From bottom to top : tweaking auxin homeostasis to deal with salt stress

Salinity of the soil is a major problem for agriculture. The number of salinized soils is rapidly increasing worldwide, and most crops are highly sensitive to salt (NaCl). To cope with salt stress, plants can adapt their growth and development. In Arabidopsis and in some crop species favorable adaptations of the root system to cope with abiotic stress have been described. Plants respond by a redistribution of root mass between main and lateral roots, yet the genetic machinery underlying this process is still largely unknown. In Chapter 2 we review the currently known root system architecture (RSA) responses in Arabidopsis and a number of crop species to a range of abiotic stresses, including nutrient limitation, drought, salinity, flooding and extreme temperatures. Although the root system is the first part of the plant to encounter salt stress, aboveground plant development and especially timing of flowering are also important factors in determining yield. The research presented in this thesis aims at further unraveling how plants adapt and acclimate to cope with salt stress both belowground (root system architecture in Chapter 2, 3, 4) and aboveground (timing of flowering, Chapter 5).Genomic screens using available genetic variation in natural populations have shown to be instrumental to identify loci with the potential of breeding more resilient crops. In this thesis we utilized natural variation in accessions of Arabidopsis to find new candidate genes involved in root development (Chapter 3) and timing of flowering (Chapter 5). In both chapters we identify candidate genes involved in auxin metabolism.CYTOCHROME P450 FAMILY 79 SUBFAMILY B2 (CYP79B2), identified in Chapter 3 to play a role in maintaining lateral root development, converts tryptophan to indole-3-acetaldoxime (IAOx). Changes in CYP79B2 expression in salt stress are positively correlated with lateral root development in Arabidopsis accessions. The IAOx pathway can produce camalexin, indole glucosinolates and indole-3-acetic acid (IAA). In Chapter 4 further investigation of this pathway puts the metabolic compound indole-3-acetonitrile (IAN) central in maintaining lateral root development during salt stress. IAN can either be synthesized as a breakdown product of indole glucosinolates, which we show to accumulate during salt stress or directly from IAOx by a novel candidate enzyme CYP71A19, which we characterize in this chapter.Salinity of the soil greatly influences the timing of flowering, but the mechanisms underlying remain largely elusive this far. In Chapter 5 we again exploit GWAS, this time to find candidate genes involved in the timing of flowering in response to salt stress. We show that a delay in flowering in response to salt stress is the common response in almost all studied accessions. Again, auxin metabolism seems to play a major role, as natural variation in the promoter of UDP-glycosyltransferase 74E2 (UGT74E2), an IBA conjugase, is linked to the extent of delay in bolting in salt stress. Further investigation of the promoter region of UGT74E2 revealed seven candidate transcription factors putatively binding the promoter and influencing its expression during salt stress. These transcription factors will provide a good starting point for further research on UGT74E2 and its upstream regulators, to obtain more insight in fine-tuning of the network of timing of flowering under salt stress.This thesis illustrates that salt induced changes in both above and belowground development are guided by changes in auxin metabolism. Chapter 6 integrates the current knowledge on auxin transport and auxin metabolism during salt stress. Our analysis stresses the importance of considering all these components together and highlights the use of mathematical modelling for predictions of plant physiological responses. In Chapter 7 we expand the discussion by taking one step back, looking at the upstream regulation of auxin metabolism under salt stress, and one step forward, discussing how altered root development might lead to salt tolerance.

Bibliographic Details

Main Author:	Koevoets, Iko Tamar
Other Authors:	Testerink, C.S.
Format:	Doctoral thesis biblioteca
Language:	English
Published:	Wageningen University
Subjects:	Life Science,
Online Access:	https://research.wur.nl/en/publications/from-bottom-to-top-tweaking-auxin-homeostasis-to-deal-with-salt-s
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