( ( ( The ( mutualistic relationship ) ) ( between ( plants humans ) ) ) ( ( is ( a ( central theme ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (JJ mutualistic) (NN relationship)) (PP (IN between) (NP (NNS plants) (NNS humans)))) (VP (VBZ is) (NP (DT a) (JJ central) (NN theme))) (. .)))
The mutualistic relationship between plants humans is a central theme.
(ROOT (S (NP (JJ Mutualistic) (NNS relationships)) (VP (VBP are) (NP (NP (DT a) (NN kind)) (PP (IN of) (NP (NP (JJ beneficial) (NNS relationships)) (PP (IN with) (NP (JJ other) (NNS organisms))))) (SBAR (WHNP (WDT that)) (S (VP (VBP are) (ADJP (JJ common) (PP (IN in) (NP (DT the) (NN plant) (NN kingdom))))))))) (. .)))