Physiological and biochemical studies on growth and low phosphorus tolerance in low-phytate soybean lines

概要

Doctoral Thesis

Physiological and biochemical studies on growth and low phosphorus
tolerance in low-phytate soybean lines

Summary

QIN DONG

Graduate School of Biosphere Science
Hiroshima University
September 2020





1. Introduction

Phytic acid (myo-inositol-1,2,3,4,5,6-hexakisphosphate; InsP6, phytate-P), a storage form
of phosphorus (P) in plant seeds, occurs mainly in the phytate anion form, and accounts for
approximately 70–90% of the total seed P content. However, phytate is poorly utilized by
monogastric animals, such as chickens, swine, and humans, which have low phytase activity
in their digestive system. Improving P and mineral bioavailability is important for stock
farming and human food security. As a result, low-phytate (LP) crops and legumes have been
developed.
P is a vital macronutrient for plant growth, and soil P levels are generally high. However,
despite this abundance, plant growth is limited by P uptake because the forms of P in the soil
environment are relatively less available and immobile owing to the transformation of P into
organic forms, such as phytates. Under P deficiency, plants can improve their root system by
producing longer primary roots and more extensive lateral roots to improve their P acquisition
ability. Investigations of the utilization of these potentially valuable ‘low phytate’ traits and of
P utilization under P deficiency are important for the development of stock farming practices,
to achieve food security, and to ensure a sustainable global P supply. This study investigated
(1) the physiological and biochemical characteristics and bioavailability of soybean LP and NP
lines during germination; (2) root and root nodule N fixation, phytase and phosphatase
activities, and the accumulation of biomass and minerals in LP and NP soybean seedlings; and
(3) the physiological and biochemical characteristics, and secretion of organic acids (OAs) and
acid phosphatase (APase) in LP soybean lines in response to P deficiency.
2. Changes in physiological characteristics, phytase activity, and mineral bioavailability
of a low-phytate soybean line during germination
To determine if the germination of the LP and NP soybean lines was affected by a low
phytate content, plants were grown in vermiculite the growth, phytase activity, mineral
concentration and bioavailability, and mineral extractability of the seedlings were evaluated.
Seedling growth did not differ significantly between the NP and LP lines before exhausting all
nutrients in the cotyledon. Phytase activities at 11 to 17 days after sowing were 1.3- to 2.6folds higher in the NP line than in the LP line. The phytate concentration decreased by 18.6%
in the NP line and by 32.0% in the LP line 7 days after sowing. In addition, the phytate was
hydrolyzed by the LP line 2 days earlier than by the NP line. The LP line had higher molar
ratios of phytic acid to Ca, Mg, and K than the NP line. These results suggested that
germination and early seedling growth are not affected by low phytate levels in seeds; however,
decreased phytase activity might be related to the lower phytate content in LP seeds.
Furthermore, these results indicated that the LP line benefits from a higher bioavailability of P
and macro-minerals, unlike the NP line.
3. Changes in seedling growth, physiological characteristics, nitrogen fixation, and root
and nodule phytase and phosphatase activity of a low-phytate soybean line
To clarify the ‘low phytate’ effects on the physiological and biochemical characteristics
of the LP and NP soybean lines at the seedling stage, this study evaluated the seed P and mineral
contents, seedling dry weight, carbon (C) and nitrogen (N) accumulation, nitrogen fixation,
and root and nodule phytase and phosphatase activity levels at 21 days after sowing. Seedling
dry weight and C and N accumulations were 31%, 38%, and 54% higher, respectively, in the
LP line than in the NP line. The N fixation levels were 46% higher in the LP nodules than in
the NP nodules. The phytase and phosphatase levels were 1.4-folds and 1.3-folds higher,
respectively, in the LP roots than in the NP roots. The phosphatase levels in the LP nodules
were 1.5-folds higher than those in the NP nodules. The mineral levels were substantially
higher in the LP seeds and seedings than in those of the NP line. The HCl extractabilities of P,
S, Fe, Cu, and Mn were higher in the LP seeds than in the NP seeds. These results indicated
that the LP line showed superior seedling growth and N fixation relative to the NP line. The

LP line showed relatively higher root phytase and root and nodule phosphatase activity levels
than the NP line. These results indicate that the LP line is be better suited and more adaptable
to low P conditions.
4. Changes in organic acids and phosphatase secreted from the roots of low-phytate
soybean lines under low-phosphorus conditions
To exploit and utilize these potentially valuable ‘low phytate’ traits, it is necessary to
understand the physiological and biochemical characteristics of different LP soybean lines (i.e.,
LP-1 and LP-2 in the present study) as adaptations to low P stress compared with the NP
cultivar “Natto Kotsubu” The LP lines showed relatively higher root dry weights and P
contents than the NP cultivar. The total root exudation of LP-2 was higher than that of the LP1 line and the NP cultivar under low-P conditions. The LP lines secreted more APase and
produced more extensive lateral roots than the NP cultivar. The APase levels were higher in
the roots of both LP lines under low-P conditions than in the roots of the NP cultivar. The leaf
photosynthesis rate and stomatal conductance were also higher in the LP lines than those of
the NP cultivar under low-P conditions. In summary, the LP lines exhibited more extensive
root development, and higher total P and internal APase levels than the NP cultivar. The LP
lines also showed relatively higher OA levels and APase exudation from the roots than the NP
cultivar.
5. General discussion and conclusions
These findings revealed that the LP and NP lines showed similar plant growth traits while
reliant on the nutrients supplied by the cotyledon at the germinative stage. However, after the
germinative stage, the LP line showed higher root and nodule mineral bioavailability, higher
P-related enzyme activity levels, better physiological performance, and secreted more OAs and
APase than the NP line. Moreover, LP cultivars may have a higher P-acquisition ability and
thus have a higher adaptive potential to low P conditions than NP lines and the “Natto Kotsubu”
cultivar, which is the breeding base cultivar of LP progeny.