A tale of two pumps: Blue light and abscisic acid alter Arabidopsis leaf hydraulics via bundle sheath cell H + -ATPases

The bundle sheath cell (BSC) layer tightly enveloping the xylem throughout the leaf is recognized as a major signal-perceiving "valve" in series with stomata, regulating leaf hydraulic conductance (Kleaf) and thereby radial water flow via the transpiring leaf. The BSC blue light (BL) signaling pathway increases Kleaf and the underlying BSC water permeability. Here, we explored the hypothesis that BSCs also harbor a Kleaf-downregulating signaling pathway related to the stress phytohormone abscisic acid (ABA). We employed fluorescence imaging of xylem sap in detached leaves and BSC protoplasts from different genotypes of Arabidopsis (Arabidopsis thaliana) plants, using pH and membrane potential probes to monitor physiological responses to ABA and BL in combination with pharmacological agents. We found that BL-enhanced Kleaf required elevated BSC cytosolic Ca2+. ABA inhibited BL-activated xylem-sap-acidifying BSC H + -ATPase AHA2 (Arabidopsis H + -ATPase 2), resulting in depolarized BSCs and alkalinized xylem sap. ABA also stimulated BSC vacuolar H + -ATPase (VHA), which alkalinized the BSC cytosol. Each pump stimulation, AHA2 by BL and VHA by ABA (under BL), also required Ca2+. ABA stimulated VHA in the dark depending on Ca2+, but only in an alkaline external medium. Taken together with earlier findings on the pH sensitivity of BSC osmotic water permeability (i.e., aquaporin activity), our results suggest a Ca2+-dependent and pH-mediated causative link between the BL- and ABA-regulated activities of two BSC H + -ATPases and Kleaf.

Woolly mutation with Get02 locus overcomes the polygenic nature of trichome-based pest resistance in tomato

Type-IV glandular trichomes, which only occur in the juvenile developmental phase of the cultivated tomato (Solanum lycopersicum), produce acylsugars that broadly protect against arthropod herbivory. Previously, we introgressed the capacity to retain type-IV trichomes in the adult phase from the wild tomato, Solanum galapagense, into the cultivated species cv. Micro-Tom (MT). The resulting MT-Get (MicroTom-Galapagos enhanced trichome) introgression line contained five loci associated with enhancing the density of type-IV trichomes in adult plants. We genetically dissected MT-Get and obtained a sub-line containing only the locus on chromosome 2 (MT-Get02). This genotype displayed about half the density of type-IV trichomes compared to the wild progenitor. However, when we stacked the gain-of-function allele of WOOLLY, which encodes an HD-ZIP IV transcription factor, Get02/Wo exhibited double the number of type-IV trichomes compared to S. galapagense. This discovery corroborates previous reports positioning WOOLLY as a master regulator of trichome development. Acylsugar levels in Get02/Wo were comparable to the wild progenitor, although the composition of acylsugar types differed, especially regarding fewer types with medium-length acyl chains. Agronomical parameters of Get02/Wo, including yield, were comparable to MT. Pest resistance assays showed enhanced protection against silverleaf whitefly (Bemisia tabaci), tobacco hornworm (Manduca sexta), and the fungus Septoria lycopersici. However, resistance levels did not reach that of the wild progenitor, suggesting the specificity of acylsugar types in the pest resistance mechanism. Our findings in trichome-mediated resistance advance the development of robust, naturally resistant tomato varieties, harnessing the potential of natural genetic variation. Moreover, by manipulating only two loci, we achieved exceptional results for a highly complex, polygenic trait, such as herbivory resistance in tomato.

Out of the blue: Phototropins of the leaf vascular bundle sheath mediate the regulation of leaf hydraulic conductance by blue light

The Arabidopsis (Arabidopsis thaliana) leaf veins bundle-sheath cells (BSCs)-a selective barrier to water and solutes entering the mesophyll-increase the leaf radial hydraulic conductance (Kleaf) by acidifying the xylem sap by their plasma membrane H+-ATPase,  AHA2. Based on this and on the BSCs' expression of phototropins PHOT1 and PHOT2, and the known blue light (BL)-induced Kleaf increase, we hypothesized that, resembling the guard cells, BL perception by the BSCs' phots activates its H+-ATPase, which, consequently, upregulates Kleaf. Indeed, under BL, the Kleaf of the knockout mutant lines phot1-5, phot2-1, phot1-5 phot2-1, and aha2-4 was lower than that of the wild-type (WT). BSC-only-directed complementation of phot1-5 or aha2-4 by PHOT1 or AHA2, respectively, restored the BL-induced Kleaf increase. BSC-specific silencing of PHOT1 or PHOT2 prevented such Kleaf increase. A xylem-fed kinase inhibitor (tyrphostin 9) replicated this also in WT plants. White light-ineffective in the phot1-5 mutant-acidified the xylem sap (relative to darkness) in WT and in the PHOT1-complemented phot1-5. These results, supported by BL increase of BSC protoplasts' water permeability and cytosolic pH and their hyperpolarization by BL, identify the BSCs as a second phot-controlled water conductance element in leaves, in series with stomatal conductance. Through both, BL regulates the leaf water balance.