学

論文題目位論文の要

旨

Physiological Responses of Rice to Saline-Alkaline Stress(塩アルカリストレスに対するイネの生理的応答)

広島大学大学院統合生命科学研究科
Bioresource プログラム
学生番号D192378
氏
名
Rowland Maganizo Kamanga

Salt stress is a major abiotic stress globally, affecting over 800 million ha of land.
Depending on soil pH, salt stress can be categorized into neutral salt stress, popularlyknown as “salinity stress” and saline-alkaline stress, also known as “alkali” stress. Overthe years, salinity stress has been extensively studied, hence physiological and molecularmechanisms underpinning plants’ responses to salinity stress have been thoroughly
elucidated. Contrarily, relatively less is known regarding how saline-alkalinity confronts
plant growth. Meanwhile, physiological mechanisms regulating plants’ resistance undersaline-alkaline stress remain hugely conjectural. Rice (Oryza sativa L.) represents a cropof prominent worldwide food value in addition to being a key model research crop, yet
known for its reputation of extreme sensitivity to salt stress. This study was aimed atunravelling comparative physiological mechanisms in which saline-alkalinity confronts
growth and physiology of rice plants. Furthermore, the study explored the potential ofacclimation to different stress components in developing cross-tolerance to saline
alkaline stress.

In order to evaluate comparative physiological responses of rice to saline-alkalinity, 21
day old seedlings of rice cultivar Hinohikari were subjected to control conditions (0 mMNa+, pH 5.5) neutral salinity stress (50 mM Na+, pH 5.5), moderate and severe alkalinity(0 mM Na+, pH 7.5 and 8.5 respectively) and moderate and severe saline-alkalinity (50mM Na+, pH 7.5 and 8.5 respectively) for 16 days. Time course changes in growth weremonitored every 4 days. The study revealed that saline-alkaline stress was the mostgrowth limiting, with growth nearly ceasing on the fourth day of stress imposition,
whereas under salinity stress growth decreases were only apparent from the 12th day.

Furthermore, at the same growth media pH, sole alkalinity was less detrimental relative
to saline-alkalinity, but more lethal than salinity stress. Therefore, effects ofrhizospheric Na+ are aggravated by growth media pH, rendering saline-alkalinity more
harmful on growth than sole salinity and sole alkalinity.

Next, comparative physiology underlying saline-alkaline damage on rice seedlings wasinvestigated. Plant grown under both alkaline and saline-alkaline conditions wascharacterized by chlorosis, which was confirmed to have resulted from Fe deficiency,
particularly in younger leaves owing to immobile nature of Fe. In older leaves, alkaline
stressed plants exhibited healthier leaves, whereas saline-alkaline stressed plants wereseverely chlorotic and nearly dying off. This chlorosis was ascribed to leaf blade Na+toxicity, which was inordinately high in saline-alkaline stressed plants, resulting intopremature senescence of older leaves. Additionally, saline-alkaline stress incited K+
deficiency predominantly in roots, due to tremendous root K+ leakage, thus, while
resistance to saline-alkaline stress may be complex and multigenetic, ability to regulate
root K+ loss may be a valuable trait. Furthermore, it was observed that undersaline-alkaline stress, leaf blade membrane integrity was substantially compromised,
due to toxic accumulation of Na+ in leaf blades, and resultant oxidative stress as shownby high accumulation of H2O2 in leaf blades. Hence, it was concluded that at the used
experimental conditions, rice plants are more sensitive to high growth media pH, andthis sensitivity is aggravated under mixed saline-alkaline conditions. Primarily,
hyperaccumulation of Na+ in leaf blades, accelerated root K+ deficiency due to root K+leakage, Fe deficiency in younger leaves, and oxidative stress mediated membrane
damage were crucial causes of severity under saline-alkalinity.

Premised on the these findings, the study further explored the potential of acclimation indeveloping cross-tolerance to saline-alkalinity. Rice plants were pre-treated (PT) withthree concentrations of each of NaCl, NaHCO2, and H2O2 for one week, then subjected to50 mM Na+ at pH 8.25 for 14 days and investigated growth and physiological responses
relative to non-pretreated (NPT) plants. Acclimation to 10 mM NaCl (salinity) and 10 µM
H2O2 (oxidative stress) were the most effective, hence were the focus for furtherinvestigation. These PTs developed cross-tolerance to saline-alkaline stress, in addition

to promoting quicker recovery and resumption of growth upon conclusion of stress.
Physiologically, both PTs noticeably maintained a lower growth medium pH, employing agrowth medium acidification strategy which was not further investigated. The lower
growth medium pH also facilitated iron acquisition, whose uptake is else constrained
under high pH. On ionic relations, NaCl pretreated plants accumulated much lower Na +in leaf blades by enhancing root Na+ efflux. In H2O2 pretreated plants, despite higher
root Na+ efflux, leaf blade Na+ accumulation was not reduced. However, experiments onNa+ uptake in excised leaf blades showed lower accumulation of Na+. These results may
suggest that the high leaf blade Na+ accumulation in H2O2 pretreated plants emanatedfrom processes relating to either uptake by roots and/ or resultant transport to the shoots.
That

notwithstanding,deleterious

effectsofNa+ wereameliorated

andleaf

photosynthetic competency was preserved by both H2O2 and NaCl PTs. Additionally, bothPTs were unable to lower saline-alkaline – triggered root K+ leakage, but efficiently
reduced oxidative stress – triggered membrane damage and lipid peroxidation, andimproved cell viability by constituting a complete shield of antioxidant enzymes SOD,CAT, APX and GR, establishing an ultimate equilibrium between ROS generation andscavenging.

These findings propose a possible crosstalk between salinity stress, oxidative stress andsaline-alkaline stress, such that exposure to salinity stress or oxidative stress develops
cross-tolerance to saline-alkaline stress. This is an important finding particularly in lightof a myriad of abiotic stresses, offering a prospect for breeding plants resistant to a rangeof abiotic stresses. Besides, upon elimination of stress and resumption of normal growth
conditions, pretreated plants, particularly those pretreated with H2O2 quickly resumed
growth and physiologically recovered, a crucial attribute especially under recurrent
abiotic stresses with some prolonged inter stress recovery periods. In order to provide afairly broad picture of the mechanism of cross-tolerance, this study recommends a further
exploration of molecular and perhaps epigenetic marks underpinning cross-tolerance,
and an investigation of overlapping signaling elements between salinity, oxidative andsaline-alkaline stress.