Preliminary Studies On Inverted Resistance Mechanism Of Cotton To Cotton Fusarium Wilt

In order to disclose why the susceptible cotton variety can become so resistant to F. oxysporum f.sp.vasinfectum only in few years by directional selection in disease plot,dipping cotton seedling roots cultured in liquid with spore suspension inoculation method was used to study the pathologic changes and differentially expressed cDNAs between susceptible cotton varieties (Xinluzao 7 and Tianjiu 63) and their resistant descendants obtained by directional selection.1. Two non-injured cotton root inoculation methods: the whole stage field disease plot appraisal and dipping cotton seedling roots cultured in liquid with spore suspension were compared for their effectiveness in infection and development of Fusarium wilt of cotton. It was showed that by dipping cotton seedling roots cultured in liquid with spore suspension (the inoculation concentration was 107 conidias/ml), the cotton varieties'resistance to Fusarium wilt could be appraised in short period(only 30 days). The inoculation time(20 min,30 min,40 min,60 min) was also optimized, and it was found that the inoculation time lasted for 40min was the most optimum and got the consistent results with those by field disease plot.2. The pathologic changes between susceptible cotton varieties (Xinluzao 7) and their resistant descendants were observed by electron microscope and optics microscope. The results showed that there had some differences in both macroscopical symptoms and microcosmic structures.As far as the macroscopical symptom is concerned, the susceptible cotton varieties showed symptom after inoculated for 7 days, and became severe after 25 days,all the vascular tissues turned brown; However, there had only slight symptom or no symptom.in the resistant descendants at the meantime, and the brown regions were only restricted below the hypocotyl. As to the cytology and cell ultrastructural of host, in the invading prophase, the cell wall of the invaded part of resistant descendants were congregated with wall apposition; In the invading metaphase, the hyphae of the pathogen can invade the cortical area of cells of susceptible varieties, but only can invade the outer cortex cells of resistant descendants;In the invading anaphase,some fungal cells were surrounded by a little aggregated deposits and secretion in susceptible varieties while a few fungal cells were surrounded by abundant aggregated deposits and secretion in resistant descendants, so that the fungal can not spread. 3. The mRNA differential display method was used to study the differentially expressed cDNAs between susceptible cotton variety (Tianjiu63) and its resistant descendants. The results indicated that the transcript abundance of some cDNA fragments between susceptible cotton variety (Tianjiu63) and its resistant descendants were different. 8 differently expressed cDNA fragments were cloned and sequenced. The sequences BLAST results showed that there had no homologous sequence of 14-3 in GenBank;10-5 and 22-11 had 100% identity with cDNA of phage of Pinus taeda; 22-6 and 22-2 had 96% and 100% identity with cDNA of roots minus nitrogen Pinus taeda,respectively, which are deduced to participate in regulation and synthesize course of protection material;13-10 had 100% identity with mRNA of cotton fibre protein, which codes for the pyridoxamine 5'-phosphate oxidase-related genes, and may take part in the synthesize course of enzyme genes related to cotton resistance; 6-3 had 99% identity with cDNA of cotton root tissue and fibre,98% with cDNA of cotton ovule; 10-7 had 98% identity with mRNA of putative senescence-associated protein. The functions of these genes need further determined.