Advances in the Study of Vesicular-Arbuscular Mycorrhiza

The study of vesicular-arbuscular (VA) mycorrhiza is expanding rapidly. Since Gerdemann's review in 1968, over a hundred papers have been pub­ lished. During previous five year periods numbers were: 14 (1930-4), 22 1935-9), 17 (1948-52), 43 (1953-7), 56 (1958-62) and 40 (1963-7). These are small numbers considering how long it has been known that VA mycorrhiza are probably the most widespread root infections of plants. With some justification they have been described as the "mal aimee des microbiolo­ gistes" (22). The increase in publications has been accompanied by a shift in subject matter. Most papers, until recently, described the anatomy and recorded the occurrence of VA mycorrhiza, and many efforts were made to culture the fungi; since 1968, 37 papers have dealt with effects of the infection on plant growth. Several factors probably account for the increased popularity of the sub­ ject. The long-standing speculation about the identity of VA endophytes (47, 56) has largely been resolved in favor of one or another species of Endogone (32,46,95). Very impure inocula consisting of infected roots or of soil con­ taining a normal population of other soil micro-organisms, have been re­ placed by Endogone spores, sporocarps, or "sterilized" soil inoculated with them in the presence of a host plant. Such inocula now regularly produce typical VA infections in experimental plants. With improved techniques, very striking effects of inoculation on plant growth and phosphate uptake have . been demonstrated beyond doubt, and this has led to studies of the uptake mechanism and the source of the extra phosphate. These results have also stimulated interest in the fungi themselves, their ecology and taxonomy. The technique of clearing roots in KOH before staining (24, 77, 120, 121) is now