The overexpression ofOsNAC9alters the root architecture of rice plants enhancing drought resistance and grain yield under field conditions

Abstract

SummaryDrought conditions limit agricultural production by preventing crops from reaching their genetically predetermined maximum yields. Here, we present the results of field evaluations of rice overexpressing OsNAC9, a member of the rice NAC domain family. Root-specific (RCc3) and constitutive (GOS2) promoters were used to overexpress OsNAC9 and produced the transgenic RCc3:OsNAC9 and GOS2:OsNAC9 plants. Field evaluations over two cultivating seasons showed that grain yields of the RCc3:OsNAC9 and the GOS2:OsNAC9 plants were increased by 13%-18% and 13%-32% under normal conditions, respectively. Under drought conditions, RCc3:OsNAC9 plants showed an increased grain yield of 28%-72%, whilst the GOS2:OsNAC9 plants remained unchanged. Both transgenic lines exhibited altered root architecture involving an enlarged stele and aerenchyma. The aerenchyma of RCc3:OsNAC9 roots was enlarged to a greater extent than those of GOS2:OsNAC9 and non-transgenic (NT) roots, suggesting the importance of this phenotype for enhanced drought resistance. Microarray experiments identified 40 up-regulated genes by more than threefold (P < 0.01) in the roots of both transgenic lines. These included 9-cis-epoxycarotenoid dioxygenase, an ABA biosynthesis gene, calcium-transporting ATPase, a component of the Ca 2+ signalling pathway involved in cortical cell death and aerenchyma formation, cinnamoyl CoA reductase 1, a gene involved in lignin biosynthesis, and wall-associated kinases¸genes involved in cell elongation and morphogenesis. Interestingly, O-methyltransferase, a gene necessary for barrier formation, was specifically up-regulated only in the RCc3:OsNAC9 roots. Such up-regulated genes that are commonly and specifically up-regulated in OsNAC9 transgenic roots may account for the altered root architecture conferring increased drought resistance phenotype.

Overexpression of the OsERF71 Transcription Factor Alters Rice Root Structure and Drought Resistance

“…Similar phenotypes have been reported in previous studies. Specifically, vigorous root radial growth was shown to be closely connected to high plant water potential under drought conditions in rice (Ekanayake et al, 1985;Price et al, 1997), and overexpression of OsNAC5, OsNAC9, and OsNAC10 promoted thicker radial root growth and enhanced drought resistance in rice (Jeong et al, , 2013Redillas et al, 2012). Interestingly, root aerenchyma were recently reported to improve drought tolerance by reducing root metabolic costs and optimizing root growth and water uptake from drying soil (Zhu et al, 2010;Yang et al, 2012).…”

“…Several studies have shown that overexpression of droughtresponsive transcription factors can lead to enhanced drought resistance. For example, plants overexpressing Arabidopsis (Arabidopsis thaliana) AtMYB96 or HARDY (an AP2/ERF transcription factor) or transgenic rice (Oryza sativa) lines overexpressing OsAP37, OsNAC5, OsNAC9, OsNAC10, OsbZIP12, OsbZIP12, or OsbZIP23 all showed strong resistance to drought stress (Karaba et al, 2007;Oh et al, 2009;Jeong et al, 2010Jeong et al, , 2013Seo et al, 2011;Redillas et al, 2012;Joo et al, 2014;Park et al, 2015). Drought-responsive genes are often categorized into abscisic acid (ABA)-dependent or ABAindependent groups.…”

<strong>When water runs dry and temperature heats up: </strong><strong>Understanding the mechanisms in rice tolerance to drought and </strong><strong>high temperature stress conditions</strong>

“…For example, overexpression of OsNAC6, OsNAC10, OsNAC9 and OsNAC5 in rice improved drought tolerance through increased root number and diameter (Lee et al, 2016a). Tolerance conferred by overexpression analysis was suggested to be mediated either by nicotianamine (NA) biosynthesis, glutathione relocation, or by incorporating and controlling factors involved in cell wall biosynthesis (Lee et al, 2016a, Lee et al, 2016b, Jeong et al, 2010, Redillas et al, 2012, Jeong et al, 2013. Transgenic rice plants overexpressing SNAC1 showed drought resistance both at the vegetative and reproductive stage by reducing water loss through regulation of stomatal opening possibly by regulating the expression of OsSRO1c but with no compromise in photosynthesis , Hu et al, 2006.…”

Rice OsERF71-mediated root modification affects shoot drought tolerance

“…Upon drought perception, plants give rise to root structural modification including length, number and radial expansion. [1][2][3][4][5][6][7][8] Alternatively or simultaneously, plant roots release uncharacterized drought-inducible signals that move to the aerial parts of the plants and confer drought tolerance to the shoot. [9][10][11][12][13][14] Although several studies have documented the root adaption for drought tolerance, little is known about the underlying molecular mechanisms not only that give rise to root morphological modification but also by which the root morphological adaptation affects plant capacity to drought stresses.…”

“…The larger aerenchyma are a root modification commonly found in drought tolerant rice plants, such that overexpression of OsNAC5, OsNAC9 and OsNAC10 activates radial root growth that enhances tolerance to drought stress. 4,6,7 Maize roots with large cortical aerenchyma also promote drought tolerance since it reduces the metabolic cost of soil exploration under water stress, permitting greater root growth and water acquisition from drying soil. 16 In addition to radial root growth, root elongation and high number of roots are associated with root structural adaptation to drought stresses.…”