Differential Expression and Turnover of the Tomato Polyphenol Oxidase Gene Family during Vegetative and Reproductive Development.

Polyphenol oxidases (PPOs) are encoded by a highly conserved, seven-member gene family clustered within a 165-kb locus on chromosome 8 of tomato (Lycopersicon esculentum). Using gene-specific probes capable of differentiating between PPO A/C, PPO B, PPO D, and PPO E/F, we examined the spatial and temporal expression of this gene family during vegetative and reproductive development. RNA blots and in situ hybridization using these probes showed that although PPO expression is primarily confined to early stages of development, the steady-state mRNA levels of these genes are subject to complex patterns of spatial and temporal regulation in vegetative and reproductive organs. Young tomato leaves and flowers possess the most abundant PPO transcripts. PPO B is the most abundant in young leaves, whereas in the inflorescence PPO B and E/F transcripts are dominant. Differential expression of PPOs is also observed in various trichome types. PPO A/C are specifically expressed in type I and type IV trichomes. In contrast, PPO D is only expressed in type VI trichomes. Type I, IV, and VI trichomes possess PPO E/F transcripts. Immunolocalization verified the translational activity of PPOs identified by in situ hybridization and suggested cell-type-specific, developmentally programmed PPO turnover. In addition, immunolocalization demonstrated the accumulation of PPO in specific idioblast cells of stems, leaves, and fruits.

Tomato Polyphenol Oxidase (Differential Response of the Polyphenol Oxidase F Promoter to Injuries and Wound Signals).

Tomato (Lycopersicon esculentum Mill.) polyphenol oxidases (PPOs) are encoded by a seven-member gene family that exhibits complex patterns of differential expression during growth and differentiation. Antisense down-regulation of constitutive and induced PPO expression results in hypersusceptibility to pathogens, suggesting a critical role for PPO-mediated phenolic oxidation in plant defense. However, the nature and extent of PPO induction and its contribution to resistance are unclear. In this study we examined the inducibility of the tomato PPO gene family. In mature plants PPO transcript levels systemically increased in young leaves (nodes 1-3) when mature leaflets (node 5) were injured. Transcripts hybridizing to PPO E/F-specific probes were the predominant wound-induced PPO mRNAs in young leaves. Analysis of PPO promoter: GUS fusion constructs shows that mechanical wounding and infection by fungal and bacterial pathogens induced transcription of PPO F. Different injuries, salicylic acid, ethylene, and jasmonates elicited distinct, cell-specific and developmental stage-specific patterns of PPO F expression. Whereas jasmonates and mechanical wounding significantly induced PPO F only in young leaves (nodes 1-3), and ethylene induced PPO F only in older leaves (node 7), salicylic acid induced PPO F in stems and foliage at all developmental stages. These results demonstrate that cis-element(s) sufficient for PPO F inducibility reside in the 5[prime] flanking region, and these elements are responsive to a broad range of signals.