Rice blast, caused by the fungal pathogen Magnaporthe oryzae, is one of the most devastating crop diseases worldwide. The avirulence gene corresponding to rice blast resistance gene Pi7 in field isolate CHL346 was inherited as a single gene, designated AvrPi7, in a segregating population consisting of 189 ascospore progenies derived from a cross between field isolates CHL346 and CHL42. In order to determine the chromosomal location of the AvrPi7 locus, a total of 121 simple sequence repeat (SSR) markers were developed based on the whole-genome sequence of reference isolate 70-15 of M. oryzae. Linkage analysis of the locus with these SSR markers showed that eight SSR markers on chromosome 1 were linked to the locus, among which the closest flanking markers MS1-9 and MS1-15 were 3.2 and 16.4 cM from the locus, respectively. For fine mapping, additional PCR-based makers including eight SSR markers and three candidate avirulence gene (CAG) markers were developed in the region flanking both markers. The AvrPi7 locus was genetically delimited within a 1.6-cM region flanked by markers MS1-21 and MS1-22, and co-segregated with the marker CAG2. To construct a physical map of the AvrPi7 locus, molecular markers linked to the Avr gene were mapped on the supercontigs of the ref-erence isolate 70-15 through bioinformation analysis (BIA). Consequently, the AvrPi7 locus was delim-ited to a 75-kb interval flanked by markers MS1-21 and MS1-22 based on the reference sequence. Merodiploids observed in this study are also discussed.
To gain an understanding of the molecular ba-sis of signaling pathways in herbivore-induced maize plant defense, three key genes, ZmAOS, ZmAOC and ZmHPL, which are involved in the biosynthesis of oxylipin signals, have been cloned using RT-PCR in this study. Beet army-worm (BAW) infestation induced the systemic expression of the key genes involved in the biosynthesis of oxylipin signals similar to exogenous methyl jasmonate (MeJA). Moreover, the systemic expression patterns of maize defense-related genes were similar between maize leaves induced by jas-monic acid (JA) and damaged by BAW. Previous treatment with salicyhydroxamic acid (SHAM), an inhibitor of jas-monates (JAs) signal pathway followed by BAW infestation did not induce the systemic expression of the defense-related genes. Exposure to the vapors of green leafy volatiles (GLVs, (Z)-3-hexen-1-ol, (E)-2-hexenal, (E)-3-hexenal) and β-oci- mene induced the expression of the defense-related genes, as well as the key genes involved in biosynthesis of JAs. How-ever, previous treatment with SHAM clearly decreased the transcript levels of the defense genes induced by (Z)-3-hexen-1-ol, (E)-2-hexenal and (E)-3- hexenal. These results demonstrate the major role of oxylipin signal path-way in herbivore-induced maize chemical defense. JA was the endogenous signal in the process of herbivore-induced maize systemic defense. GLVs, another group of oxylipin, played an important role in the process of herbivore-induced systemic defense outside the plant. Furthermore, the expres-sion of defense-related genes induced by GLVs was partially dependent on JAs signal pathway, while β-ocimene induction was independent of JAs signal pathway.
