Cells of round gall
Discussion of Byers et al. 1976:
Needle morphology
High levels of auxin and/or gibberellin will cause cell expansion (hypertrophy) and cell
division (hyperplasia) in many plants (Sachs, 1961; Jablonski & Skoog, 1954; Nitsch, 1968). The
mesophyll parenchyma cells of galled needles show a remarkable hypertrophy when compared to
cells of young needles (Fig. 8A and B). The average mesophyll parenchyma cell in the round gall increased 13 times in volume
and divided 1.5 times more often than a cell from a young needle (Table 1). The results, however, were
calculated assuming a spherical cell which only approximates the actual cell shape. There also
appeared to be an alteration of plastids and the chloroplasts may have undergone some type of
degradation involving the loss of chlorophyll. However, the latter may have been an artifact
of the staining technique.
The stunting of the needle length (from 3/4 to 1/2 or normal) during gall formation is partly due to
transverse growth to cells as opposed to longitudinal growth. Burg & Burg (1966) have reported that auxin
interacting with ethylene promotes transverse growth of cells. Therefore, the
high levels of auxin found in these galls may possibly induce ethylene production and help stunt
needle elongation.
The cellulose content of epidermal cell walls was found to decrease in round galls as determined by inspection of cross sections with a polarizing microscope
(Stamm, 1964). Auxin has been reported to induce several polysaccharidases such as Beta-1,3 glucanase and exogalactanases (Cleland, 1971).
These hydrolase enzymes could weaken the cell walls, allowing expansion by decreasing the amounts and lengths of
cellulose. Thus, some of the increase in cell size may have resulted from an unfolding of the inner
ridges of normal cells. The cellular changes in round galls can be explained by increased levels of auxin and gibberellin.
Cytokinins also may be involved in allowing cell hyperplasia to occur but auxin alone will cause the cell
to automatically divide once it becomes too large (Jablonski & Skoog, 1954).
Numerous starch grains, probably within chloroplasts, were found throughout the mesophyll of the round gall but were absent in young
needles. The starch grains were easily observed due to an "x" interference pattern in
polarized light because of the concentric layers of starch (Winton, 1906). This starch may provide
a rich source of food for the midge larvae. However, Schnetzler et al. (1962), working on a cynipid gall of oak found that when IAA (Indole acetic acid = auxin) was applied to forming galls
from which the larvae were removed, the starch reserves disappeared while the gall continued to enlarge. Gibberellin acid in barley seeds causes starch
breakdown but in pinyon needles the physiological effects are probably different.
Auxins and gibberellin-like substances were found to increase dramatically as the normal needle begins
to form a gall. In the still-forming gall tissue there was about as much as 17 times more auxin and 21 times more gibberellin-like substances per gall than per
normal needle. Calculated on a weight basis, gall tissue had from 2.2 to 3.7 more IAA equivalents and from 2.4 to 4.8 times more GA3 equivalents than
normal tissue. The quantities of these growth hormones are not known with certainty to be
physiologically active in pinyon gall formation although the amounts are within the range
of quantities found in other plant tissues (Overbeek, 1966). If the gibberellin-like substances and auxin are increased due to secondary response from gall
formation, they still could affect the morphogenesis of the gall.
This study indicates that the cecidomyiid larvae probably do not secrete auxin, but my secrete gibberellins. However, the gibberellin activity found in larval extracts may
have been the insect molting hormone, ecdysone, producing gibberellin-like activity in the bioassay.
These needle galls senesce and fall off the pinyon tree during the winter following gall growth. Most needles remain living on the tree from 4 to 9 years;
therefore auxin and/or gibberellin may induce senescent changes, or change the physiology of the gall to the extent that death results. Auxin apparently has been
shown to induce ethylene production in several plant tissues and to alter senescence and abscission (Abeles & Rubinstein, 1964; Burg & Burg, 1966; Morgan & Hall, 1962). Therefore,
auxin could cause changes in cell growth and consequently gall formation and later induce ethylene production and
senescence. The transverse gall growth may also be aided by ethylene.
Cellular hypertrophy and hyperplasia in the gall possibly support the theory that auxin and gibberellin-like materials cause gall formation. Round gall cells increase in volume at least 13 fold from normal needle cells. The
amount of cellulose in epidermal cells and osmotic concentration of cellular contents decreased in
round galls showing classical effects of auxin and gibberellin. Other cecidomyiid pinyon galls such as the spindle gall (Houseweart & Brewer, 1972) are probably formed in much the same way as the round gall, since the
causative agents are closely related. However, different hormone concentrations and different times of hormone inducement may cause the different morphological galls.
References in:
Byers, J.A., Brewer, J.W., & Denna, D.W. 1976. Plant growth hormones in
pinyon insect galls. Marcellia 39:125-134.
Chemical Ecology
