Chromatin interacting factor OsVIL2 increases biomass and rice grain yield

Grain number is an important agronomic trait. We investigated the roles of chromatin interacting factor Oryza sativa VIN3‐LIKE 2 (OsVIL2), which controls plant biomass and yield in rice. Mutations in OsVIL2 led to shorter plants and fewer grains whereas its overexpression (OX) enhanced biomass production and grain numbers when compared with the wild type. RNA‐sequencing analyses revealed that 1958 genes were up‐regulated and 2096 genes were down‐regulated in the region of active division within the first internodes of OX plants. Chromatin immunoprecipitation analysis showed that, among the downregulated genes, OsVIL2 was directly associated with chromatins in the promoter region of CYTOKININ OXIDASE/DEHYDROGENASE2 (OsCKX2), a gene responsible for cytokinin degradation. Likewise, active cytokinin levels were increased in the OX plants. We conclude that OsVIL2 improves the production of biomass and grain by suppressing OsCKX2 chromatin.     

The overexpression of OsNAC9 alters the root architecture of rice plants enhancing drought resistance and grain yield under field conditions

Drought 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 Ca²⁺ 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.     

SHY2 as a node in the regulation of root meristem development by auxin,brassinosteroids, and cytokinin

In multicellular organisms, the balance between cell division and differentiation determines organ size, and represents a central unknown in developmental biology. In Arabidopsis roots, this balance is mediated between cytokinin and auxin through a regulatory circuit converging on the IAA3/SHORT HYPOCOTYL 2 (SHY2) gene. Here, we show that crosstalk between brassinosteroids (BRs) and auxin occurs in the vascular transition zone to promote root meristem development. We found that BR increases root meristem size by up‐regulating expression of the PINFORMED 7 (PIN7) gene and down‐regulating expression of the SHY2 gene. In addition, BES1 could directly bind to the promoter regions of both PIN7 and SHY2, indicating that PIN7 and SHY2 mediate the BR‐induced growth of the root meristem by serving as direct targets of BES1. Moreover, the PIN7 overexpression and loss‐of‐function SHY2 mutant were sensitive to the effects of BR and could partially suppress the short‐root phenotypes associated with deficient BR signaling. Interestingly, BRs could inhibit the accumulation of SHY2 protein in response to cytokinin. Taken together, these findings suggest that a complex equilibrium model exists in which regulatory interactions among BRs, auxin, and cytokinin regulate optimal root growth.     

OsNAC5 overexpression enlarges root diameter in rice plants leading to enhanced drought tolerance and increased grain yield in the field

Drought conditions are among the most serious challenges to crop production worldwide. Here, we report the results of field evaluations of transgenic rice plants overexpressing OsNAC5, under the control of either the root‐specific (RCc3) or constitutive (GOS2) promoters. Field evaluations over three growing seasons revealed that the grain yield of the RCc3:OsNAC5 and GOS2:OsNAC5 plants were increased by 9%–23% and 9%–26% under normal conditions, respectively. Under drought conditions, however, RCc3:OsNAC5 plants showed a significantly higher grain yield of 22%–63%, whilst the GOS2:OsNAC5 plants showed a reduced or similar yield to the nontransgenic (NT) controls. Both the RCc3:OsNAC5 and GOS2:OsNAC5 plants were found to have larger roots due to an enlarged stele and aerenchyma at flowering stage. Cell numbers per cortex layer and stele of developing roots were higher in both transgenic plants than NT controls, contributing to the increase in root diameter. The root diameter was enlarged to a greater extent in the RCc3:OsNAC5, suggesting the importance of this phenotype for enhanced drought tolerance. Microarray experiments identified 25 up‐regulated genes by more than three‐fold (P < 0.01) in the roots of both transgenic lines. Also identified were 19 and 18 up‐regulated genes that are specific to the RCc3:OsNAC5 and GOS2:OsNAC5 roots, respectively. Of the genes specifically up‐regulated in the RCc3:OsNAC5 roots, GLP, PDX, MERI5 and O‐methyltransferase were implicated in root growth and development. Our present findings demonstrate that the root‐specific overexpression of OsNAC5 enlarges roots significantly and thereby enhances drought tolerance and grain yield under field conditions.     

