Figure 6.

U. maydis Penetrates the Plant Surface before the Appressorial Tip Compartment Has Been Sealed by Septum Formation.

(A) A typical appressorium formed by wild-type strains (1 and 2) or by Δgas1 mutant strains (3 and 4). Young maize seedlings were infected with a combination of wild-type (JSD2 and JSF6) or Δgas1 mutant (JSA8 and JSB11) strains expressing cytoplasmic GFP. Appressoria were stained with calcofluor 1 d after inoculation and visualized by fluorescence microscopy (1 and 3) or GFP fluorescence (2 and 4). In wild-type appressoria, regular septation continues up to the appressorial tip (1, arrows), which is filled with cytoplasm (2). In appressoria of Δgas1 strains, the regular septation stops before reaching the tip (3, arrows), leaving an elongated tip cell with irregular wall deposits (3), which is filled with cytoplasm (4).

(B) Wild-type appressoria penetrate the plant surface before septation has reached the appressorial tip. A penetrating early appressorium of a wild-type dikaryon (left column) and a Δgas1 dikaryon (right column) expressing cytoplasmic GFP as visualized by calcofluor staining (1 and 4), GFP fluorescence (2 and 5), and GFP fluorescence in a different focal plane (3 and 6). At this early stage, both wild-type and Δgas1 hyphae show an elongated appressorial cell on the plant surface that is filled with cytoplasm. The cytoplasm reaches into the plant tissue. Bars = 10 μm.