The effects of UV light or fungal elicitors on plant cells have so far been studied mostly with respect to defense-related gene activation. Here, an inverse correlation of these stimulatory effects with the activities of several cell cycle-related genes is demonstrated. Concomitant with the induction of flavonoid biosynthetic enzymes in UV-irradiated cell suspension cultures of parsley (Petroselinum crispum), total histone synthesis declined to about half the initial rate. A subclass of the histone H3 gene family was selected to demonstrate the close correlation of its expression with cell division, both in intact plants and cultured cells. Using RNA-blot and run-on transcription assays, it was shown that one arbitrarily selected subclass of each of the histone H2A, H2B, H3 and H4 gene families and of the genes encoding a p34cdc2 protein kinase and a mitotic cyclin were transcriptionally repressed in UV-irradiated as well as fungal elicitor-treated parsley cells. The timing and extent of repression differed between the two stimuli; the response to light was more transient and smaller in magnitude. These differential responses to light and elicitor were inversely correlated with the induction of phenylalanine ammonia-lyase, a key enzyme of phenylpropanoid metabolism. Essentially the same result was obtained with a defined oligopeptide elicitor, indicating that the same signaling pathway is responsible for defense-related gene activation and cell cycle-related gene repression. A temporary (UV light) or long-lasting (fungal elicitor) cessation of cell culture growth is most likely due to an arrest of cell division which may be a prerequisite for full commitment of the cells to transcriptional activation of full commitment of the cells to transcriptional activation of pathways involved in UV protection or pathogen defense. This conclusion is corroborated by the observation that the histone H3 mRNA level greatly declined around fungal infection sites in young parsley leaves.

Fungal pathogens secrete hydrolases during infection of plant tissues capable of fragmenting the primary cell wall polysaccharides of the host. Magnaporthe grisea, the fungal pathogen that causes blast disease of graminaceous monocots, secretes two distinct endo-β-1,4-D-xylanases when grown on xylan-rich rice cell walls as the carbon source. We have previously reported the cloning of the genes encoding these two xylanases, XYL1 and XYL2 (formerly XYN22 and XYN33, respectively; see S.-C. Wu, S. Kauffmann, A. G. Darvill, and P. Albersheim, Mol. Plant-Microbe Interact. 8:506–514, 1995). We now present three M. grisea mutants created by selective deletion of XYL1 and/or XYL2. The xyl1 mutant grows as well as the parent in culture medium when rice cell walls or xylan is the sole carbon source. Under the same conditions, the xyl2 mutant grows slightly slower than the parent, whereas the xyl1/xyl2 double mutant exhibits a 50% reduction in accumulation of total mycelial mass. Under conditions idealized for infection, all three mutants infect host plants as efficiently as the parent, indicating neither XYL1 nor XYL2 is required by M. grisea for infection. Endoxylanase assays showed that, at the stationary stage of growth in culture when the accumulation of total xylanase activity is at its maximum, the xyl1 mutant retains ≈ 88%, xyl2, 39%, and xyl1/xyl2, 19% of the endoxylanase activity of the parent. Partial protein purification of xylanases secreted by the xyl1/xyl2 double mutant revealed four distinct endoxylanase activities. One of the xylanases, XYL3, is present in the culture filtrate of both the parent and the mutant strains, and, like XYL1, has been identified as a member of the Family G xylanases. The other three xylanases are not found in the culture filtrates of the parent or of the xyl1 mutant. Thus, M. grisea appears capable of secreting additional xylanases and does so when XYL2, a member of the Family F glycanases, has been eliminated.

Plant endo‐β‐1,3‐glucanases and chitinases inhibit the growth of some fungi and generate elicitor‐active oligosaccharides while depolymerizing polysaccharides of mycelial walls. Overexpression of the endo‐β‐1,3‐glucanases and/ or chitinases in transgenic plants provides, in some cases, increased protection against fungal pathogens. However, most of the phytopathogenic fungi that have been tested in vitro are resistant to endo‐β‐1,3‐glucanases and chitinases. Furthermore, some phytopathogenic fungi whose growth is inhibited by these enzymes are able to overcome the effect of these enzymes over a period of hours, indicating an ability of those fungi to adapt to the enzymes. Evidence is presented indicating that fungal pathogens secrete proteins that inhibit selective plant endo‐β‐1,3‐glucanases.A glucanase inhibitor protein (GIP‐1) has been purified to homogeneity from the culture fluid of the fungal pathogen of soybeans, Phytophthora sojae f. sp. glycines (Psg), and two basic pathogenesis‐related endo‐β‐1,3‐glucanases (EnGLsoy‐A and EnGLsoy‐B) have been purified from soybean seedlings. GIP‐1 inhibits EnGLsoy‐A but not EnGLsoy‐B. Moreover, GIP‐1 does not inhibit endo‐β‐1,3‐glucanases secreted by Psg itself nor does GIP‐1 inhibit PR‐2c, a pathogenesis‐related endo‐β‐1,3‐glucanase of tobacco. Evidence is presented that Psg secretes other GIPs that inhibit other endo‐β‐1,3‐glucanase(s) of soybean. Furthermore, GIP‐1 does not exhibit proteolytic activity but does appear to physically bind to EnGLsoy‐A. The results reported herein demonstrate specific interactions between gene products of the host and pathogen and establish the need to consider fungal proteins that inhibit plant endo‐β‐1,3‐glucanases when attempting to use the genes encoding endo‐β‐1,3‐glucanases to engineer resistance to fungi in transgenic plants.

Magnaporthe grisea, a destructive ascomycetous pathogen of rice, secretes cell wall-degrading enzymes into a culture medium containing purified rice cell walls as the sole carbon source. From M. grisea grown under the culture conditions described here, we have identified an expressed sequenced tag, XYL-6, a gene that is also expressed in M. grisea-infected rice leaves 24 h postinoculation with conidia. This gene encodes a protein about 65% similar to endo-␤-1,4-D-glycanases within glycoside hydrolase family GH10. A M. grisea knockout mutant for XYL-6 was created, and it was shown to be as virulent as the parent strain in infecting the rice host. The proteins secreted by the parent strain and by the xyl-6⌬ mutant were each fractionated by liquid chromatography, and the collected fractions were assayed for endo-␤-1,4-D-glucanase or endo-␤-1,4-D-xylanase activities. Two protein-containing peaks with endo-␤-1,4-D-xylanase activity secreted by the parent strain are not detectable in the column eluant of the proteins secreted by the mutant. The two endoxylanases (XYL-6␣ and XYL-6␤) from the parent were each purified to homogeneity. N-terminal amino acid sequencing indicated that XYL-6␣ is a fragment of XYL-6␤ and that XYL-6␤ is identical to the deduced protein sequence encoded by the XYL-6 gene. Finally, XYL-6 was introduced into Pichia pastoris for heterologous expression, which resulted in the purification of a fusion protein, XYL-6H, from the Pichia pastoris culture filtrate. XYL-6H is active in cleaving arabinoxylan. These experiments unequivocally established that the XYL-6 gene encodes a secreted endo-␤-1,4-D-xylanase.