Regulated partitioning of fixed carbon (14C), sodium (Na+), potassium (K+) and glycine betaine determined salinity stress tolerance of gamma irradiated pigeonpea [Cajanus cajan (L.) Millsp].

Soil salinity is a major constraint that limits legume productivity. Pigeonpea is a salt sensitive crop. Seed gamma irradiation at a very low dose (2.5 Gy) is known to enhance seedling establishment, plant growth and yield of cereals and other crops. The present study conducted using two genetically diverse varieties of pigeonpea viz., Pusa-991 and Pusa-992 aimed at establishing the role of pre-sowing seed gamma irradiation at 0, 0.0025, 0.005, 0.01, 0.02, 0.05 and 0.1 kGy on plant growth, seed yield and seed quality under salt stress at 0, 80 and 100 mM NaCl (soil solution EC equivalent 1.92, 5.86 and 8.02 dS/m, respectively) imposed right from the beginning of the experiment. Changes in carbon flow dynamics between shoot and root and concentration of osmolyte, glycine betaine, plant uptake and shoot and root partitioning of Na+ and K+ and activity of protein degrading enzyme protease were measured under the combined effect of gamma irradiation and salt stress. Positive affect of pre-sowing exposure of seed to low dose of gamma irradiation (<0.01 kGy) under salt stress was evident in pigeonpea. Pigeonpea variety, Pusa-992 showed a better salt tolerance response than Pusa-991 and that the radiated plants performed better than the unirradiated plants even at increasing salinity level. Seed yield and seed protein and iron content were also positively affected by the low dose gamma irradiation under NaCl stress. Multiple factors interacted to determine physiological salt tolerance response of pigeonpea varieties. Gamma irradiation caused a favourable alteration in the source-sink (shoot-root) partitioning of recently fixed carbon (14C) under salt stress in pigeonpea. Gamma irradiation of seeds prior to sowing enhanced glycine betaine content and reduced protease activity at 60-day stage under various salt stress regimes. Lower partitioning of Na+and relatively higher accumulation of K+ under irradiation treatment was the other important determinants that differentiated between salt-tolerant and salt-susceptible variety of pigeonpea. The study provides evidence and physiological basis for exploring exploitation of pre-sowing exposure of seeds with low-dose gamma ray for enhancing the salt tolerance response of crop plants.

Overexpression of an S-like ribonuclease gene, OsRNS4, confers enhanced tolerance to high salinity and hyposensitivity to phytochrome-mediated light signals in rice.

S-like ribonucleases (S-like RNases) are homologous to S-ribonucleases (S-RNases), but are not involved in self-incompatibility. In dicotyledonous plants, S-like RNases play an important role in phosphate recycling during senescence and are induced by inorganic phosphate-starvation and in response to defense and mechanical wounding. However, little information about the functions of the S-like RNase in monocots has been reported. Here, we investigated the expression patterns and roles of an S-like RNase gene, OsRNS4, in abscisic acid (ABA)-mediated responses and phytochrome-mediated light responses as well as salinity tolerance in rice. The OsRNS4 gene was expressed at relatively high levels in leaves although its transcripts were detected in various organs. OsRNS4 expression was regulated by salt, PEG and ABA. The seedlings overexpressing OsRNS4 had longer coleoptiles and first leaves than wild-type seedlings under red light (R) and far-red light (FR), suggesting negative regulation of OsRNS4 in photomorphogenesis in rice seedlings. Moreover, ABA-induced growth inhibition of rice seedlings was significantly increased in the OsRNS4-overexpression (OsRNS4-OX) lines compared with that in WT, suggesting that OsRNS4 probably acts as a positive regulator in ABA responses in rice seedlings. In addition, our results demonstrate that OsRNS4-OX lines have enhanced tolerance to high salinity compared to WT. Our findings supply new evidence on the functions of monocot S-like RNase in regulating photosensitivity and abiotic stress responses.

The circadian clock-associated small GTPase LIGHT INSENSITIVE PERIOD1 suppresses light-controlled endoreplication and affects tolerance to salt stress in Arabidopsis.

