What causes fruit set failure in pink trumpet trees? Look at pistils to find out more

Spring is finally here (in the northern hemisphere) and cheerful blossoms are everywhere. One tree that you might have come across along the streets is the pink trumpet tree, Handroanthus heptaphyllus that belongs to the plant family, Bignoniaceae. It is native to Central and South America from Mexico to Argentina and they are liked as ornamentals due to their mass flowering. 

Dr Marta Bianchi and colleagues from Universidad Nacional de Rosario and the University of St Andrews investigated the pink trumpet tree pollination system. The scientists tested a pollination-crossing system to test why fruit setting is sometimes unsuccessful (e.g. self-incompatibility or cross-incompatibility). The researchers collected fallen flowers for microscopic observation and suggest that late-acting self incompatibility (LSI) is the underlying mechanism behind fruit-set failure. Previously, Dr Peter Gibbs,  one of the co-investigators of the latest study, called LSI the “pariah breeding system” in flowering plants due to its complexity and confusions about the mechanism. 

Handroanthus flowers have a bifid (two-lobed) stigma that closes after receiving conspecific (from the same species) pollen. In Bignoniaceous species, self-pollen tubes grow to the ovary, penetrate it which then go through double fertilisation. Sometimes fruit setting fails following self-pollination despite self-pollen tubes reaching the ovary. This phenomenon was first described in 1986 as late-acting self-incompatibility (LSI). Another type of fruit-set failure is caused by early-acting inbreeding depression (EID) that mimics LSI but can still lead to fruit production after self-pollinations.

Bianchi and colleagues made diallel crosses from three seed lots in Argentina in 2010 where one seed lots was used as a parent for all crosses. The plants flowered after 12 and 15 months and the researchers tested cross- and self-pollinations over five years. The pistils (female parts of the flowers) fell off after 2-12 days after pollination. Hundreds of these pistils were collected and examined under a microscope to look at pollen germination and pollen tube growth. Based on these observations, the scientists grouped crosses into reciprocally cross compatible (RCC), not reciprocally compatible (NRC) and reciprocal incompatibility (RIC) groups.

Bianchi and colleagues found that twice as many ovules were penetrated by pollen tubes during compatible crosses than incompatible ones. The pistils from incompatible pollinations were shorter than the ones from successful crosses, making pistil length a distinguishable feature. No fruits were produced from incompatible crosses. 

“In conclusion, our diallel crosses support the mechanism of late-acting self incompatibility (LSI) over early-acting inbreeding depression (EID),” Bianchi and colleagues wrote. 

“The former could be driven by a single locus whereas the latter would presumably be driven by a whole genome effect. If crossing success rates were a consequence of EID, one would expect to see a broad distribution of success rates in crosses.”

There were some male-sterile plants from the hybridised parents and based on dominant self-incompatibility genes, the researchers proposed a genetic model, explaining underlying mechanisms of LSI. 

This study has shown how testing self- and cross-pollinations of a tree species can answer fundamental questions about plant reproduction and that pistil length can be used as a reliable feature to distinguish pollination incompatibility.