Foliar endophytic fungi alter patterns of nitrogen uptake and distribution in Theobroma cacao

Colonization by foliar endophytic fungi can affect the expression of host plant defenses and other ecologically important traits. However, whether endophyte colonization affects the uptake or redistribution of resources within and among host plant tissues remains unstudied. We inoculated leaves of Theobroma cacao with four common colonizers that range in their effect from protective to pathogenic (Colletotrichum tropicale, Pestalotiopsis sp., Colletotrichum theobromicola, or Phytophthora palmivora). We pulsed the soil with nitrogen-15 (¹⁵ N) and then traced ¹⁵ N uptake and its subsequent distribution to whole plants and individual leaves. At a whole-plant level, C. tropicale-inoculated plants showed significantly greater ¹⁵ N uptake than endophyte-free plants did in the same pot. Among leaves within plants, younger leaves were particularly enriched in ¹⁵ N, but endophyte inoculation at the individual leaf level did not alter ¹⁵ N distribution within plants. However, leaves co-inoculated with pathogenic Phytophthora and protective C. tropicale experienced significantly elevated ¹⁵ N content as pathogen damage increased, compared with leaves inoculated only with the pathogen. Further, endophyte-pathogen co-infection also increased total plant biomass. Our results indicate that colonization by foliar endophytes significantly affects N uptake and distribution among and within host plants in ways that appear to be context dependent on other microbiome components.