Integrated hormone and transcriptome analyses reveal the mechanisms underlying growth and development changes of Tamarix chinensis parasitized by Cistanche tubulosa

Cistanche tubulosa (Schenk) R. Wight, a precious traditional Chinese medicine, exclusively parasitizes on the roots of Tamarix spp. Parasitism usually causes various morphological and physiological changes in host plants. However, the effects of C . tubulosa parasitism on root development and flowering of Tamarix chinensis Lour. have rarely been studied. Here, the phenotypic characteristics and anatomical structure of T. chinensis roots and leaves were compared between parasitized and unparasitized by C. tubulosa . In parasitized T. chinensis , the plant height and internode length were obviously shorter, leaves appeared dark green with higher chlorophyll content, and the flowering percentage dramatically decreased. Parasitism increased phloem and root thickness, and thickened with growing and number of C. tubulosa . The endogenous hormone levels and gene expression profiles in roots and leaves were further determined. The results showed that the root thickening in T. chinensis was primarily caused by a substantial increase in trans-Zeatin (tZ) and cis-Zeatin (cZ) type cytokinins (CKs), probably originating from C. tubulosa . Additionally, C. tubulosa parasitism triggered salicylic acid-mediated defense response in roots. In leaves of parasitized T. chinensis , decreased levels of senescence-inducing hormones, abscisic acid, salicylic acid, ethylene precursor 1-aminocyclopropane-1-carboxylic acid and jasmonic acid, potentially slowing down the aging process. Furthermore, genes associated with chlorophyll biosynthesis and photosynthesis were strongly activated. The tZ- and cZ-type CKs in PL were extremely low, and the key floral-inducing MADS-box gene AGAMOUS-LIKE 24 ( AGL24 ), WUSCHEL-RELATED HOMEOBOX ( WOX ), YABBY ( YAB ), and transcription factors promoting flower primordium initiation were all significantly down-regulated. It was speculated that the combined effects of hormones and flowering regulators resulted in non-flowering phenotype in parasitized T. chinensis . Taken together, this study illustrated the underlying mechanisms of root thickening and non-flowering phenotype in parasitized T. chinensis at physiological, biochemical, and molecular levels, providing novel molecular insights in plant parasitism. • Cistanche tubulosa parasitism affect root growth and flowering of Tamarix chinensis . • Zeatin-type cytokinins derived from Cistanche tubulosa promote host root thicken. • Parasitism induces hormone changes and expression of key floral-inducing genes. • Hormones and flowering factors synergistically inhibit flowering of host plants.