The nucleotide sequence of the putative DNA-binding domain of SlARF6A was amplified and fused to that of the glutathione S-transferase tag in a pGEX-4T-1 bacterial expression vector and expressed using Escherichia coli strain BM Rosetta . Isopropyl-β-D-thiogalactopyranoside was used to induce recombinant protein expression, and a GST-Tagged Protein Purification Kit was used to purify the protein. Purified recombinant proteins and biotin-labeled fragments of the target promoters were used to conduct EMSA with a Light Shift Chemiluminescent EMSA kit based on the method described in detail by Han et al.. The Pierce Biotin 3’ End DNA Labeling Kit was employed to label the probe containing the TGTCTC sequence with biotin. The unlabeled same sequence was used in the assay as a competitor. To generate the mutant probe, the TGTCTC DNA fragment was changed to AAAAAA. Biotin-labeled DNA was assayed via a ChemiDoc™ MP Imaging System based on the manufacturer’s procedures. All primers for the EMSA are listed in Supplementary Table S3.Many varieties of Citrus grown commercially will reproduce reasonably true to type from seed through a phenomenon known as nucellar embryony as described by Frost and Soost . These include sweet orange, Rough lemon, Cleopatra mandarin, Troyer citrange and others. Other varieties such as the pummelos, citrons, Algerian tangerine, Temple orange, etc. are monoembryonic and will not reproduce true to type from seed since they only produce hybrid progeny. Still others, such as certain lemons, Mexican lime, nansho-daidai, yuzu, etc. have relatively low polyembryony. On the other hand, some varieties such as the Washington navel orange, Bearss lime, Satsuma mandarin,vertical hydroponic and Pixie mandarin are seedless, or nearly so, and it would be impossible to obtain seed in adequate quantities.
Seedling trees of most varieties are vigorous upright growers, extremely thorny, and are late in coming into bearing. Many citrus species and varieties also do not inherently possess adequate hardiness to cold, resistance to soil-borne diseases, tolerance to salinity or high water tables, and other desirable qualities that would enable them to survive long in their planted environment as seedlings. Consequently, some are therefore occasionally propagated by some vegetative means such as cuttings, layers, or marcots, but generally by grafting or budding onto a root stock of some closely related variety, species, or even genera, or hybrids thereof, to take advantage of the root stock’s influence. The latter method is the one generally used in the propagation of citrus varieties, and the general technique is described by Platt and Opetz . The root stock and the scion interact with each other to produce stionic effects which may have a three-way influence. First, there is the influence of the scion upon the stock. For example, the scion may increase the sodium uptake of the root stock. The depth of root penetration or the extent and configuration of the root system may vary on a given root stock, depending upon the scion variety budded upon it. The root stock may greatly alter the scion. It may dwarf or invigorate it. Yields may be increased or decreased; fruit size may be altered; fruit quality can be affected; hardiness of the scion may also be influenced; and maturity and precociousness of the scion are other considerations. The union of a stock and a scion may give rise to a combination which may be affected by an external factor which by itself affects neither the stock nor the scion individually. Such a situation exists when the virus disease tristeza is present. The sweet orange by itself is not measurably affected; the sour orange by itself is not viscerally affected. However, when sweet orange is budded upon sour orange and tristeza virus is present, the combination will decline or die from the disease. If sour orange is budded upon sweet orange, there is no expression of the disease, even though the virus is present.
While it has long been recognized that the stock and scion have a reciprocal influence on each other, there must be a certain affinity or congenial relationship between them for healthy development of the composite plant. Different root stocks vary in their adaptability to grow on different soils and under different climatic conditions, as well as with different scion varieties. All of these factors will be thoroughly discussed in depth individually at a more appropriate place in this monograph. It is ridiculous to assume that any one root stock will have the general qualities to meet each grower’s needs, yet each individual grower’s specific root stock need is a critical choice for the success of his orchard. The successful choice of a root stock is important because it is to be a permanent part of that orchard and cannot be changed at will like a cultural practice, a fertilizer program, and irrigation schedule or pest control procedures. A considerable fund of information has accumulated in recent years throughout the citrus producing regions of the world concerning root stock reactions under different conditions. A recent comprehensive, but concise, review of much of the body of knowledge has been made by Wutscher . The last previous detailed treatment of the subject was by Webber and Batchelor and Rounds . The purpose of the present monograph, therefore, is that it will emphasize the importance of careful root stock selection for different commercial citrus varieties. It is an attempt to evaluate the response of the root stock to the influence of its total environment; describe the difficulties commonly encountered in choosing desirable root stocks; stress the need not only of considering the results of scientifically planned field experiments, but also the findings and observations of discriminating growers; and review critically the results of root stocks and related experiments. In addition, much original data and observations on trials conducted by the writer during a 40 year period at the Citrus Research Center of the University of California, Riverside are presented.
