The ZmASR1 Protein Influences Branched-Chain Amino Acid Biosynthesis and Maintains Kernel Yield in Maize under Water-Limited Conditions    

Abstract

Abscisic acid-, stress-, and ripening-induced (ASR) proteins were first described about 15 years ago as accumulating to high levels during plant developmental processes and in response to diverse stresses. Currently, the effects of ASRs on water deficit tolerance and the ways in which their physiological and biochemical functions lead to this stress tolerance remain poorly understood. Here, we characterized the ASR gene family from maize (Zea mays), which contains nine paralogous genes, and showed that maize ASR1 (ZmASR1) was encoded by one of the most highly expressed paralogs. Ectopic expression of ZmASR1 had a large overall impact on maize yield that was maintained under water-limited stress conditions in the field. Comparative transcriptomic and proteomic analyses of wild-type and ZmASR1-overexpressing leaves led to the identification of three transcripts and 16 proteins up-or down-regulated by ZmASR1. The majority of them were involved in primary and/or cellular metabolic processes, including branched-chain amino acid (BCAA) biosynthesis. Metabolomic and transcript analyses further indicated that ZmASR1-overexpressing plants showed a decrease in BCAA compounds and changes in BCAA-related gene expression in comparison with wild-type plants. Interestingly, within-group correlation matrix analysis revealed a close link between 13 decreased metabolites in ZmASR1-overexpressing leaves, including two BCAAs. Among these 13 metabolites, six were previously shown to be negatively correlated to biomass, suggesting that ZmASR1-dependent regulation of these 13 metabolites might contribute to regulate leaf growth, resulting in improvement in kernel yield.

ASR1 transcription factor and its role in metabolism

“…Similarly, leaves from Asr1-overexpressing maize plants showed reductions in amino acid levels including branchedchain amino acids (BCAAs: isoleucine, leucine, valine), aromatic amino acids (phenylalanine, tryptophan) and the glutamate family amino acids (asparagine, glutamine, proline). 29 Interestingly, in the same study, the protein levels of some enzymes belonging to different metabolic pathways were altered in the maize leaves, whereas their mRNA levels remained unaltered. In agreement with these results, the authors suggest that the mechanism of action of ASR1 on certain enzymes is through its chaperone-like activity.…”

“…ASR1 levels are inversely related to glucose levels in tobacco and maize leaves of Asr1 transgenic plants. 29,41 Moreover, Ht1 mRNA levels are reduced in the Asr1-silenced tobacco leaves. 41 HT1 is a plasma membrane transporter with high affinity for glucose, and is involved in the retrieval of hexoses from the apoplast in the phloem region of vascular bundles of grape leaves, petioles and fruits.…”

“…Members of the ASR family are present in Solanaceae, grape, maize and conifers, but surprisingly the model plant Arabidopsis thaliana lacks structural homologs. 5 The organization of the family varies among the species: 5 paralogs have been described in tomato, 6,7 9 in maize, 8 3 in potato, 9 6 in rice, 10 4 in banana 11 and one in melon. 12 In general, the ASR proteins are small (molecular mass of tomato ASR1 is 13 kDa) and, specifically, tomato ASR1 is intrinsically unstructured.…”

“…Among others, this increase occurs in tobacco and tomato by osmotic and drought stress; 4,16,[20][21][22] in rice by drought, saline and osmotic stress; 10,23,24,25 and in plantain and banana by osmotic and biotic stress. 11,26,27 Only a few physiological studies with transgenic tomato, 28 tobacco, 22 maize, 29 and rice 18 plants have proved the involvement of ASR1 in stress tolerance.…”

“…26 In addition, Asr1 overexpression in maize plants increased yield and reduced sugar and amino acid levels; which suggests that ASR1 also participates in plant growth through the regulation of this kind of metabolites. 29 The Role of Asr1 in Sugar Metabolism Differs Among Species ASR1 functional analyses through the use of transgenic plants have shown that the metabolic effects depend not only on the species but also on the tissue under study. ASR1 levels are inversely related to glucose levels in tobacco and maize leaves of Asr1 transgenic plants.…”