Breadfruit (Artocarpus altilis) gibberellin 20-oxidase genes: sequence variants, stem elongation and abiotic stress response

Breadfruit (Artocarpus altilis) is a traditional staple tree crop throughout the tropics. Susceptibility to windstorm damage is the primary constraint on breadfruit cultivation. Significant tree loss due to intense tropical windstorm in the past decades has driven an increasing interest in developing dwarf varieties of breadfruit. As a first step toward understanding the molecular mechanism of growth regulation in the species, we investigated the role of gibberellin and the regulation of GA20-oxidase genes in breadfruit. We provided first evidence that the stem elongation in breadfruit could be manipulated by exogenous gibberellin-related growth regulators. We then cloned six GA20-oxidase cDNAs, AaGA20ox1-AaGA20ox6, in full length from breadfruit. Sequence analysis showed that the predicted proteins of the AaGA20ox1-AaGA20ox3 bear all the hallmarks of functional GA20oxidase of other species, but predicted AaGA20ox4-AaGA20ox6 as expressed, unprocessed pseudogenes closely related to AaGA20ox2. AaGA20ox1, AaGA20ox3 and AaGA20ox4 were predominantly expressed in green vegetative organs, but displayed different expression pattern in roots and reproductive organs. AaGA20ox2, AaGA20ox5 and AaGA20ox6 were expressed mainly in leaves at low level. AaGA20ox1, AaGA20ox3-AaGA20ox6 were subjected to GA feedback regulation following treatment of exogenous gibberellin and/or gibberellin biosynthesis inhibitors. AaGA20ox1 and AaGA20ox3 were down-regulated under drought and salinity stress, but AaGA20ox2 was upregulated under salt stress. Pseudogenes AaGA20ox4 and AaGA20ox5 were up-regulated under drought or/and salt stress condition. The function of AaGA20oxs is discussed with particular reference to their role in stem elongation and involvement in abiotic stress response in breadfruit.