These are based on nearly 500 root stocks and 100,000 trees grown by the author and supplemented by numerous observations in commercial orchards throughout the world. It is just as important to use carefully selected root stocks of superior performance as it is to use selected superior fruit varieties. The selection of a root stock should be for the purpose of enhancing the merits of a scion variety, or adapting it to its total environment, rather than merely to follow local custom. The selection of improved fruit varieties has been in progress for centuries, but the choice of the best root stocks to use has not received much attention prior to a hundred years ago, and most of it has been in the last fifty years. Although propagation by budding and grafting was understood many centuries ago in China and elsewhere, it was mostly considered a curiosity rather than a practical measure,vertical lettuce tower and most commercial citrus trees throughout the world were grown as seedlings. Schenk states that citrus was budded in China before the time of Christ. The author finds this acceptable but extremely difficult to document. In Han Yen-chih’s “Chü Lu” written in 1178 A.D., and translated into English in 1923, he does describe the grafting process. However, he does indicate the method for grafting trees will be found in the work called “Ssu Shih Tsuan Yao,” which I have not been able to find a record of. Greek and Roman references are numerous. According to Condit the grafting and budding of fruit trees were common practices in the time of Theophrastus, who said, “The ingrafted part uses the other as an ordinary plant uses the ground. Whenever they have split the trunk, they insert the scion which they have fashioned to a wedge-shape; then with a mallet they drive it in to fit as snugly as possible.” Virgil in his Georgics and Ecologues, according to Condit , provided an explicit as well as a poetic account of grafting. Briefly it is as follows: “Nor is there one sole way to graft and bud, for where young eyes from the trees bark swell forth, bursting their slender sheaths, a slit is made just as the knot; and here they fasten in the shoot from stranger tree and bid it thrive in the moist sapwood. Or, smooth trunks are gashed and wedges through the solid timber driven. Then fruit scions set; in no long time the tall trees skyward lifts its laden boughs and sees with wonder what strange leaves it bears and fruitage not its own.” According to Gallesio , Palladius, who is thought to have written at some time in the 5th century states, “They graft the citron in April in warm districts and in May in colder latitudes, placing the graft not upon the bark, but opening the stem or trunk near the ground.” Also, wood cuts from Ferrari’s “Hesperides” clearly show the grafting technique being practiced in an ancient herbarium . In the early history of citrus culture, especially in the western world, consumption of the fruit was principally restricted to the area in which it was produced. As faster and more convenient transportation developed, new consumer markets developed, which in turn stimulated new plantings. New plantings created new problems.
Probably the general use of grafted or budded trees first became an accepted practice as a result of the outbreaks of “foot-rot,” in the middle of the 19th century, as pointed out by Fawcett as it occurred in the Azores in 1834. It was observed in the Mediterranean area that the sour orange was resistant to the disease and could be successfully used as a root stock in place of the susceptible sweet orange varieties. The introduction of seedless commercial varieties such as the Valencia Orange, Pera, or Marsh grapefruit to the industry also gave impetus to propagation by budding. Since the middle of the 20th century, considerable attention has been given to the choice of the best root stocks for the different varieties and soil conditions. However, for the most part, relevance has been placed on the results obtained in a given area with one stock, or at best with only two or three others for comparison. For nearly half a century in California, root stock choice was based on the adage, “Use sweet orange on the light soils and sour orange on the heavy soils.” If a root stock gave commercially successful results, it was generally considered satisfactory, with little incentive to search for a better one. This attitude still prevails in many citrus producing countries today. The greater part of the information available in any locality, with reference to successful stocks, was based on the experience of the growers, and often this is the most reliable available. It must be recognized, however, that such local experiences are inadequate, as they do not include replicated trials with a sufficient number of stocks or scion sources over a long enough period of time to supply valid comparisons. This is a situation which has prevailed in nearly every citrus growing area, but since the 1940’s growing emphasis has been given to systematic trials, especially in the United States and Brazil. Probably the one single factor which has given more impetus than any other to the recent emphasis on citrus root stock trials around the world is the occurrence of tristeza, and the intricate response of various stock-scion combinations to its presence. While the disease has primarily affected the sweet orange and certain other scions on sour orange root stock, many other root stocks and combinations can be affected. See the section on tristeza elsewhere in this monograph. Studies on the etiology of tristeza focused greater attention on other diseases caused by transmissible agents which are often influenced in their reaction by certain root stock-scion combinations or the root stock itself . The inroads of disease usually necessitate growing citrus where other citrus has previously grown, and the complex replant problem becomes a serious consideration. Rising production costs economically demand greater production per acre either in tons of fruit or pounds of soluble solids. Greater production necessitates new markets and the competition increases demands for better quality fruit, both for fresh fruit and for concentrate or other purposes. In order to solve these problems, it becomes increasingly necessary to obtain the greatest production at the least possible cost.