Production and roles of IAA and ABA during development of superior and inferior rice grains

Abstract

<jats:p> Current understanding of the role of plant hormones during cereal grain filling is confounded by contradictory reports on hormone production that is based on poor methodology. We report here on the accurate measurement of indole-3-acetic acid (IAA) and abscisic acid (ABA) by combined liquid chromatography-tandem mass spectrometry in multiple reaction-monitoring mode with heavy isotope labelled internal standards. ABA and IAA contents of superior versus inferior rice grains (ABA maxima 159 ng g–1 FW and 109 ng g–1 FW, IAA maxima 2 µg g–1 FW and 1.7 µg g–1 FW respectively) correlated with the expression of biosynthetic genes and with grain fill. Results confirm that grain ABA is produced primarily by OsNCED2(5), but suggest that ABA import and metabolism also play important roles in ABA regulation. The IAA content of grains is primarily influenced by OsYUC9 and OsYUC11. However, the distinct expression profile of OsYUC12 suggests a specific role for IAA produced by this enzyme. Co-expression of OsYUC12 with OsIAA29 indicates their involvement in a common signalling pathway. Co-expression and cis-element analysis identified several aleurone-specific transcriptional regulators as well as glutelin as strong candidates for detailed investigation for direct regulation by the auxin-signalling pathway. </jats:p>

Expression of TaTAR2.3-1B, TaYUC9-1 and TaYUC10 correlates with auxin and starch content of developing wheat grains

“…Interestingly, ZmYUC1 appears to be primarily responsible for IAA production in developing maize grains (Chourey et al 2010), whereas three genes, OsYUC9, OsYUC11 and OsYUC12 are all up-regulated in rice grains. These rice genes have differences in their expression profiles, with earlier and more restricted up-regulation of OsYUC12 (Nonhebel and Griffin 2020). A similar situation may occur in wheat; TaYUC9-1 genes were maximally up-regulated between 10 and 15 DAA and remained active at 20 DAA, similar to the orthologous OsYUC9 (Nonhebel and Griffin 2020).…”

“…The TAR/YUCCA pathway has been demonstrated as the main route of IAA synthesis in plants (Mashiguchi et al 2011;Won et al 2011). TAR and YUCCA activity has also been reported as the major source of IAA in maize and rice cereal grains (Bernardi et al 2012;Chourey et al 2010;Nonhebel and Griffin 2020). However, there has been no comprehensive study of these genes in wheat.…”

“…These rice genes have differences in their expression profiles, with earlier and more restricted up-regulation of OsYUC12 (Nonhebel and Griffin 2020). A similar situation may occur in wheat; TaYUC9-1 genes were maximally up-regulated between 10 and 15 DAA and remained active at 20 DAA, similar to the orthologous OsYUC9 (Nonhebel and Griffin 2020). Expression of the TaYUC9-1 group also increased in parallel with TaTAR2.3-1B and correlated most closely with the increasing IAA content of grains suggesting that these genes may be primarily responsible for IAA production during grain fill and similar again to the situation in rice.…”

“…This expression profile is similar to that of OsYUC12 in rice. Although TaYUC10 and OsYUC12 are not directly orthologous, their expression profile distinct from that of TaYUC9-1 suggests a requirement for very precise and localised IAA production regulated by separate genes during early grain fill, as suggested by Nonhebel and Griffin (2020).…”

“…As in maize and rice, IAA in wheat grains accumulates particularly during early grain fill and coincides with the initiation of starch production. In other seeds, it has been proposed to initiate the differentiation of endosperm transfer cells and starch production (Bernardi et al 2019;McAdam et al 2017;Nonhebel and Griffin 2020). On the other hand, a negative effect of IAA on grain fill has been reported in publications relating to the TGW6 genes in rice (Ishimaru et al 2013) and wheat (Hu et al 2016a), and TaTGW-7A in wheat (Hu et al 2016b).…”