Pathways associated with FAW damage levels
The pathway analysis was run in four separate tests on FAW damage.
Pathways associated with reduced FAW damage scores at the p <
0.05 level were identified in the 7-day and 14-day ratings datasets, and
pathways associated with increased FAW damage scores were run on the
ratings datasets collected at 7 and 14 days. Following these pathway
analyses, 31 pathways were associated with increased FAW damage: 26 with
the 7-day ratings, 18 with the 14-day ratings, and 5 in common. Another
42 pathways were associated with decreased FAW damage: 29 with the 7-day
ratings, 22 with the 14-day ratings, and 9 in common (Supplemental Table
4). There were no common pathways between the analyses for increased
damage vs. decreased damage, but three related pathways were found
between the increased and decreased analyses. Triacylglycerol
biosynthesis was associated with reduced FAW damage, while
triacylglycerol degradation was associated with increased damage;
phospholipid biosynthesis was associated with increased damage, and
phospholipid conversion to another compound was associated with reduced
damage. On the other hand, UDP-β-L-arabinose biosynthesis was associated
with both increased and decreased damage (via different pathways).
Some pathways share genes, and one or both pathways may become
significantly associated with reductions in FAW damage scores. Which
causes the reduction (or if both do) may not be readily apparent. For
example, the top pathway for damage scores collected on both 7 and 14
days is the sporopollenin precursors biosynthesis pathway (32 genes
total), at p < 0.001 and 0.006, respectively, but the suberin
biosynthesis pathway (55 genes total) is also significantly associated
with a reduction in FAW damage rated at 14 days, at p < 0.05,
and the two pathways have 26 genes in common. The cutin biosynthesis
pathway (42 genes) is significantly associated with reduced damage at 7
days, and cutin and sporopollenin precursors biosynthesis pathways also
have 26 genes in common. However, suberin biosynthesis is not associated
with reduced damage at 7 days and cutin is not associated with reduced
damage at 14 days. All three pathways could plausibly be associated, but
sporopollenin precursors (which are carotenoids and carotenoid esters)
may have better statistical evidence for association. Most pathways with
shared genes create related compounds, such as homoserine and
L-homoserine biosynthesis. Most pathways contained several genes that
jointly contributed to association with the trait. Few pathways had only
one gene contributing to the trait and these, such as acyl-CoA
hydrolysis, generally had only a few genes in the pathway so statistical
evidence that they are actually associated is lower.
Looking at the 31 pathways associated at a more stringent significance
threshold p < 0.02 (Table 2), mechanisms that may be
highlighted by the current study as involved with FAW feeding resistance
include the biosynthesis of carotenes including zeaxanthin and other
pathways that also utilize geranylgeranyl pyrophosphate, including
ent-kaurene and chlorophyll (Fig. 1). These are highly associated with
FAW damage levels, both increasing and decreasing. Other mechanisms of
resistance may include hormone signaling, as the IAA and sterol
biosynthesis (of which brassinosteroids are a subset) as well as the
ent-kaurene biosynthesis pathway (which can be converted to
gibberellins, Fig. 1) were all identified as associated with damage
levels. Reactions associated with FAW damage (Table 2) include the
S-adenosyl-L-methionine cycle, which was associated with increased
damage; acyl-CoA hydrolysis (decreasing damage); and thioredoxins and
glutathione redox reaction (related reactions, associated with decreased
damage). The production of lipids and waxes, including cuticular wax,
phosphatidylethanolamine, and phospholipids (compounds in biological
membranes) and the essential oil linalool and β-caryophyllene), all
associate with increased levels of damage. The biosynthesis of several
other compounds is associated with decreased FAW feeding damage,
including ascorbate, flavin or riboflavin, myo-inositol and phytate, and
the amino acid cysteine. Some compounds associated with an increase in
damage ratings include ammonia, 1,4-dihydroxy-2-naphthoate, the phenolic
compound coumarin, and the amino acids homoserine, methionine, and
homocysteine.
Figure 1 : Many of the pathways associated with Fall Armyworm
leaf damage rating scores were in the carotenoid biosynthesis pathway or
pathways branching off from or downstream of it. Pathways significantly
associated with damage scores are circled in red (solid line circles p
< 0.02; dashed line p < 0.05).