Killing two birds with one stone

Abstract

Plants, being immobile, must adjust development in response to various stresses, external and internal. Although much has been learned about the mechanisms that regulate development and sugar signaling and response, how the two processes are coordinated is poorly understood. GRAS-family transcriptional regulators SHORT-ROOT (SHR) and SCARECROW (SCR) are crucial to radial patterning and stem-cell renewal in the Arabidopsis root. We found that they directly control genes involved not only in development but also in stress responses and that SCR is pivotal in modulating sugar homeostasis and response. Our data suggest that SHR and SCR promote root growth by suppressing the deleterious effects of stress and that ABI4 has a dual role in sugar response and root growth. Other transcriptional regulators have also been reported to play dual roles in plant growth and stress responses. I therefore propose that regulation of both development and stress responses by single transcriptional regulators is a general and efficient mechanism of adaptive response in plants.

Plants cannot move; therefore, to survive a changing environment, they must be able to adjust development in accordance with the stresses to which they are exposed. Because plants are composed of multiple organs that execute distinct functions, to ensure optimal growth, they must also orchestrate various biological processes. As the energy source for all cellular activities, sugar is essential for plant growth and development, but at elevated levels it inhibits growth. Plants have multiple sugar-signaling pathways,¹ and many sugar-sensing and -response factors have been identified,² such as the hexokinase family members HXK1,³ HXK2,² HXK3,⁴ the transcription factor ABI4,⁵ and the ABA biosynthesis gene ABA2/GIN1.⁶ Nevertheless, how plants balance the trade-offs between the need for sugar and its growth-inhibitory effects is poorly understood. Our recent study demonstrates a role for the development regulator SCARECROW in modulating sugar response. This provides an important piece to this trade-offs puzzle.

SCARECROW (SCR) is a plant-specific GRAS-family transcriptional regulator with a critical role in stem-cell renewal and radial patterning in the Arabidopsis root.⁷ SHORT-ROOT (SHR), another GRAS family member, plays a similar role in root development but has an expression pattern distinct from that of SCR.⁸ SCR is primarily expressed in the endodermis, the cortex/endodermis initial cell, and the quiescent-center cells, which are the ultimate source of all cell types in the root.⁷ In contrast, SHR is expressed in the vascular tissue. The SHR protein however moves outwards to the adjacent cell layer, where it forms a complex with SCR and together they maintain a high level of SCR through a positive feedback loop.^9,10 SCR in turn sequesters SHR in the nucleus and restricts its movement. Because SHR is required for the specification of the endodermis, the SHR/SCR-dependent accumulation of SCR ensures that all SHR molecules are trapped in the endodermis, thereby ensuring the formation of a single layer of endodermis.⁹

Although much has been learned about the mechanism by which SHR and SCR control radial patterning, how these development regulators promote stem-cell renewal is still unclear. To address this question, we recently determined the direct targets of SHR at the genome scale by ChIP-chip.^11,12 This study has led us to the finding that SHR regulates root vascular differentiation and patterning by modulating cytokinin homeostasis.¹² Because cytokinin is known to reduce root meristematic activity, we hypothesized that the failure by the shr mutant to maintain the root stem cells is due to an elevated level of cytokinin, but our results showed that this is not the case, as reduction of cytokinin does not alleviate the root-meristem defect in shr.¹² Interestingly, among the SHR direct targets are a number of genes that are involved not only in development but also in stress responses, raising the possibility that SHR and SCR also play a role in stress response and that they promote root growth by suppressing stress response. Consistent with this notion, we found that the shr and scr mutants are hypersensitive to high levels of glucose.¹³ The sugar hypersensitivity is unlikely to be a general response to high osmoticum, because shr and scr respond to high levels of salt and mannitol similarly to the wild type and because soluble sugar levels are elevated in both mutants. Most importantly, we showed that SCR binds to the promoters of ABI4 and ABI5 and that the transcript levels of ABI4 and ABI5 are dramatically increased in scr. Because ABI4 is a positive regulator of sugar signaling, this observation indicates that SCR has a direct role in modulating sugar response. Supporting this hypothesis, scr is no longer hypersensitive to high sugar when ABI4 is mutated.

Surprisingly, we found that shr is much less sensitive to high sugar than scr, even though SHR acts upstream of SCR and the two mutants have similar developmental defects. An explanation for this observation arose from our further studies showing that SCR, but not SHR, regulates ABI4 and ABI5 directly. SHR is essential for SCR expression at a level required for radial patterning, but it does not determine the cell-type-specific expression pattern of SCR, as SCR is still expressed in the mutant ground tissue layer in shr, albeit at a much lower level.⁹ Because of this SHR-independent regulatory mechanism, ABI4 would remain largely repressed in shr, thereby conferring less sugar sensitivity. In the wild type, SCR is expressed at a much higher level than SHR as a consequence of the SHR/SCR-dependent feedback loop, and inevitably some SCR protein could not form a complex with SHR. The superfluous SCR molecules would act as a transcriptional repressor, unlike those in the SHR/SCR complex, which activates gene transcription.

Sugar response is unlikely to be the only type of stress in which SCR is involved, however, because most of the confirmed SCR direct targets are associated with other kinds of stress,¹³ and T-DNA insertion in these genes did not seem to cause sugar hypersensitivity (data unpublished). To determine whether SHR and SCR promote root growth by suppressing the harmful effects of stresses on root meristematic activity, we therefore introduced the aba2/gin1 mutation into shr or scr so that all stress responses could be blocked. Interestingly, both the shr aba2 and scr aba2 double mutants had a much larger root than the scr and shr single mutants (unpublished), lending strong support to our hypothesis.

Our recent study not only uncovered a dual role for SHR and SCR in stress response but also revealed a positive role for ABI4 in root development. ABI4 is known to be sugar inducible and to suppress plant growth¹⁴ and chloroplast development.¹⁵ We also showed that, when ectopically expressed in the quiescent center and endodermis, ABI4 inhibited root growth. Intriguingly, we found that, under normal growth conditions, the abi4 mutation caused reduction in root growth in the wild type and scr. These results suggest that ABI4 inhibits plant growth at high expression levels but is required for root growth at low levels.

In addition to SHR, SCR, and ABI4, other transcriptional factors that appear to have a dual role in plant growth and stress response have been reported.^16-20 Transcriptome profiling with individual cell types in the Arabidopsis root suggests that many development regulators are involved in stress response as well.²¹ In light of these findings, we propose that regulation of development and stress response by the same transcriptional factors is a general and efficient mechanism by which plants “kill two birds with one stone” in orchestrating various processes for optimal growth or survival in a changing environment.

Cui H, Hao Y, Kong D. SCARECROW has a SHORT-ROOT independent role in modulating the sugar response. Plant Physiol. 2012;158:1769–78. doi: 10.1104/pp.111.191502.