Figure 4.

In situ localization of mRNA transcript for AP (A and B) and histochemical localization of ascorbate (C and D) in alfalfa nodules. A, In situ hybridization with antisense RNA probe for AP showing enrichment in the nodule endodermis when viewed by dark-field illumination. B, Similar in situ hybridization showing high levels of mRNA labeling in an infected cell (I) and lower levels in an adjacent, uninfected cell (U). The label is evident as small bright dots. Other features include numerous bacteroids (nearly continuous bumps in I) and starch grains in uninfected cells. C, Ag particles precipitated from AgNO₃, indicating the presence of ascorbate in high levels in the endodermis region. D, Negative control for C in which the tissue was preincubated in pH 9.0 buffer to oxidize the ascorbate before exposure to AgNO₃. Other abbreviations are as in Figure 2. Bar indicates 100 μm in A, C, and D; 10 μm in B.