**Gene Expression Profiling Reveals Enhanced Defense Responses in an Invasive Weed Compared to Its Native Congener During Pathogenesis**

**Bharani Manoharan 1,2, Shan-Shan Qi 1,2,3, Vignesh Dhandapani <sup>4</sup> , Qi Chen <sup>2</sup> , Susan Rutherford <sup>5</sup> , Justin SH Wan 3,5, Sridharan Jegadeesan <sup>6</sup> , Hong-Yu Yang <sup>2</sup> , Qin Li <sup>2</sup> , Jian Li 1,2, Zhi-Cong Dai 1,2,3,7,\* and Dao-Lin Du 1,2,\***


Received: 23 August 2019; Accepted: 1 October 2019; Published: 3 October 2019

**Abstract:** Invasive plants are a huge burden on the environment, and modify local ecosystems by affecting the indigenous biodiversity. Invasive plants are generally less affected by pathogens, although the underlying molecular mechanisms responsible for their enhanced resistance are unknown. We investigated expression profiles of three defense hormones (salicylic acid, jasmonic acid, and ethylene) and their associated genes in the invasive weed, *Alternanthera philoxeroides*, and its native congener, *A*. *sessilis*, after inoculation with *Rhizoctonia solani*. Pathogenicity tests showed significantly slower disease progression in *A*. *philoxeroides* compared to *A. sessilis*. Expression analyses revealed jasmonic acid (JA) and ethylene (ET) expressions were differentially regulated between *A*. *philoxeroides* and *A. sessilis*, with the former having prominent antagonistic cross-talk between salicylic acid (SA) and JA, and the latter showing weak or no cross-talk during disease development. We also found that JA levels decreased and SA levels increased during disease development in *A*. *philoxeroides*. Variations in hormonal gene expression between the invasive and native species (including interspecific differences in the strength of antagonistic cross-talk) were identified during *R. solani* pathogenesis. Thus, plant hormones and their cross-talk signaling may improve the resistance of invasive *A*. *philoxeroides* to pathogens, which has implications for other invasive species during the invasion process.

**Keywords:** invasive weed; *Rhizoctonia solani*; gene expression; phytohormone signaling; systemic signal; plant defense
