Host‐plant resistance of sorghum: Differential hydrolysis of sorghum pectic substances by polysaccharases of greenbug biotypes (Schizaphis graminum, homoptera: Aphididae)

Abstract

Pectic substances extracted from different varieties of sorghum are hydrolyzed at differing rates by unfractionated polysaccharases isolated from two biotypes (C, GBC; and E, GBE) of the sorghum pest, Schizaphis graminurn (the greenbug). A higher degree of susceptibility of a sorghum variety i s associated with a greater rate of hydrolysis of sorghum pectic substances by a greenbug biotype. increases in the specific activity of polysaccharases on the pectic substances from a resistant sorghum variety are dependent on the duration that a biotype is maintained as a colony on that variety.Polysaccharase activity of CBE on arabinogaiactan was significantly greater than GBC. However, there were no differences between the biotypes on the depolymerization of a variety of other plant matrix polysaccharides and a synthetic polysaccharide. The sequence of substrates of increasing refractoriness to hydrolysis are: arabinogalactan < microcrystalline cellulose < xylan c pectin < 2,3-diacetyl pectin < a-1,4-galacturonan. Pectic substances from sorghum varieties resistant to GBC but susceptible to GBE are relatively lower in arabinogalactan with elevated levels of uronic acid (UA) compared to varieties susceptible to both biotypes. A sorghum variety resistant to both GBC and GBE was lowest in levels of arabinogalactan, highest in UA, and highest in fructan content, which in the other varieties occurred only in trace amounts. Pectic composition of rhamnose, xylose, and glucose showed no relationship to resistance. Bound phenolics (potential inhibitors of enzyme activity) were not detected in any of the sorghum pectic substances. The relationship of plant matrix polysaccharides to host-plant aphid biotype compatibility is discussed.

Activity of endo-polygalacturonases in mirid bugs (Heteroptera: Miridae) and their inhibition by plant cell wall proteins (PGIPs)

“…Among piercing-sucking species, no PG activity was found in Metopoplax ditomoides (Heteroptera: Lygaeidae), Nezara viridula (L.) (Heteroptera: Pentatomidae) or Aphis fabae Scopoli (Hemiptera: Aphididae), although PGs have been recorded for other aphid species (Adams & McAllan, 1956;Laurema & Nuorteva, 1961;Miles, 1972;Campbell & Dreyer, 1985;Ma et al, 1990;Tjallingii & Esch, 1993;Miles, 1999;Cherqui & Tjallingii, 2000).…”

Possible involvement of the phloem sealing system in the acceptance of a plant as host by an aphid

“…The protein subunits in sieve-tube exudates clearly differ from species to species [13,32]. Aphid saliva characteristics also vary from one species to another [33], and even within biotypes (activity of pectinases [34]). Thus, some specificity in aphid-plant interactions could arise from compatible relationships between the phloem sealing system and the aphid saliva composition.…”

Life History, Functional Anatomy, Feeding and Mating Behavior

“…While again presence of an enzyme in whole body homogenates does not necessarily imply its presence in saliva, pectin is an insoluble polysaccharide found primarily in plants, not animals; therefore, it seems likely that if whiteflies produce a pectinase, it would have to be injected into the plant via the saliva in order to have an effect. Furthermore, pectinases have been detected in the saliva of many, although not all, aphid species that have been examined (Adams and McAllan 1958;McAllan and Adams 1961;Campbell and Dreyer 1985;Miles 1999;Cherqui and Tjallingii 2000). Pectinases have been hypothesized to aid stylet penetration through the pectin-rich middle lamella that serves as the intercellular cement between adjacent cell walls in plant tissue (McAllan and Adams 1961;Campbell and Dreyer 1985), although this hypothesis has been challenged (Tjallingii and Hogen Esch 1993).…”

“…Furthermore, pectinases have been detected in the saliva of many, although not all, aphid species that have been examined (Adams and McAllan 1958;McAllan and Adams 1961;Campbell and Dreyer 1985;Miles 1999;Cherqui and Tjallingii 2000). Pectinases have been hypothesized to aid stylet penetration through the pectin-rich middle lamella that serves as the intercellular cement between adjacent cell walls in plant tissue (McAllan and Adams 1961;Campbell and Dreyer 1985), although this hypothesis has been challenged (Tjallingii and Hogen Esch 1993). To date, the only enzyme definitively proven to be a component of whitefly saliva is alkaline phosphatase that was detected in adult Bemisia principal and accessory salivary glands (especially the former) as well as in saliva secreted into an artificial medium (Funk 2001).…”