The rice OsNAC6 transcription factor orchestrates multiple molecular mechanisms involving root structural adaptions and nicotianamine biosynthesis for drought tolerance

Drought has a serious impact on agriculture worldwide. A plant's ability to adapt to rhizosphere drought stress requires reprogramming of root growth and development. Although physiological studies have documented the root adaption for tolerance to the drought stress, underlying molecular mechanisms is still incomplete, which is essential for crop engineering. Here, we identified OsNAC6‐mediated root structural adaptations, including increased root number and root diameter, which enhanced drought tolerance. Multiyear drought field tests demonstrated that the grain yield of OsNAC6 root‐specific overexpressing transgenic rice lines was less affected by drought stress than were nontransgenic controls. Genome‐wide analyses of loss‐ and gain‐of‐function mutants revealed that OsNAC6 up‐regulates the expression of direct target genes involved in membrane modification, nicotianamine (NA) biosynthesis, glutathione relocation, 3′‐phophoadenosine 5′‐phosphosulphate accumulation and glycosylation, which represent multiple drought tolerance pathways. Moreover, overexpression of NICOTIANAMINE SYNTHASE genes, direct targets of OsNAC6, promoted the accumulation of the metal chelator NA and, consequently, drought tolerance. Collectively, OsNAC6 orchestrates novel molecular drought tolerance mechanisms and has potential for the biotechnological development of high‐yielding crops under water‐limiting conditions.     

Synergistic action of auxin and cytokinin mediates aluminum‐induced root growth inhibition in Arabidopsis

Auxin acts synergistically with cytokinin to control the shoot stem‐cell niche, while both hormones act antagonistically to maintain the root meristem. In aluminum (Al) stress‐induced root growth inhibition, auxin plays an important role. However, the role of cytokinin in this process is not well understood. In this study, we show that cytokinin enhances root growth inhibition under stress by mediating Al‐induced auxin signaling. Al stress triggers a local cytokinin response in the root‐apex transition zone (TZ) that depends on IPTs, which encode adenosine phosphate isopentenyltransferases and regulate cytokinin biosynthesis. IPTs are up‐regulated specifically in the root‐apex TZ in response to Al stress and promote local cytokinin biosynthesis and inhibition of root growth. The process of root growth inhibition is also controlled by ethylene signaling which acts upstream of auxin. In summary, different from the situation in the root meristem, auxin acts with cytokinin in a synergistic way to mediate aluminum‐induced root growth inhibition in Arabidopsis.     

Role of AUX1 in the control of organ identity during in vitro organogenesis and in mediating tissue specific auxin and cytokinin interaction in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Classic plant tissue culture experiments have shown that exposure of cell culture to a high auxin to cytokinin ratio promotes root formation and a low auxin to cytokinin ratio leads to shoot regeneration. It has been widely accepted that auxin and cytokinin play an antagonistic role in the control of organ identities during organogenesis in vitro. Since the auxin level is highly elevated in the shoot meristem tissues, it is unclear how a low auxin to cytokinin ratio promotes the regeneration of shoots. To identify genes mediating the cytokinin and auxin interaction during organogenesis in vitro, three allelic mutants that display root instead of shoot regeneration in response to a low auxin to cytokinin ratio are identified using a forward genetic approach in Arabidopsis. Molecular characterization shows that the mutations disrupt the AUX1 gene, which has been reported to regulate auxin influx in plants. Meanwhile, we find that cytokinin substantially stimulates auxin accumulation and redistribution in calli and some specific tissues of Arabidopsis seedlings. In the aux1 mutants, the cytokinin regulated auxin accumulation and redistribution is substantially reduced in both calli and specific tissues of young seedlings. Our results suggest that auxin elevation and other changes stimulated by cytokinin, instead of low auxin or exogenous auxin directly applied, is essential for shoot regeneration. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Improving rice tolerance to potassium deficiency by enhancing OsHAK16p:WOX11‐controlled root development

Potassium (K) deficiency in plants confines root growth and decreases root‐to‐shoot ratio, thus limiting root K acquisition in culture medium. A WUSCHEL‐related homeobox (WOX) gene, WOX11, has been reported as an integrator of auxin and cytokinin signalling that regulates root cell proliferation. Here, we report that ectopic expression of WOX11 gene driven by the promoter of OsHAK16 encoding a low‐K‐enhanced K transporter led to an extensive root system and adventitious roots and more effective tiller numbers in rice. The WOX11‐regulated root and shoot phenotypes in the OsHAK16p:WOX11 transgenic lines were supported by K‐deficiency‐enhanced expression of several RR genes encoding type‐A cytokinin‐responsive regulators, PIN genes encoding auxin transporters and Aux/IAA genes. In comparison with WT, the transgenic lines showed increases in root biomass, root activity and K concentrations in the whole plants, and higher soluble sugar concentrations in roots particularly under low K supply condition. The improvement of sugar partitioning to the roots by the expression of OsHAK16p:WOX11 was further indicated by increasing the expression of OsSUT1 and OsSUT4 genes in leaf blades and several OsMSTs genes in roots. Expression of OsHAK16p:WOX11 in the rice grown in moderate K‐deficient soil increased total K uptake by 72% and grain yield by 24%–32%. The results suggest that enlarging root growth and development by the expression of WOX11 in roots could provide a useful option for increasing K acquisition efficiency and cereal crop productivity in low K soil.