Differential RNA expression of alpha-expansin gene family members in the parasitic angiosperm Triphysaria versicolor (Scrophulariaceae).

Haustoria are parasitic plant specific organs that locate, attach to, and invade host plant tissues. Parasitic species of the Scrophulariaceae develop haustoria on their roots in response to chemical signals released by host plant roots. Haustorium development was induced in vitro in roots of the parasitic Scrophulariaceae Triphysaria versicolor by treating them with exudates obtained from maize roots, the chemical 2,6-dimethoxybenzoquinone (DMBQ) or the cytokinin 6-benzylaminopurine (BAP). Morphological responses of T. versicolor roots to these haustoria inducing factors (HIFs) included localized swelling and epidermal hair proliferation near the root tips. These responses were not observed when roots of the non-parasitic Scrophulariaceae Lindenbergia muraria were similarly treated. Because expansin proteins are closely associated with plant cell wall expansion and growth, we examined the expression of expansin genes in response to HIFs. We isolated cDNAs homologous to transcripts encoding three distinct alpha-expansin proteins in T. versicolor. Northern-blot analyses indicated that these transcripts were differentially abundant in different tissues. Steady-state levels of two expansin transcripts increased in T. versicolor roots exposed to BAP, but not DMBQ or maize root exudates. Expansin transcript abundance also increased in L. muraria in response to BAP treatment. These results suggest that the expansins examined fulfill functions distinct from haustorium development.