RICE SALT SENSITIVE3 Forms a Ternary Complex with JAZ and Class-C bHLH Factors and Regulates Jasmonate-Induced Gene Expression and Root Cell Elongation  

Abstract

Plasticity of root growth in response to environmental cues and stresses is a fundamental characteristic of land plants. However, the molecular basis underlying the regulation of root growth under stressful conditions is poorly understood. Here, we report that a rice nuclear factor, RICE SALT SENSITIVE3 (RSS3), regulates root cell elongation during adaptation to salinity. Loss of function of RSS3 only moderately inhibits cell elongation under normal conditions, but it provokes spontaneous root cell swelling, accompanied by severe root growth inhibition, under saline conditions. RSS3 is preferentially expressed in the root tip and forms a ternary complex with class-C basic helix-loop-helix (bHLH) transcription factors and JASMONATE ZIM-DOMAIN proteins, the latter of which are the key regulators of jasmonate (JA) signaling. The mutated protein arising from the rss3 allele fails to interact with bHLH factors, and the expression of a significant portion of JAresponsive genes is upregulated in rss3. These results, together with the known roles of JAs in root growth regulation, suggest that RSS3 modulates the expression of JA-responsive genes and plays a crucial role in a mechanism that sustains root cell elongation at appropriate rates under stressful conditions.

RICE SALT SENSITIVE3 binding to bHLH and JAZ factors mediates control of cell wall plasticity in the root apex

“…Expression of nearly half of salt stress-responsive genes is modified in rss3. 3 The largely altered salt stress responses are rather likely to be responsible for drastically impaired cell elongation in rss3 under the salinity conditions.…”

“…4 Under conditions without salinity stress, rss3 shows a moderate reduction in root tip cell elongation but exhibits severe inhibition of root growth under salinity stressed conditions. 3 This is accompanied by aberrant cellular arrangement, formation of oblique cell plates, swelling of cells, and impaired root flexibility. Cell swelling and abnormal cytokinesis are also observed in cellulose-deficient mutants such as radial swelling1 (rsw1) and rsw2.…”

“…3 These include genes encoding biosynthetic enzymes for lignin, such as PAL, COMT, and CAD (Fig. 1A).…”

“…RSS3 expresses and functions in the meristematic zone (MZ) and in the region that borders the MZ within the elongation zone (EZ) of the root tip, 3 we monitored lignin contents in the cell wall areas in the MZ, by measuring the UV absorbance (278 nm) of ultra-thin sections under a UV microscope. 10 As shown in Figure 2, UV absorption in the cell wall areas was detected only at low levels in the wild type (WT), but at considerable levels in rss3.…”

“…asterisk indicates P value < 0.05 (the Student t-test). 3 Genes encoding lignin biosynthetic enzymes or naC transcription factors that were upregulated in rss3 were selected, based on the microarray data (GEo repository: GSE41442), 3 and levels of gene expression were visualized with tiGr multi-experiment Viewer (meV; http://www.tm4.org/mev/). stress in rss3, in spite of activation of lignin synthetic gene expression.…”