Sci. Rep. 5, 11274 (2015)

Heterosis, or ‘hybrid vigour’, causes cross-bred offspring to have growth or other characteristics that are more advantageous than either of their purebred parents. Though helpful in agriculture, the heterotic advantage is rarely passed beyond the first generation. Now, Singh et al. have developed a genetic system to ensure consistent production of hybrid seed.

A number of approaches exist to suppress self-fertilization and ensure hybrid seed production, including the laborious removal of anthers from ‘female’ parents, chemical poisoning of the pollen, and biotechnological systems involving ribonuclease and ribonuclease inhibitor expression.

This new approach relies on sabotaging pollen production in the female parent. The maternal line is transfected with an expression cassette containing the gene for BECLIN1 an autophagy related protein from Arabidopsis. BECLIN1 is under the control of a promoter specific for the tapetum — a layer of cells which supports the developing male gametophyte. This promoter is mutated so that it requires a variant form of transcription factor (TBPm3) not present in the normal plant but co-supplied in the expression cassette. BECLIN1 interferes with programmed cell death in the tapetum; an essential part of its function. The male sterility thus induced is rescued in hybrids because TBPm3 is tagged with an additional protein fragment used by the ubiquitin ligase, COP1, to target proteins for degradation. The intended ‘male’ parent has a second expression cassette containing COP1 also under the control of a tapetum-specific promoter. In hybrid plants (which contain both cassettes), COP1 drives degradation of TBPm3, preventing BECLIN1 synthesis, thus restoring tapetal cell death and male fertility.

This system is less labour intensive than mechanical or chemical approaches and should circumvent some of the biosafety concerns that accompany existing biotechnological approaches. Singh et al. demonstrated the functioning of their system in Nicotiania tabacum but its elegant simplicity should ensure its utility in commercial cereal and pulse breeding.