Physiological and Molecular Responses of 'Dusa' Avocado Rootstock to Water Stress: Insights for Drought Adaptation

Abstract

Avocado consumption is increasing year by year, and its cultivation has spread to many countries with low water availability, which threatens the sustainability and profitability of avocado orchards. However, to date, there is not much information on the behavior of commercial avocado rootstocks against drought. The aim of this research was to evaluate the physiological and molecular responses of 'Dusa' avocado rootstock to different levels of water stress. Plants were deficit irrigated until soil water content reached 50% (mild-WS) and 25% (severe-WS) of field capacity. Leaf water potential (Ψ_w), net CO₂ assimilation rates (A_N), transpiration rate (E), stomatal conductance (g_s), and plant transpiration rates significantly decreased under both WS treatments, reaching significantly lower values in severe-WS plants. After rewatering, mild- and severe-WS plants showed a fast recovery in most physiological parameters measured. To analyze root response to different levels of drought stress, a cDNA avocado stress microarray was carried out. Plants showed a wide transcriptome response linked to the higher degree of water stress, and functional enrichment of differentially expressed genes (DEGs) revealed abundance of common sequences associated with water stress, as well as specific categories for mild-WS and severe-WS. DEGs previously linked to drought tolerance showed overexpression under both water stress levels, i.e., several transcription factors, genes related to abscisic acid (ABA) response, redox homeostasis, osmoprotection, and cell-wall organization. Taken altogether, physiological and molecular data highlight the good performance of 'Dusa' rootstock under low-water-availability conditions, although further water stress experiments must be carried out under field conditions.

Keywords: Persea americana; abiotic stress; microarray; water deprivation.

Figure 1

Schematic illustration of the experimental design. Control plants were maintained at field capacity (Fc) throughout the experiment, and water-stressed plants were subjected to substrate drying-up until they reached 50% of Fc (mild-WS; t₁) and 25% of Fc (severe-WS; t₂), respectively. Subsequently, plants were fully irrigated to assess the drought recovery response (t₂ and t₃).

Figure 2

Time-course of mean values (± SE; n = 4 to 16) of volumetric soil moisture (A), relative water content (B), predawn (C) and midday leaf water potential (Ψ_w; D), net CO₂ assimilation rate (A_N; E), transpiration rate (E; F), stomatal conductance (gs; G), and intrinsic water use efficiency (A_N/g_s; H) of ‘Dusa’ plants subjected to three water treatments: control, mild-WS (50% Fc), and severe-WS (25% Fc). Solid arrows indicate the timepoints where measurements and samplings were done, while dotted arrows show rewatering days. Different letters indicate significant differences among treatments for the specific timepoints (days) (p < 0.05; ns: no statistical differences).

Figure 3

Predawn and midday water potential (Ψw; A), plant transpiration rate (B), and plant hydraulic conductance (K_h; C) from ‘Dusa’ avocado rootstock subjected to two different levels of water stress: mild-WS (50% of Fc) and severe-WS (25% of Fc (mean ± SE; n = 4) and their corresponding recovery. Within each series, different capital or lowercase letters indicate significant differences among treatments (p < 0.05).

Figure 4

Venn diagram of differentially expressed genes. Numbers of common and specific differentially expressed genes (DEGs) obtained in the microarray analysis of ‘Dusa’ avocado rootstocks subjected to two different levels of water stress (50% of Fc, mild-WS and 25% of Fc, severe-WS). Statistically significant DEGs (p < 0.05) were filtered above and below fold change values of 2 and −2, respectively. Unique DEGs are shown only in one of the two circles, while shared transcripts are shown in their intersection. Arrows indicate the number of genes up- and downregulated.

Figure 5

Gene Ontology (GO) enrichment analysis of differentially expressed genes (DEGs) in the microarray analysis of ‘Dusa’ avocado rootstocks subjected to two different levels of water stress: mild-WS (50% of Fc) and severe-WS (25% of Fc). Enrichment GO terms were obtained by Blast2GO (p < 0.05). BP, biological process; MF, molecular function; CC, cellular component. Black bars represent test set (mild and severe-WS) while gray bars represent the reference set.

Figure 6

Hierarchical clustering (HCL) of differentially expressed genes (DEGs) in the microarray analysis of ‘Dusa’ avocado rootstock subjected to two different levels of water stress: mild-WS (50% of Fc) and severe-WS (25% of Fc).

Figure 7

Comparative response of ‘Dusa’ avocado rootstock subjected to two different levels of water stress: mild-WS (50% of Fc) and severe-WS (25% of Fc). Physiological response to the different levels of water stress are represented with changes in net CO₂ assimilation rate (A_N), stomatal conductance (g_s), transpiration rate (E), relative water content (RWC), leaf water potential (Ψ_w), plant transpiration rate, and plant hydraulic conductance (K_h). At a molecular level, mild and severe-WS treatment trigger the overexpression of genes encoding detoxification enzymes (REDOX), transcription factors (TFs), abscisic acid pathway (ABA), osmoprotectants (OSP), cell-wall organization (CWO), heat-shock proteins (HSPs), and chaperones (CHPs).