石榴籽粒硬度研究进展

Progresses in research on pomegranate seed hardness

石榴果实营养丰富、风味独特,具有良好的营养保健功能。随着人们对石榴营养保健功能认识的进一步加深,石榴的市场需求逐渐增大,产业得到了迅猛发展。籽粒硬度作为鲜食石榴的重要品质指标,直接影响着果实的感官品质和消费者的认可度。籽粒硬度已成为近年来石榴研究的热点。从石榴籽粒硬度的形成、石榴籽粒硬度的评价、影响石榴籽粒硬度的因素、石榴籽粒硬度的遗传基础研究等方面进行了全面综述,并介绍了我国自主选育的软籽及半软籽石榴新品种。结合已有研究基础,提出了研究中存在的不足及今后的研究展望。旨在为进一步开展石榴籽粒硬度形成机制与调控技术研究提供方向,为进一步开展石榴籽粒硬度的遗传改良及新品种选育提供指导。

Pomegranate(Punica granatum L.)is a fruit species cultivated worldwide.Pomegranate fruits possess rich nutrition,unique flavor and important economic value,and it is favorite by people for its nutrients benefiting to human health.As more attentions are being paid to the nutrition and health protection functions of pomegranate fruits,its market demand has increased rapidly and the industry has developed gradually.Pomegranate cultivars can be divided into soft-seeded cultivars,semi-softseeded cultivars,hard-seeded cultivars,and so on.Seed hardness is one of the important quality characteristics of fresh pomegranate,which determines the sensory quality of the fruit and consumer acceptance.In recent years,with the increasing attentions on the seed hardness of pomegranate fruits,there were many studies focusing on the seed hardness formation and soft-seed pomegranate breeding,so as to understand the reason for pomegranate seed hardness,and provide a reference to further study the genetic mechanism of seed hardness formation in pomegranate.In the present review,seed hardness formation and its influencing factors are reviewed,and candidate genes involving in seed hardness formation and the heredity patterns of seed hardness are summarized.As we known,there are two layers of seed coats for pomegranate,that is,outer and inner seed coats.The soft and juicy outer seed coat is rich in nutrients,which is the main edible part of pomegranate fruits.The inner seed coat is rich in lignin and cellulose,which forms the seed hardness.The seed hardness formation is a process of lignin accumulation in endoderm cells and thickening of secondary cell walls,namely,a process of lignification of endoderm cell walls.The seed hardness is depended on cultivars,growing technology and environment,while the genetics of cultivars plays the decisive roles.Lignin is composed of the primary monolignols p-coumaryl alcohol,coniferyl alcohol,and sinapyl alcohol,which form predominantly by oxidative polymerization of the three major monolignols to generate the hydroxyphenyl(H),guaiacyl(G),and syringyl(S)lignin subunits.Nontarget metabolic profiling revealed that coniferyl alcohol and sinapyl alcohol were main monolignols accumulated in seed coat of pomegranate,while little p-coumaryl alcohol was detected in the inner seed coat.Thus,S lignin and G lignin appeared to be the dominant lignin subunits in pomegranate seed coats.In addition,the accumulation of cellulose and hemicellulose in seed coat was significantly different between hard-seed and soft-seed pomegranates,which suggested that the accumulation difference of cellulose and hemicellulose in seed coat was also an important factor in the formation of pomegranate seed hardness.Several genes involving in lignin,cellulose and hemicellulose synthesis and degradation had special expression during seed hardness formation,which were identified and considered as the potential candidate genes involving in seed hardness formation.Pgr011171.1 encoding POD,Pgr013634.1 encoding F5 H,Pgr022328.1,Pgr006310.1,Pgr006318.1,and Pgr006319.1 encoding CAD,and Pgr011171.1 encoding POD had lots of transcript accumulation,accompanied with the sinapyl alcohol and coniferyl alcohol accumulation,were candidate genes involving in seed hardness formation.Basing on the results of co-expression network analysis of lignin,cellulose and hemicellulose metabolism-related genes and the transcription factors for inner seed coats at different developmental stages with both hard-seed cultivar and soft-seed cultivar,Pgr000815.1 encoding AUX-IAA might be involved in the lignin metabolism,and Pgr011491.1 and Pgr022940.1 encoding AUX-IAA2,and Pgr017424.1 encoding NAC66 might be involved in the metabolism of cellulose and hemicellulose.PgL0137670 encoding NAC transcript factor has a SNP site(T-C)at 166 bp,and the allelic variant T site of PgL0137670 is associated with the characteristics of soft-seed,according to the comparative transcriptome analysis of soft-seeded cultivar and hard-seeded cultivar.Besides,a number of selective sites related to seed hardness including SUC8,SUC6,FOXO and MAPK genes were selected basing on the whole genome resequencing analysis with different seed hardness varieties.However,the gene functions need further studies.The growth environment also has a certain impact on the seed hardness of pomegranate.Analysis of the seed hardness of pomegranate grown in different soils found that the seed hardness of pomegranate grown in sandy soil was the highest,followed by loam soil and the smallest in red loam soil.The study on the effect of different fertilization treatments on the seed hardness of pomegranate found that the application of phosphorus fertilizer could reduce the seed hardness,while the application of fertilizer such as urea,compound fertilizer and potash fertilizer had no significant effect on the seed hardness.Gibberellin,2,4-dichlorophenoxyacetic acid(2,4-D)and naphthalene acetic acid are widely used plant growth regulators,and they have significant effects on improving fruit quality.Research showed that foliar spraying of these growth regulators during pomegranate flowering could significantly reduce the seed hardness of pomegranate.Although the study of hormones in regulating the seed hardness of pomegranate has been reported in related studies,in the actual production process,there is still a lack of suitable plant growth regulator application technology to promote pomegranate seed softening.In conclusion,seed hardness is one of the characteristics of pomegranate.Although different cultivation environments or artificial measures have a certain impact on the seed hardness of pomegranate,this effect is relatively limited,and genetics is the fundamental factor that determines seed hardness.Finally,the new pomegranate cultivars,bred independently in China and having soft or semi-soft seeds,are introduced in the review.Summarily,the review will provide not only the insight to further study on mechanism and regulation of seed hardness formation,but also the guidance for genetic improvement and breeding in pomegranate.