Plant physiology and biotechnology for the study and improvement of Mediterranean japonica rice varieties

Abstract

Rice belongs to the Poaceae family and Oryza genus. The genus Oryza comprehends 24 species, being 22 wild and 2 cultivated. These two cultivated species correspond to Oryza sativa, originated in Asia, nowadays cultivated and consumed worldwide, and Oryza glaberrima, originated in Africa, but cultivated and consumed limitedly in West Africa, both are diploid (2n=24) (Bernis & Pamies, 2006; Wei & Huang, 2019). Traditionally, Oryza sativa has been classified into two subspecies, indica and japonica. Molecular analyses, as well as biochemical and hybrid sterility analyses, point out at the separate domestication of Oryza rufipogon populations as the origin for these two subspecies (Garris et al., 2005; Rakshit et al., 2007; Wei et al., 2012), with a gene flow present from japonica to indica (Yang et al., 2012). Indica rice is usually long grained, less sticky and with a lower level of amylopectin than japonica rice, which is short grained and sticky. There are also phenotypical differences between these subspecies, for example, japonica plants are generally shorter, the leaves have a lighter color and shaper shape. Also, the tiller number is lower in japonica than in the indica plants (Wei & Huang, 2019).

The O. sativa plant is a semiaquatic annual monocot grass although rice can live as perineal in the tropics (McLean et al., 2013). The plant has a height that varies from 80 to 150 centimeters depending on the variety and growing conditions (Bernis & Pamies, 2006), or even less in modern varieties. Its morphology varies according to the development stage, this being the vegetative phase or the reproductive phase. The vegetative phase includes the germination, the seedling and the tillering stage. The reproductive phase comprehends panicle initiation, also called heading, and flowering stages. As a food, rice contributes to the 20% of calories ingested worldwide (Kubo & Purevdorj,

2004), reaching half of the calories ingested daily in some areas (Counce et al., 2000). In terms of consumption, as well as production, Asia is the main continent (McLean et al., 2013). The rice production and consumption in Europe is minor compared to Asia. Despite that, rice holds an important sociocultural role since it is one of the basic foods of the Mediterranean diet. Some regions have developed famous rice dishes, like risotto in Italy or paella in Spain. The annual rice consumption per capita in Europe is 6-18

kilograms in the southern regions, and 3.5-5.5 kilograms in the northern regions (McLean et al., 2013). The main European producer is Italy, followed by Spain. These two countries hold more than 75% of the total rice production in Europe. Rice in Europe is also of ecological importance, due to the great biodiversity that inhabits and benefits from the paddy fields. Around 70-80% of rice cultivated in Europe are japonica varieties, and the rest indica (Bernis & Pamies, 2006; Ferrero, 2007). Agriculture is the most important bioeconomy sector in Spain, with a value of 43.8 million of euros in 2015 and coping the 50.9% employment of all bioeconomy sectors (Lániez & Periago, 2016). Rice mobilized

258.766 millions of euros in Spain in 2019 (MINECO, 2019). Thus, the studies dedicated to improving different aspects of the rice production in Spain are crucial. This thesis has made a multidisciplinary approach, from genetics to the field, in order to study Mediterranean rice varieties considering its actual applicability for the Spanish rice industry through three experimental approaches: (i) an improvement of the anther culture protocol for two Mediterranean temperate japonica varieties and two tropical japonica varieties trough the addition of additives to the media; (ii) testing the salt-tolerance of Saltol-introgressed varieties to select those that will help to fight the apple snails plague in the Ebro Delta; and (iii) a study of genetic and physicochemical aspects of the rice pearl in five Mediterranean varieties, an important feature in gastronomical terms.