Inhibition of auxin biosynthesis in early rice grains leads to extensive post-fertilisation grain abortion

In spite of its abundant presence in rice grains, auxin functions during grain development are not understood well. Absence of grain-specific auxin biosynthesis mutants in rice further limits our understanding in this respect. Here, we report a chemical biology approach to inhibit auxin biosynthesis specifically in early rice grains as well as its subsequent effects on final grain yield. Exogenous application of two auxin biosynthesis inhibitors, namely L-kynurenine (L-Kyn) and 4-phenoxyphenylboronic acid (PPBo), to spikelets daily from three to ten days after pollination (DAP) significantly reduced grain levels of indole-3-acetic acid (IAA), the predominant in planta auxin. The inhibitor-treated panicles showed extensive post-fertilisation seed abortion, leading to drastic reduction in total panicle weight at maturity. Locally synthesised auxin during early grain development may therefore play a crucial role in grain retention. This chemical biology approach can be an easy and cost-effective way to study auxin biosynthesis and signalling during grain development in rice and possibly other cereal crops. Highlight Auxin synthesised in early rice grains may play a crucial role in grain retention.