PRESENCE OF XYLELLA FASTIDIOSA IN SWEET ORANGE FRUIT AND SEEDS AND ITS TRANSMISSION TO SEEDLINGS

Authors: LI, WENBIN - 1275-11-00; PRIA, WOLNEY - UNIV OF SAO PAULO; LACAVA, P.M. - UNIV OF SAO PAULO; Hartung, John

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2003
Publication Date: 8/1/2003
Citation: Li, W., Pria, W., Lacava, P., Hartung, J.S. 2003. Presence of xylella fastidiosa in sweet orange fruit and seeds and its transmission to seedlings. Phytopathology. 93(8):953-953.

Interpretive Summary: Citrus variegated chlorosis is a serious disease of sweet orange in Brazil, home of the world's largest citrus industry. The disease is caused by the bacterium Xylella fastidiosa. Related strains of the pathogen cause important diseases of other tree fruit and vine crops in the New World. The pathogen is unusual because it is believed to occur only in the water conducting vessels of infected plants and in the mouthparts of insect vectors and because individual strains have exceptionally broad natural host ranges. In this work, we have demonstrated that Xylella fastidiosa completely colonizes the water conducting vessels of sweet orange fruit, including the water conducting vessels of the sweet orange seed. We also show, for the first time, that the pathogen can be transmitted to the next generation of plants through infected seed. This result is surprising because there is no physical connection between the embryos in the seed, which give rise to the seedlings, and the water conducting vessels of the mother plant. Thus many researchers have assumed that such transmission by seed could not occur. Citrus variegated chlorosis does not occur in the United States, and based on our results, care should be taken whenever seed is exchanged for research purposes to prevent the inadvertent introduction of the disease into the United States. Our results will change the way researchers think about the many diseases caused by Xylella fastidiosa, and will therefore be of value to both the scientific and plant health regulatory communities.

Technical Abstract: Xyella fastidosa, a xylem-limited bacterium, causes several economically important diseases in North, Central, and South America. These diseases are transmitted by sharpshooter insects, contaminated budwood, and natural root-grafts. Xylella fastidiosa extensively colonizes the xylem vessels of susceptible plants. Citrus fruit have a well developed vascular system, which is continuous with the vascular system of the plant. Citrus seeds develop very prominent vascular bundles, which are attached through ovular and seed bundles to the xylem system of the fruit. Sweet orange (Citrus sinensis (L.) Osbeck) fruit of the varieties 'Pera', 'Natal', and 'Valencia' with characteristic symptoms of citrus variegated chlorosis disease were collected for analysis. X. fastidiosa was detected by PCR in all main fruit vascular bundles, as well as in the seed and in dissected seed parts. No visual abnormalities were observed in seeds infected with the bacterium. However, the embryos of the infected seeds weighed 25% less than those of healthy seeds, and their germination rate was lower than uninfected seeds. There were about 2500 cells of X. fastidiosa per infected seed of sweet orange, as quantified using Real-Time PCR techniques. The identification of X. fastidiosa in the infected seeds was confirmed by cloning and sequencing the specific amplification product, obtained by standard PCR with specific primers. X. fastidiosa was also detected in and recovered from seedlings by isolation in vitro. Our results show that X. fastidiosa can infect and colonize fruit tissues including the seed. We have also shown that X. fastidiosa can be transmitted from seeds to seedlings of sweet orange. This is apparently the first report of the presence of X. fastidiosa in seeds and its transmission to seedlings.