The occurrence of hydroxamic acids (Hx) and their affects on take-all have
been investigated in this study. An improved HPLC procedure for the
separation and quantification of Hx in wheat, rye and triticale roots was
established. This method completely separated 2,4-Dihydroxy—1,4—
benzoxazin—3-one (DIBOA), 2 , 4-Dihydroxy - 7- methoxy - 1 4- benzoxazin
-3-one (DIMBOA), 2(3)-benzoxazolinone (BOA) and 6-
methoxybenzoxazolinone (MBOA) within 17 min.
DIMBOA was the only Hx found in wheat roots, whereas both DIMBOA
and DIBOA were present in the roots of triticale and rye. The Hx content of
whole roots of wheat, rye and triticale reached a maximum 3 to 4 days after
germination, depending on species. The DIMBOA content of wheat roots
ranged from 0.4 to 1.5 umoles / g f.wt in the varieties studied. The DIMBOA
content of the triticale varieties ranged from 0.9 to 2.0 umoles/ g f.wt, and
DIBOA from 0.26 to 1.1 umoles / g f.wt. DIMBOA concentrations in rye
roots ranged from 0.3 to 0.5 umoles/ g f.wt, whereas DIBOA levels ranged
from zero to 1.1 umoles/g.f.wt.
The Hx content of Wheat, rye and triticale roots was highest in the
youngest parts of the root. The root tip of these cereals always contained
significantly higher levels of Hx than the older parts of the root.
When extracts prepared from triticale and rye roots were incorporated into
the nutrient media, growth of two isolates of Gaeumannomyces graminis
var.tritici (th) (E31 and WP 28) was inhibited. Similar extracts prepared
from wheat did not inhibit the growth of th. The fungal strain WP 28
actually grew more rapidly on medium containing extracts from wheat
(cv. Sunstar) roots. The inhibitory effect of triticale and rye extracts was
attributed to the presence of DIBOA. The inhibitory effect of these particular extracts correlated to the resistance of the respective plant to take-all in the field as reported by Hollins et al . (1986).
Hydroxamic acids inhibited the growth of th when incorporated into the
growth media. DIBOA and BOA significantly inhibited the growth of both
strains of the fungus at concentrations as low as 0.5 mM. DIMBOA and 6-
methoxybenzoxazolinone (MBOA) did not significantly inhibit the growth
of th EBI at 0.5 mM. However, at higher concentrations DIMBOA and
MBOA were inhibitory. The Hx at the concentrations studied (0.5 to 5.0
mM) were only fungistatic, though, as the fungal colonies resumed
growth when removed from the inhibitor. There was no significant
difference in the growth of the two fungal isolates on media containing
extracts or Hx.
The wheat variety with the lowest DIMBOA content was the most
susceptible to infection by the fungus. Wheat contained only DIMBOA,
which was undetectable by 21 days. The cereals, rye and triticale, which
contined both DIMBOA and DIBOA were more resistant to the take-all
fungus. Hydroxamic acid levels in triticale and rye were low or not
detectable at 21 and 35 days. Rye was the more resistant species out of the
two. Increased synthesis of Hx was not observed in roots of these cereals as
a response to infection by the take—all fungus. On the basis of these results,
it was concluded that DIBOA was more effective than DIMBOA in
conferring resistance to take-all.
Wheat varieties which had an individual rye chromosome inserted were
assayed for Hx content. All lines contained DIMBOA but one line (CSB 5R)
also contained DIBOA. This preliminary result indicates that the gene(s)
responsible for DIBOA synthesis may be on chromosome 5 of rye
