Effects of drought and the role of auxin on barley stamen maturation

Abstract

Drought is a major abiotic stress that reduces crop yields all over the world. Climate change is predicted to increase the frequency and intensity of future drought events. Concomitantly, the increasing world population will require a rise in global crop production. To prevent a severe food insecurity situation, it is a pressing need to create varieties with robust yields under water deficit conditions, in economically important crops such as barley. Drought stress, occurring during the phase shortly before anthesis, is especially harmful to male reproductive development, which takes place in the stamens. Thus, understanding the molecular aspects that are essential for stamen development and that are also disrupted by drought stress should provide basis for targeted crop improvement. <br /> First, this work investigated the effects of transient drought stress on barley stamen development during the post-meiotic maturation phase. Drought treatment started around the phase of microspore release caused the highest reduction in barley spikelet fertility and was mainly caused by impaired male fertility, although female organ development was also negatively affected. Stamens that showed an altered morphology upon drought harbored viable but starch-less and infertile pollen. Accordingly, transcriptome analysis of stamens revealed that drought downregulates genes involved in central carbon metabolism, ranging from sucrose degradation to ATP synthesis, and in starch metabolism. Furthermore, drought-stressed stamens contained reduced levels of sucrose, hexoses and succinate. Drought also downregulated the expression of genes related to auxin synthesis, response and signaling in stamens. Thus, this work supports that auxin upregulates central carbon metabolism to ensure pollen starch accumulation, hence male fertility. Therefore, controlling auxin response and signaling in stamens may offer a valuable approach to improve pollen maturation and yield under drought. Moreover, screening of a subset of the wild barley introgression line population S42IL and the genotypes Golden Promise Fast (GPF) and GB2 identified potential drought-tolerant accessions that could serve as genetic resources in future crop improvement programs. <br /> Next, comparative omics analyses between stamens of the auxin-deficient mutant <em>male sterile genetic 38 (msg38)</em> and the fertile cultivar Bowman were conducted to study the role of auxin in barley stamen maturation, with a special focus on auxin response and central carbon metabolism factors. Transcriptome analysis indicated that auxin predominantly regulates primary targets of auxin signaling at early stages of stamen maturation. However, auxin extensively regulates gene expression during the phase of pollen starch accumulation, particularly boosting genes related to energy production. Decreased expression of auxin-dependent energy production genes in <em>msg38</em> stamens often correlated well with lower levels of the corresponding proteins. Moreover, auxin was found to control the phosphorylation of sugar utilization enzymes and other factors of energy generation. These findings underline the importance of auxin as major regulator of the stamen energy production pathway. In sum, this work provides a basis for future analysis of the auxin signaling cascade as well as targeted molecular engineering in stamens that could help to improve male fertility under adverse environmental conditions such as drought.