Proceedings of the XLVII Italian Society of Agricultural Genetics - SIGA Annual Congress

Verona, Italy - 24/27 September, 2003

ISBN 88-900622-4-X

Poster Abstract - 5.17

RESISTANCE OF LIMA BEAN TO BEAN WEEVIL LARVAE MAY BE ACCOUNTED FOR BY THE STABILITY OF ITS SEED PROTEINS IN THE GUT OF THIS BRUCHID SPECIES

M. FILEPPI, F. SPARVOLI, R. BOLLINI

Istituto di Biologia e Biotecnologia Agraria, CNR, Via Bassini 15, 20133 Milan, Italy

Acanthoscelides obtectus, lectin-related proteins, Phaseolus seeds, resistance

The bean weevil Acanthoscelides obtectus represents a major thread for bean seeds during storage.

On common bean seeds (Phaseolus vulgaris), each female lays several eggs, the larvae then penetrate the seed to emerge as mature adults in about thirty days. This cycle can be repeated a number of times on each seed and may lead to complete loss of the stored commodity.

A diffuse level of resistance to this bruchid has been found in lima bean (Phaseolus lunatus), where adult emergence occurs in a number of days almost doubled compared to common bean, known to be a susceptible species.

Common bean seeds contain a family of abundant lectin-related proteins that can be regarded as defense proteins. Among them, arcelin has been related to resistance against Mexican bean weevil (Zabrotes subfasciatus) and resistance apparently is due to poor digestibility of this protein.

In lima bean seeds, proteins of this class, namely ARL (arcelin-like) and AIL (alpha-amylase inhibitor-like) are particularly abundant.

As a preliminary attempt to identify resistance factors in lima bean, the digestibility of its proteins has been investigated. The presence of undigested seed proteins has been estimated by western blot analysis of total protein extracts from developing larvae guts. Large amounts of both phaseolin and lectin-rlated proteins apparently survive in the guts. On the contrary, in control common bean seeds only faint amounts of phaseolin survive digestion. In an attempt to correlate one or more major lima bean proteins to resistance, two protein fractions, enriched in phaseolin and in the lectin-related ARL and AIL respectively, have been prepared from dry seeds and their toxicity assayed in artificial seeds. At the concentration investigated, no correlation between protein fractions and resistance has been found. Proteins in the partially purified protein fractions survive a long time in the gut, suggesting their poor digestion is due to intrinsic features rather than to the presence of proteinase inhibitors. Indeed, lima bean seed proteins remained undigested when incubated in vitro with proteolytic enzymes extracted from the gut of third instar larvae raised on common bean seeds whereas they were promptly digested if heat denatured before the incubation.