R. Perl-Treves

The broad mite (BM) Polyphagotarsonemus latus is a minute polyphagous mite that inhibits plant growth, and causes leaf distortion and fruit damage. Its mechanisms of injury and subsequent plant responses are still unclear. Time-dependent effects of BM infestation on leaf anatomy were evaluated by light microscopy and transmission electron microscopy (TEM). Using Northern analysis, we examined the expression of two genes involved in lignin synthesis, representing plant-induced defense mechanisms: peroxidase (PRX) and phenyl alanin ammonia lyase (PAL). Comparisons were made between plants infested with BM and two control groups: untreated and mechanically wounded, looking at the infested leaf and an upper leaf that was free of mites. Severe BM infestation led to complete loss of epidermis and failure of mesophyll differentiation. Hypertrophy and hyperplasia were also apparent, resulting in thick and very compact tissue, lacking air spaces. Cell walls appeared thick and distorted. A condensed layer of collapsed cells covered both sides of the infested leaf, instead of the epidermal layers. Safranin - fast green staining suggested that the wall material contained high levels of phenolic substances, presumably lignin and/or suberin. Northern analysis revealed that PRX and PAL were induced by mite feeding, whereas wounding did not induce these transcripts. PAL was also induced in the isolated upper leaf, suggesting a systemic plant response. How these responses affect the susceptibility of plants to BM infestations is yet to be studied. [L]