Plant species vary in nectar peroxide, an antimicrobial defense tolerated by some bacteria and yeasts

Summary Floral nectar attracts pollinators but can also harbor microbes including plant pathogens. Nectar contains antimicrobial constituents including hydrogen peroxide, yet it is unclear how widespread nectar hydrogen peroxide might be among plant species or how effective against common nectar microbes. Here, we surveyed 45 flowering plant species distributed across 23 families and reviewed the literature for measurements of 13 other species to assess the field-realistic range of nectar hydrogen peroxide (Aim 1) and explored whether plant defense hormones induce nectar hydrogen peroxide upregulation (Aim 2). Further, we tested the hypotheses that variation in microbial tolerance to peroxide is predicted by microbe isolation environment (Aim 3), that increasing hydrogen peroxide in flowers alters microbial abundance and community assembly (Aim 4), and that microbial community context affects microbial tolerance to peroxide (Aim 5). We found that sampled plants contained nectar hydrogen peroxide ranging from 0 to 2940 µM, although half of the sampled species concentrations were less than 100 µM. While plant defensive hormones did not change the concentration of hydrogen peroxide in floral nectar, enzymatically upregulated concentrations in living flowers significantly reduced microbial growth. Together, our results suggest that nectar peroxide is a common but not pervasive antimicrobial defense among nectar-producing plants. In addition, nectar microbes vary in tolerance and detoxification ability, and co-growth can facilitate the survival and growth of less tolerant species, suggesting a key role for community dynamics in microbial colonization of nectar.