Glucose-dependent acidification strategy by nectar-dwelling bacteria mediates pollen tube burst.

Abstract

Floral nectary gland serves as an ecological hub, including a habitat for microorganisms introduced by pollinators. Although nectar supplies abundant carbon sources, it is limited in nitrogen sources. To address this unbalanced nutritional condition, it is considered that some nectar-dwelling bacteria including those belong to genus Acinetobacter obtain nutrients from pollen protoplasm by inducing pollen tube burst. Here, using gas chromatography-mass spectrometry, we identify gluconic acid in A. nectaris and A. pollinis as the substance responsible for triggering pollen tube burst via acidification of the surrounding environments. Through whole-genome sequencing and functional analysis, we identify a membrane-bound pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase which produces gluconic acid. Notably, nectar-dwelling Acinetobacter species consistently retain PQQ biosynthetic genes, and expression of the PQQ precursor peptide gene are upregulated in the presence of glucose, suggesting that these bacteria can synthesize PQQ and gluconic acid immediately upon entering the nectary gland. Taken together with previous findings, this glucose-dependent acidification is beneficial for both efficient carbon source utilization and nutrient landscape modification, a bacterial adaptive strategy well-suited to the competitive nectar environment.