Submitted to: Physiological and Molecular Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/12/2002
Publication Date: 6/1/2002
Citation: Interpretive Summary: To solve a mystery you need to collect and interpret clues. The mystery here is, how do plants resist disease? A particular chemical test, the Weisner test, reveals striking differences between susceptible and disease-resistant plant tissues. Weisner developed this test in the late 19th century, and it has proven to be diagnostic for resistant tissues in a huge variety of plants, including ferns, pines, palms, and many of our food and fiber crops. But what does this test tell us about plant resistance? In healthy plants the Weisner test is diagnostic for lignin, the material that makes wood rigid and cherry pits hard. For decades plant scientists have believed that the material produced to resist disease was also lignin. We have isolated large quantities of this disease resisting material from squash fruit, purified it, and found that its properties are very different from those of lignin. When boiled in water, the material releases an aldehyde, which is responsible for its reaction in the Weisner test. Lignin does not do this. By reexamining the clues, we have revealed a new mystery, what is this disease resisting substance? Our current thinking is that plants produce a gum infused with toxic aldehydes. Further experimentation is needed to be certain, but by correctly interpreting the clues, we will gain a better understanding of plant resistance.
Technical Abstract: Pathological tissues from a variety of plants turn red when treated with the histochemical reagent phloroglucinol-HCl(PG-HCl). This induced PG-HCl reacting material has been termed both wound gum and, more recently, induced lignin or lignin-like material. Previous chemical analyses, based on lignin models, concluded that the induced material was an unusual lignin, derived primarily from p-coumaryl alcohol. Recent findings contrary to this interpretation suggest the wound gum model may have merit. To help resolve the identity of the induced material, we explored the use of alternative approaches. In this paper we describe a method for purifying the PG-HCl reactive material from solvent washed tissue preparations and quantify the amount of p-coumaryl aldehyde released by hot water extraction. Fresh acorn squash fruit wall tissue (which contains none of the induced material) and tissue elicited with pectinase (to promote the formation of the induced material) and incubated at 27 C and >95% RH for 24 to 72 h, was extensively washed with organic solvents. Driselase and pectinase were used to remove cell wall material, and DMSO was used to dissolve starch. Refluxing the purified material in water released 8100 plus/minus 1100 ug p-coumaryl aldehyde/g from samples of elicited tissue incubated for 48 h, whereas <6 ug p-coumaryl aldehyde/g was release from preparations of fresh tissue. If the PG-HCl reactive material is not lignin, then what is it? Is it produced to sequester phytoalexins?