3.4.20
COLONIZATION OF SUGARCANE STALKS BY CLAVIBACTER XYLI SUBSP. XYLI AND ITS EFFECTS IN WATER UPTAKE THROUGH THE XYLEM VESSELS OF CULTIVARS DIFFERING IN THEIR SUSCEPTIBILITY TO RATOON STUNTING DISEASE

EA GIGLIOTI1, JC COMSTOCK2, MJ DAVIS3, S MATSUOKA1 and H TOKESHI4

1Centro de Ciências Agrárias, UFSCar, Caixa Postal 153, Araras-SP, 13600-970 Brasil; 2USDA-ARS, Sugarcane Field Station, PO Box 8, Canal Point, FL 33438, USA; 3University of Florida, Tropical Research and Education Center, 18905 SW 280th St, Homestead FL 33031, USA and 4Departamento de Fitopatologia, ESALQ/USP, Caixa Postal 9, Piracicaba-SP, 13400 Brasil

Background and objectives
The prevention of xylem water uptake as a result of colonization of metaxylem vessel by Clavibacter xyli subsp. xyli (Cxx) cells, production of gels and gums has been indicated as the main physiological trait associated with yield losses in sugarcane caused by ratoon stunting disease (RSD). The diagnostic techniques tissue blot immunoassay, TBIA [1] and staining by transpiration method, STM [2] were jointly applied to the same plants to estimate, respectively, the colonization of sugarcane stalks by Cxx and its effects on water uptake through the xylem vessels of cultivars differing in their susceptibility to RSD.

Results and conclusions
STM stained the functional vascular bundles of stalks but not the non-functional ones (NFVB), while TBIA stained imprints of the colonized vascular bundles (CVB) on nitrocelulose membranes. The safranine stain used in the STM did not interfere in the serological process of the TBIA, thus allowing both methods to be applied to the same stalks. When two cross-sections were examined from the third internode of 30 stalks, high correlation coefficients were obtained both for % NFVB (r=0.98) and for % CVB (r=0.96), which indicated the repeatability of both methods. Cross-sections of 10-mm central cores of third internodes were found to be representative of entire internode cross-sections with both methods (r=0.99), therefore cores provided a uniform sample size without the variation due to differences in stalk diameters within and between plants, and between genotypes, places, times, etc., with less work as well as costs. Infected vascular tissue was most extensive in the inner and least extensive in the outer tissues adjacent to the epidermis of a cross-section excised from the third sugarcane internodes. When eight genotypes were studied, 29.77, 15.49 and 6.69% of CVB were detected in the inner, middle and outer tissues of the third internodes, respectively. The same pattern was observed for NFVB (%NFVB=9.51, 3.67 and 1.25 in the inner, middle and outer, respectively). High correlation (r=0.995) was obtained between CVB and NFVB in inner, middle and outer tissues. Just 44.97% of all CVB were clogged by Cxx. In a group of nine moderately resistant cultivars, a good correlation between NFVB and CVB was obtained (r=0.987). However, in a second trial, using 19 genotypes that had a wider range of RSD reaction, the correlation between the two methods was just r=0.40. The main reason for the lower correlation was that some cultivars had a lower percentage of NFVB despite having a high percentage of CVB. Since STM identified all resistant genotypes it can be used as an alternative technique to screen sugarcane genotypes for RSD resistance. Also, the jointed method STM/TBIA, conjugated with histological and biochemical assays, should be useful in measuring physiological changes induced in the host by xylem inhabitant bacterial pathogens.

References
1. Harrison NA, Davis MJ, 1988. Phytopathology 78, 722-7.
2. Chagas PRR, Tokeshi H, 1994. Current Trends in Sugarcane Pathology, 163-71.