Drought-Induced Root Pressure in Sorghum bicolor

Abstract

Root pressure, also manifested as profusive sap flowing from cut stems, is a phenomenon in some species that has perplexed biologists for much of the last century. It is associated with increased crop production under drought, but its function and regulation remain largely unknown. In this study, we investigated the initiation, mechanisms, and possible adaptive function of root pressure in six genotypes of Sorghum bicolor during a drought experiment in the greenhouse. We observed that root pressure was induced in plants exposed to drought followed by re-watering but possibly inhibited by 100% re-watering in some genotypes. We found that root pressure in drought stressed and re-watered plants was associated with greater ratio of fine: coarse root length and shoot biomass production, indicating a possible role of root allocation in creating root pressure and adaptive benefit of root pressure for shoot biomass production. Using RNA-Seq, we identified gene transcripts that were up- and down-regulated in plants with root pressure expression, focusing on genes for aquaporins, membrane transporters, and ATPases that could regulate inter- and intra-cellular transport of water and ions to generate positive xylem pressure in root tissue.

Keywords: RNA-Seq; agriculture; aquaporin; root pressure; transporter; water relations; xylem transport.

Figure 1

Boxplot of total shoot biomass achieved before harvest by each genotype grown either under control or deficit conditions, as well as percent deficit shoot biomass for each genotype.

Figure 2

Boxplot of root pressure (KPa) measured in control plants vs. deficit plants. Directly before measurements, sample plants were re-watered with 100% of their daily evapotranspiration loss, or not re-watered at all.

Figure 3

Root pressure (KPa) by genotype in samples re-watered before measurement with 50 or 100% of daily evapotranspiration loss. Error bars represent standard error.

Figure 4

Principal component analysis of re-watered plants in the Deficit treatment. Loading arrows indicate the magnitude and direction of trait variation (7 traits) underlying the sample distribution. Ellipses indicate groupings by genotype.

Figure 5

Relationship between root pressure (KPa) and the root system's relative allocation to fine roots (ratio of total fine root length to total coarse root length, unitless), by genotype. Orange genotypes (Btx642, IS3620C, and SC56) are drought susceptible, and green genotypes (RTx430, Btx623, and Tx700) are drought tolerant. Note that Btx623 and Tx700 (bottom panels) have much higher proportions of fine roots than the other genotypes. Dotted regression lines indicate non-significant relationships.

Figure 6

Normalized RNA-Seq counts of gene transcripts in each of three treatment groups (Control Low, Deficit Low, and Deficit Mid) for the following differentially-expressed genes: (A) PIP1;5, (B) NIP2;3, (C) WAT1-related protein Atg64700, (D) Salt stress-induced protein, (E) Beta-galactosidase 9, (F) Beta-amylase 3, (G) Plasmodesmata callose-binding protein.