Physiological and molecular responses to drought stress in teak (Tectona grandis L.f.)

Abstract

Drought stress is an increasingly common and worrying phenomenon because it causes a loss of production in both agriculture and forestry. Teak is a tropical tree which needs alternating rainy and dry seasons to produce high-quality wood. However, a robust understanding about the physiological characteristics and genes related to drought stress in this species is lacking. Consequently, after applying moderate and severe drought stress to teak seedlings, an infrared gas analyzer (IRGA) was used to measure different parameters in the leaves. Additionally, using the root transcriptome allowed finding and analyzing the expression of several drought-related genes. As a result, in both water deficit treatments a reduction in photosynthesis, transpiration, stomatal conductance and leaf relative water content was found. As well, an increase in free proline levels and intrinsic water use efficiency was found when compared to the control treatment. Furthermore, 977 transcripts from the root contigs showed functional annotation related to drought stress, and of these, TgTPS1, TgDREB1, TgAREB1 and TgPIP1 were selected. The expression analysis of those genes along with TgHSP1, TgHSP2, TgHSP3 and TgBI (other stress-related genes) showed that with moderate treatment, TgTPS1, TgDREB1, TgAREB1, TgPIP1, TgHSP3 and TgBI genes had higher expression than the control treatment, but with severe treatment only TgTPS1 and TgDREB1 showed higher expression than the control treatment. At the end, a schematic model for the physiological and molecular strategies under drought stress in teak from this study is provided. In conclusion, these physiological and biochemical adjustments in leaves and genetic changes in roots under severe and prolonged water shortage situations can be a limiting factor for teak plantlets' growth. Further studies of those genes under different biotic and abiotic stress treatments are needed.

Fig 1. Photosynthetic rate (μmol CO₂ m^-2 s^-1) in response to the Photosynthetically Active Radiation (P.A.R.) increase (μmol m^-2 s^-1).

All measurements were obtained on the same day and light amount. External graphic (A): Each point represents the mean value of four replicates taken in different plants. The vertical bars correspond to the standard deviation from the mean. The red line is the quadratic tendency of the photosynthetic rate in relation to the irradiance of P.A.R. incident on the leaf (the corresponding equation is in the red square). The green line is the logarithmic trend of the photosynthetic rate relative to the irradiance of P.A.R. incident on the leaf (the corresponding equation is in the green square). The region shaded in blue corresponds to the approximate range of photosynthesis limitation by light in teak. The region shaded in green corresponds to the approximate range of photosynthesis limitation by CO₂ in teak. The blue arrow indicates the light saturation point (1,217 μmol photons m^-2 s^-1 on teak leaf), which is also the irradiance of P.A.R. selected for the subsequent measurements and analyses. Internal graph (B): Photosynthetic response to light only with the linear values of the initial variation range (purple arrow). The purple line is the linear trend of the photosynthetic rate relative to the irradiance of P.A.R. incident on the leaf (the corresponding equation is in the purple square). The red arrow indicates the light compensation point in teak (8.26 μmol photons m^-2 s^-1 on teak leaf).

Fig 2. Photosynthetic leaf gas exchange characteristics of teak plants under different stress treatments (moderate and severe).

A) Photosynthetic Rate, A (μmol CO₂ m^-2 s^-1). B) Stomatal conductance, gs (mol H₂O m^-2 s^-1). C) Transpiration, E (mmol H₂O m^-2 s^-1). D) Leaf temperature, TI (°C)_. Bars represent the standard deviations of the means from five replicates taken in different plants. * refers to the significant value at 0.01 probability, t-test.

Fig 3. Effect of drought stress on teak vegetative growth in the greenhouse.

A) Control plants. B). Plants under stress due to moderate water deficit (20 days). C) Plants under stress due to severe water deficit (30 days).

Fig 4. Leaf relative water and proline content under drought stress.

A) Effect of drought stress on leaf relative water content (%). B) Leaf proline content (μg g^-1 Fresh Weight of leaves) of teak plants under well-watered and water-stressed conditions (moderate and severe). Bars represent the standard deviations from the means for five replicates taken in different plants. * refers to the significant value at 0.01 probability, t-test.

Fig 5. Relative expression of drought stress genes in teak.

A) TgTPS1 (Trehalose 6-Phosphate Synthase), B) TgDREB1 (Dehydration-Responsive Element-binding Protein), C) TgAREB1 (ABA Responsive Element Binding Protein), D) TgPIP1 (Protein Intrinsic of Plasma Membrane), E) TgHSP1 (Heat shock protein), F) TgHSP2 (Heat shock protein), G) TgHSP3 (Heat shock protein), H) TgBI (Bax inhibitor) in teak under different drought stress treatments. Control treatment was used as a calibrator, and TgEF1α (Elongation Factor) was used as the control gene. Bars represent the standard deviations from the means from three technical replicates for each treatment. * refers to the significant value at 0.01 probability, t-test.

Fig 6. A schematic model for the physiological and molecular strategies under drought stress in teak deducted from this study.

Roman numerals indicate the consecutive order of processes that the plant performs during drought stress, based on previous studies and reviews [–104]: I. Signal Perception and Transduction (not studied here). II. ABA signaling. III. Physiological adjustment. IV. Water and Ion Movement. V. Osmoprotection and Metabolic Adjustment. VI. Re-establishment of Homeostasis and Plant Protection (not studied here). VII. Drought Stress Tolerance (not studied here). Green boxes denote the strategies studied in the teak leaf for some processes (at the physiological and metabolic level). Blue boxes denote the strategies studied in the teak root for some processes (at the transcriptional level).