Abstract
In cereal grains, the maternal nucellar projection (NP) constitutes the link to the filial organs, forming a transfer path for assimilates and signals towards the endosperm. At transition to the storage phase, the NP of barley (Hordeum vulgare) undergoes dynamic and regulated differentiation forming a characteristic pattern of proliferating, elongating, and disintegrating cells. Immunolocalization revealed that abscisic acid (ABA) is abundant in early non-elongated but not in differentiated NP cells. In the maternally affected shrunken-endosperm mutant seg8, NP cells did not elongate and ABA remained abundant. The amounts of the bioactive forms of gibberellins (GAs) as well as their biosynthetic precursors were strongly and transiently increased in wild-type caryopses during the transition and early storage phases. In seg8, this increase was delayed and less pronounced together with deregulated gene expression of specific ABA and GA biosynthetic genes. We concluded that differentiation of the barley NP is driven by a distinct and specific shift from lower to higher GA:ABA ratios and that the spatial-temporal change of GA:ABA balances is required to form the differentiation gradient, which is a prerequisite for ordered transfer processes through the NP. Deregulated ABA:GA balances in seg8 impair the differentiation of the NP and potentially compromise transfer of signals and assimilates, resulting in aberrant endosperm growth. These results highlight the impact of hormonal balances on the proper release of assimilates from maternal to filial organs and provide new insights into maternal effects on endosperm differentiation and growth of barley grains.
Highlights
The nucellar projection (NP) develops from the nucellus tissue facing the main vascular bundle and reveals a complex pattern of simultaneous cell division, differentiation, and disintegration (Thiel et al, 2008; Radchuk et al, 2011).The release of assimilates from the nucellus and the NP partially depends on programmed cell death (PCD) (Radchuk et al, 2006)
Assimilates are transferred from the main vascular bundle via NP and endosperm
The other predicted GA biosynthesis genes were expressed in Bowman and seg8 NPs at 7 d after fertilization (DAF) but elevated in seg8 either at 5 DAF (HvGA3ox1) or 10 DAF (HvGA20ox3, HvGA20ox5, and HvGA2ox5) (Fig. 9). These results indicated transiently increased expression of abscisic acid (ABA) biosynthesis genes in the seg8 NP at 7 DAF, namely of HvNCED2 and HvAAO2
Summary
The nucellar projection (NP) develops from the nucellus tissue facing the main vascular bundle and reveals a complex pattern of simultaneous cell division, differentiation, and disintegration (Thiel et al, 2008; Radchuk et al, 2011).The release of assimilates from the nucellus and the NP partially depends on programmed cell death (PCD) (Radchuk et al, 2006). The NP represents an important interface, which accomplishes transfer and inter-conversion of Abbreviations: 2ODD, 2-oxoglutarate dependent dioxygenase; AAO, aldehyde oxidase; ABA, abscisic acid; DAF, days after flowering; EDC, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide; GA, gibberellin; NCED, 9-cis-epoxycarotenoid dioxygenase; NP, nucellar projection; PCD, programmed cell death; qRT-PCR, quantitative reverse transcription-PCR; SE, standard error; TUNEL, terminal deoxynucleotidyl transferase dUTP nick end labelling. Analysing the development of the NP addresses important yield-related traits
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