activities on E. coli and the three on S. meliloti. In addition, cell elongation and increase of membrane permeability were also observed by the addition of the BCR peptides. Moreover, while a membrane protein BacA is known for involving in resistance to NCR peptides in S. meliloti, the authors observed higher sensitivity of a homologue of bacA (called sbtA) mutant of E. coli to BCR peptides compared to the wild type strain. This is the first report suggesting the occurrence of parallel evolution of cysteine-rich peptides in an insect and a plant for the regulation of endosymbionts. The peptides also likely function in both obligately and facultatively alpha- and gammaproteobacterial symbionts. We do not know about the distribution of such cysteine-rich peptides in other endosymbiotic systems, but the parallel evolution in the distinct lineages suggests that this system functions in diverse endosymbiotic systems. This study is quite exciting and provides novel insights into the molecular mechanisms of host-endosymbiont relationship. We all are excited to see the following reports from the group of authors.