Cross incompatibility mechanisms between wheat and rye

Abstract

In an attempt to increase the crossability of wheat (Triticum aestivum L.) with rye (Secale cereale L.), mechanisms of action of the putative glycoprotein products of the Kr genes which are expressed in wheat styles were studied.

Style extracts of Otane wheat (incompatible with rye) inhibited rye pollen tubes more than compatible Chinese Spring wheat extracts in vitro. Style extracts from the same rye plant as the pollen (self-incompatible extracts) were also more inhibitory than style extracts from different plants.

Deglycosylation of Otane style extracts alleviated incompatibility in vitro. Treatment of styles with the glycosylation inhibitor tunicamycin improved crossability 4.1 times (from 3.4% to 14%).

Some partially purified style RNases inhibited 'self' pollen more than others, suggesting that they might have a role in incompatibility. Rye 'self' pollen extracts or RNases could induce incompatibility in vitro. Treatment of Otane wheat styles with the RNase inhibitors diethyl pyrocarbonate and polyvinyl sulphate improved crossability with rye 3.8 and 5.0 times, respectively.

Lectins overcame intergeneric and self-incompatibility in vitro. The lectins UEA II and WGA-peroxidase improved crossability of Otane wheat with rye 4.9 and 5.7 times. Endogenous lectins from pollen tube diffusates, extracted with incompatible style extract-coated polystyrene latex beads, also overcame incompatibility in vitro. Beads coated with incompatible style extracts reduced in vitro pollen tube growth, probably by adsorbing stimulatory pollen tube lectins.

Pollen tubes in incompatible style extract took up only half as much calcium as pollen tubes in compatible style extract. Addition of cyclic AMP evened out the differences. Cyclic AMP, calcimycin (A23l87) and inositol-l,4,5-triphosphate overcame intergeneric and self-incompatibility in vitro. The cyclic AMP analogue, 8-bromo-cyclic AMP, and inositol-l,4,5,-triphosphate improved the crossability of Otane wheat with rye 5.8 and 4.8 times, respectively.

A hypothesis was developed in which pollen tube lectins bind to receptors on the plasma membrane of pollen tubes, possibly stimulating their growth. Interference by specific style glycoproteins would result in adverse changes in membrane fluidity and low cytoplasmic calcium and cyclic AMP levels. Pollen of any individual of a species possesses a range of lectins sufficient to match any of the S-alleles. Intergeneric incompatibility was found to operate in a similar manner to self-incompatibility .