The Important Roles of Oxalate Oxidase In Taro Disease Resistance

Taro [Colocasia esculenta (L.) Schott] is one of the most important staple food crops in the Pacific Islands. One major pathogen that threatens the sustainability of taro production in the world is Taro Leaf Blight (TLB).  TLB is caused by the oomycete pathogen Phytophthora colocasiae.  It was estimated that 25 to 50% of taro corms in Hawaii were lost due to oomycete and fungal diseases. Taro corms and leaves are known to contain abundant levels of oxalate and calcium oxalate.  In the previous research, we has transformed taro cv. Bun long with the wheat OXO gene gf2.8. Dramatically increased resistance of the transformed line to the oomycete pathogen Phytophthora colocasiae was found in the plants under tissue-culture and growth chamber conditions. Due to current controversies in Hawaii over genetic engineering of taro, we are focusing our research to identify homologous OXO genes within the taro germplasm. One pair of polymerase chain reaction (PCR) primers was designed based on the conserved sequence of the wheat and other OXO genes. Total DNAs were extracted from the leaf tissues of a TLB-susceptible cultivar Maui Lehua and TLB-resistant cultivars: 1999-6, 1999-9, Paakala, Pauakea, Palehua, P1, P2, P7, and P20.  A specific 138bp PCR product was amplified in the TLB-resistant cultivar Paakala, and the sequence of this PCR product was identical to the conserved sequence of the wheat and other OXO genes based on the NCBI BLAST. Six vegetative propagules of each taro cultivar: Paakala, Pauakea, Palehua, and Maui Lehua were planted in the greenhouse. Yellow spot lesion mimicry was observed in the mature leaves of all six eight-week-old plants of TLB-resistant cultivar Paakala and two plants of Pauakea and Palehua. However, no lesion mimicry was observed in the mature leaves of all six eight-week-old plants of TLB-susceptible cultivar Maui Lehua. Bright-yellow autofluorescent substances in the leaves of eight-week-old Paakala were observed, which indicated that they were HR-like lesions. In addition, the mature leaves of eight-week-old Paakala plants showed strong positive results in the histo-chemical assay for OXO activity. Oxalate oxidase should provide an excellent molecular marker to assist conventional taro breeding to improve disease resistance.  The expression and localization of OXO are easily analyzed using a visible histo-chemical assay.  In addition, OXO is an extreme high temperature resistant enzyme as well as an SDS PAGE-resistant protein that differentiates it from other proteins.