Abstract:
Rice (Oryza sativa L.) is the second most important cereal crop in the world but its production
suffers from saline environments in many areas since it is one of the most salt sensitive crops.
However, the large variation in tolerance between rice cultivars can be exploited to gain
insights into mechanisms of salinity tolerance. A multifaceted approach encompassing
physiology, biochemistry, transcriptomics and genetics was followed to study the mechanisms
of salt tolerance in indica rice varieties at the seedling stage. Salinity tolerance in rice, like in
other glycophytes, is a function of cellular ion homeostasis. The large divergence in ion
homeostasis between the salt tolerant and salt sensitive rice varieties indicate that salt tolerant
variety shows lower Na+ influx, reduced Na+ translocation to the shoot, and maintains a lower
Na+:K+ ratio due to a higher membrane stability index (MSI) that effectively exclude the Na+.
As such, tolerant cultivars showed relatively unaffected growth supplemented with higher rate
of productivity and better control on reactive oxygen species (ROS) under salinity stress.
Lower TBARS (thiobarbituric acid reactive substance) content in salt tolerant FL478 roots
than its sensitive counterpart IR29 under stress indicated that its cellular membrane was
relatively unaffected by ROS despite high H2O2 content recorded under salt stress.
Comparatively higher SOD activity along with a parallel increase in transcript level of
superoxide dismutase (Os07g46990) in FL478 indicates that this protein may make a vital
contribution to salt stress tolerance. Although, the content of ascorbic acid remained
unchanged in FL478, the activity of the ascorbic peroxidises (APOXs) was reduced
comparably in both cultivars. There were several transcripts of peroxidase family that showed
salt induction (0s03g25300, Os03g55410, Os04g55740, 0s05g04490, 0s05g06970,
0s07g01410 and 0s07g48020) and may be important in the rice antioxidant response.
Genome-wide root transcriptomic analysis revealed several genes encoding membrane
transporter proteins which showed differential regulation in tolerant (FL478) and sensitive
(IR29) cultivars in response to salinity stress. Genes encoding aquaporins and N transporters
are induced in both cultivars. Genes encoding monovalent ion channels (OsTPCl, OsAKTl,
OsCNGC), monovalent ion transporters (OsHAK7), antiporters (OsCHXll), Si influx
transpoters (OsLsil) and several metal and sugar transporters showed differential regulation
between the cultivars. Heterologous expression data indicated that rice cDNA encodes for
(Os06g31070), a prolamin precursor, can rescue salt sensitive yeast phenotypes (G19
and Axt3K) from high salt stress. Comparative study of in planta partitioning of Na+ ions showed
that salt tolerant mechanism adopted by Nonabokra and Bw400 are significantly deviated from that
of pokkali which relies on minimizing the apoplastic Na+ content in leaf. Three types of salt
tolerant mechanisms in rice P-type (of Pokkali), N-type (of Nonabokra) and B-type (of Bw400)
were identifiable in this study. Besides, Bw400 also showed a positive cytoplasmic effect on salt
tolerance. Significantly large hybrid vigour of the FI plants derived from Bw400 and Pokkali
suggests that these parents have high prospect in rice breeding programmes for salt tolerance.
Estimation of genetic indices of salt associated Na+ and K+ homeostasis in FI plants indicated
that Na+ and K+ homeostasis in rice was controlled by different proportions of additive and
dominance effects. Rice genome bears a considerable heterosis effect that can be harnessed in
development of salt tolerant hybrids or varieties. On whole, this study revealed several
physiological processes of salt tolerance and their underlying genomic and genetic information
which are of crucial importance in future rice improvement programmes for salt tolerance