Cytokinin induces expansin gene expression in Melilotus alba Desr. wild-type and the non-nodulating, non-mycorrhizal (NodMyc) mutant Masym3.

We previously showed that applying 10(-6) M of the cytokinin 6-benzylaminopurine (BAP) to uninoculated roots of Masym3, a Nod(-)Myc(-)Melilotus alba (Desr.) mutant, resulted in the accumulation of MaENOD40 transcripts to levels similar to BAP-treated wild-type roots. In contrast, inoculation with a Nod(-)S. meliloti mutant expressing the trans-zeatin synthase gene of Agrobacterium tumefaciens (Nod(-)/pTZS(+)) did not induce MaENOD40 transcript accumulation in either wild-type or Masym3 roots. However, Masym3 root hairs became swollen at their tips following inoculation with wild-type or Nod(-)/pTZS(+) rhizobia. Because root hair distention and elongation are often correlated with increased expansin activity, we investigated whether BAP treatment or inoculation with Nod(-)/pTZS(+)S. meliloti upregulated the expression of expansin mRNAs. We first determined that treating wild-type roots with 10(-5) or 10(-6) M BAP resulted in greater MaEXPA1 transcript accumulation than treating roots with comparable concentrations of the auxin NAA. When Masym3 roots were treated with 10(-6) M BAP, MaEXPA mRNAs accumulated to levels comparable to wild-type roots. We then showed that MaEXPA1 mRNAs accumulated in wild-type M. alba roots in response to the Nod(-)/pTZS(+)S. meliloti strain. Masym3 mutant roots inoculated with Nod(-)/pTZS(+) rhizobia were also upregulated for MaEXPA1 expression.