Circadian clocks are biochemical timers regulating many physiological and molecular processes according to the day/night cycle. The small GTPase LIGHT INSENSITIVE PERIOD1 (LIP1) is a circadian clock-associated protein that regulates light input to the clock. In the absence of LIP1, the effect of light on free-running period length is much reduced. Here, we show that in addition to suppressing red and blue light-mediated photomorphogenesis, LIP1 is also required for light-controlled inhibition of endoreplication and tolerance to salt stress in Arabidopsis (Arabidopsis thaliana). We demonstrate that in the processes of endoreplication and photomorphogenesis, LIP1 acts downstream of the red and blue light photoreceptors phytochrome B and cryptochromes. Manipulation of the subcellular distribution of LIP1 revealed that the circadian function of LIP1 requires nuclear localization of the protein. Our data collectively suggest that LIP1 influences several signaling cascades and that its role in the entrainment of the circadian clock is independent from the other pleiotropic effects. Since these functions of LIP1 are important for the early stages of development or under conditions normally experienced by germinating seedlings, we suggest that LIP1 is a regulator of seedling establishment.

Limitation of C3-CAM shift in the common ice plant under high irradiance.

In the halophytic plant Mesembryanthemum crystallinum salinity or drought can change the mode of photosynthesis from C(3) to crassulacean acid metabolism (CAM). These two stress factors are linked to oxidative stress, however, the induction of CAM by oxidative stress per se is not straightforward. Treatment with high light (HL) did not lead to the induction of CAM, as documented by a low night/day difference in malate level and a low expression of the CAM-related form of phosphoenolcarboxylase (Ppc1), despite causing some oxidative damage (elevated MDA level, malondialdehyde). In contrast to the action of high salinity (0.4M NaCl), HL treatment did not activate neither the cytosolic NADP-malic enzyme nor the chloroplastic form of NADP-dependent malate dehydrogenase (NADP-MDH). In plastids of HL-treated plants a huge amount of starch was accumulated. This was associated with a weak stimulation of hydrolytic and phosphorolytic starch-degrading enzymes, in contrast to their strong up-regulation under high salinity. It is concluded that HL alone is not able to activate starch degradation necessary for CAM performance. Moreover, in the absence of salinity in C(3)M. crystallinum plants an age-dependent increase in energy dissipation from PSII was documented under high irradiance, as illustrated by non-photochemical quenching (NPQ). Obtained data suggest that in this halophytic species several photoprotective strategies are strictly salinity-dependent.

Germination of dimorphic seeds of the desert annual halophyte Suaeda aralocaspica (Chenopodiaceae), a C4 plant without Kranz anatomy.

BACKGROUND AND AIMS: Suaeda aralocaspica is a C4 summer annual halophyte without Kranz anatomy that is restricted to the deserts of central Asia. It produces two distinct types of seeds that differ in colour, shape and size. The primary aims of the present study were to compare the dormancy and germination characteristics of dimorphic seeds of S. aralocaspica and to develop a conceptual model of their dynamics. METHODS: Temperatures simulating those in the natural habitat of S. aralocaspica were used to test for primary dormancy and germination behaviour of fresh brown and black seeds. The effects of cold stratification, gibberellic acid, seed coat scarification, seed coat removal and dry storage on dormancy breaking were tested in black seeds. Germination percentage and recovery responses of brown seeds, non-treated black seeds and 8-week cold-stratified black seeds to salt stress were tested. KEY RESULTS: Brown seeds were non-dormant, whereas black seeds had non-deep Type 2 physiological dormancy (PD). Germination percentage and rate of germination of brown seeds and of variously pretreated black seeds were significantly higher than those of non-pretreated black seeds. Exposure of seeds to various salinities had significant effects on germination, germination recovery and induction into secondary dormancy. A conceptual model is presented that ties these results together and puts them into an ecological context. CONCLUSIONS: The two seed morphs of S. aralocaspica exhibit distinct differences in dormancy and germination characteristics. Suaeda aralocaspica is the first cold desert halophyte for which non-deep Type 2 PD has been documented.