MIC3 Over-Expression Reduces Cotton Susceptibility to Root-Knot Nematode

The inheritance of root-knot nematode (RKN) resistance in cotton has been the focus of much research; however, the mechanism of the resistance at the molecular level remains largely unknown.  To date, increased transcript and protein levels of MIC3 in galls of resistant plants remains the only example of a gene whose expression is correlated with the onset of RKN resistance.  In this report, we further validate this correlation via over-expression of MIC3 in the RKN-susceptible line Coker312.  A MIC3 overexpression cassette driven by the CaMV35S promoter was constructed using the binary vector pBI121.  Transgenic cotton lines harboring this cassette were created using Agrobacterium tumefaciens.  Six (6) homozygous T2 lines were identified that showed a range of elevated MIC3 transcript and protein levels in roots and leaves compared to non-transgenic controls.  For the RKN assays, data from two independent experiments showed that both high and low levels of MIC3 overexpression affected RKN egg production.  We found that the transgenic line 11-1-1Top, which showed the highest level of MIC3 transcript in uninfected roots, reduced RKN eggs/plant by 70% compared to the non-transgenic susceptible control Coker312.  In contrast, the transgenic line 14-11-1A, which showed the lowest level of MIC3 overexpression, reduced RKN eggs/plant by only 35% compared to Coker312.  None of the transgenic lines showed gall index scores that were significantly different from the susceptible controls.  In contrast to RKN, MIC3 overexpression did not affect plant susceptibility to the reniform nematode.